Minimize Roman Space Telescope

Roman Space Telescope / former WFIRST (Wide Field Infrared Survey Telescope)

Development Status   Spacecraft   Launch    Sensor Complement    Ground Segment    References

 

WFIRST is a NASA observatory designed to perform wide-field imaging and surveys of the NIR (Near Infrared ) sky and to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. The telescope has a primary mirror that is 2.4 m in diameter, and is the same size as the Hubble Space Telescope's primary mirror. WFIRST will have two instruments, the WFI (Wide Field Instrument), and the CGI (Coronagraph Instrument).

WFIRST is the top-ranked large space mission in the ”New Worlds, New Horizon Decadal Survey of Astronomy and Astrophysics,” written by the U.S. National Research Council in 2010. That study, which laid out a blueprint for ground- and space-based astronomy and astrophysics for the decade of the 2010s, rated WFIRST as the top-priority large-scale mission. NASA's current plans call for WFIRST to perform an extraordinarily broad set of scientific investigations: studying the newly-discovered phenomenon of dark energy, measuring the history of cosmic acceleration, completing the exoplanet census begun by NASA's Kepler Space Telescope and demonstrating technology for direct imaging and characterization of exoplanets. 1)

After years of preparatory studies, NASA is formally starting an astrophysics mission designed to help unlock the secrets of the universe — the WFIRST (Wide Field Infrared Survey Telescope). NASA's Agency Program Management Council, which evaluates the agency's programs and projects on content, risk management, and performance, made the decision to move forward with the mission on February 17, 2016. 2)

The Wide Field Instrument will provide a field of view of the sky that is 100 times larger than images provided by HST (Hubble Space Telescope). The coronagraph will enable astronomers to detect and measure properties of planets in other solar systems. 3)

On 20 May 2020, NASA renamed WFIRST in honor of Nancy Grace Roman, NASA’s first chief astronomer, who paved the way for space telescopes focused on the broader universe.

The newly named Nancy Grace Roman Space Telescope – or Roman Space Telescope, for short – is set to launch in the mid-2020s. It will investigate long-standing astronomical mysteries, such as the force behind the universe’s expansion, and search for distant planets beyond our solar system.

Considered the “mother” of NASA’s Hubble Space Telescope, which launched 30 years ago, Roman tirelessly advocated for new tools that would allow scientists to study the broader universe from space. She left behind a tremendous legacy in the scientific community when she died in 2018.

“It is because of Nancy Grace Roman’s leadership and vision that NASA became a pioneer in astrophysics and launched Hubble, the world’s most powerful and productive space telescope,” said NASA Administrator Jim Bridenstine. “I can think of no better name for WFIRST, which will be the successor to NASA’s Hubble and Webb Telescopes.”

Former Senator Barbara Mikulski, who worked with NASA on the Hubble and WFIRST space telescopes, said, "It is fitting that as we celebrate the 100th anniversary of women’s suffrage, NASA has announced the name of their new WFIRST telescope in honor of Dr. Nancy Roman, the Mother of Hubble – well deserved. It recognizes the incredible achievements of women in science and moves us even closer to no more hidden figures and no more hidden galaxies."

Who Was Nancy Grace Roman?

Born on May 16, 1925, in Nashville, Tennessee, Roman consistently persevered in the face of challenges that plagued many women of her generation interested in science. By seventh grade, she knew she wanted to be an astronomer. Despite being discouraged about going into science – the head of Swarthmore College’s physics department told her he usually dissuaded girls from majoring in physics, but that she “might make it” – Roman earned a bachelor’s degree in astronomy from Swarthmore in 1946 and a doctorate from the University of Chicago in 1949.

She remained at Chicago for six years and made discoveries about the compositions of stars that had implications for the evolution of our Milky Way galaxy. Knowing that her chances of achieving tenure at a university as a woman were slim at that time, she took a position at the U.S. Naval Research Laboratory and made strides in researching cosmic questions through radio waves.

Roman came to NASA in 1959, just six months after the agency had been established. At that time, she served as the chief of astronomy and relativity in the Office of Space Science, managing astronomy-related programs and grants.

“I knew that taking on this responsibility would mean that I could no longer do research, but the challenge of formulating a program from scratch that I believed would influence astronomy for decades to come was too great to resist,” she said in a NASA interview.

This was a difficult era for women who wanted to advance in scientific research. While Roman said that men generally treated her equally at NASA, she also revealed in one interview that she had to use the prefix “Dr.” with her name because “otherwise, I could not get past the secretaries.”

But she persisted in her vision to establish new ways to probe the secrets of the universe. When she arrived at NASA, astronomers could obtain data from balloons, sounding rockets and airplanes, but they could not measure all the wavelengths of light. Earth’s atmosphere blocks out much of the radiation that comes from the distant universe. What’s more, only a telescope in space has the luxury of perpetual nighttime and doesn’t have to shut down during the day. Roman knew that to see the universe through more powerful, unblinking eyes, NASA would have to send telescopes to space.

Through Roman’s leadership, NASA launched four Orbiting Astronomical Observatories between 1966 and 1972. While only two of the four were successful, they demonstrated the value of space-based astrophysics and represented the precursors to Hubble. She also championed the IUE (International Ultraviolet Explorer), which was built in the 1970s as a joint project between NASA, ESA (European Space Agency) and the United Kingdom, as well as the COBE (COsmic Background Explorer), which measured the leftover radiation from the big bang and led to two of its leading scientists receiving the 2006 Nobel Prize in Physics.

Above all, Roman is credited with making the Hubble Space Telescope a reality. In the mid-1960s, she set up a committee of astronomers and engineers to envision a telescope that could accomplish important scientific goals. She convinced NASA and Congress that it was a priority to launch the most powerful space telescope the world had ever seen.

Hubble turned out to be the most scientifically revolutionary space telescope of all time. Ed Weiler, Hubble’s chief scientist until 1998, called Roman “the mother of the Hubble Space Telescope.”

“Nancy Grace Roman was a leader and advocate whose dedication contributed to NASA seriously pursuing the field of astrophysics and taking it to new heights,” said Thomas Zurbuchen, NASA’s associate administrator for science. “Her name deserves a place in the heavens she studied and opened for so many.”

What is the Roman Space Telescope?

The Roman Space Telescope will be a NASA observatory designed to settle essential questions in the areas of dark energy, exoplanets and infrared astrophysics. The telescope has a primary mirror that is 2.4 meters in diameter and is the same size as the Hubble Space Telescope's primary mirror. The Roman Space Telescope is designed to have two instruments, the Wide Field Instrument and a technology demonstration Coronagraph Instrument. The Wide Field Instrument will have a field of view that is 100 times greater than the Hubble infrared instrument, allowing it to capture more of the sky with less observing time. The Coronagraph Instrument will perform high contrast imaging and spectroscopy of individual nearby exoplanets.

The WFIRST project passed a critical programmatic and technical milestone in February, giving the mission the official green light to begin hardware development and testing. With the passage of this latest key milestone, the team will begin finalizing the mission design by building engineering test units and models to ensure the design will hold up under the extreme conditions during launch and while in space.

NASA’s Fiscal Year 2020 Consolidated Appropriations Act funds the WFIRST program through September 2020. It is not included in the Fiscal Year 2021 budget request, as the administration wants to focus on completing the James Webb Space Telescope.

Table 1: NASA Telescope Named For ‘Mother of Hubble’ Nancy Grace Roman 4)

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Figure 1: NASA’s Wide Field Infrared Survey Telescope (WFIRST) is now named the Nancy Grace Roman Space Telescope, after NASA’s first Chief of Astronomy (image credit: NASA)

Figure 2: Scheduled to launch in the mid-2020s, the Nancy Grace Roman Space Telescope, formerly known as WFIRST, will function as Hubble’s wide-eyed cousin. While just as sensitive as Hubble's cameras, the Roman Space Telescope's 300-megapixel Wide Field Instrument will image a sky area 100 times larger. This means a single Roman Space Telescope image will hold the equivalent detail of 100 pictures from Hubble (video credit: NASA)


Science objectives:

• Dark Energy: In 1998, scientists discovered that the expansion of the universe is accelerating, causing them to reconsider their models for the formation of the universe. In order to explain their observations, cosmologists have to assume that 3/4 of the universe is filled with mass and energy that cannot be observed through traditional means. This 'dark matter' and 'dark energy' has revolutionized cosmology. WFIRST will contribute to our understanding of the nature of dark energy by addressing two questions:

- Is cosmic acceleration caused by a new energy component or by the breakdown of General Relativity on cosmological scales?

- If the cause is a new energy component, is its energy density constant in space and time, or has it evolved over the history of the universe?

WFIRST will conduct 3 different types of surveys in order to answer these questions.

1) HLSS (High Latitude Spectroscopic Survey): The HLSS will measure accurate distances and positions of a very large number of galaxies. By measuring the changes in the distribution of galaxies, the evolution of dark energy over time can be determined. The High Latitude Survey will measure the growth of large structure of the universe, testing theory of Einstein's General Relativity.

2) SNe (Type Ia Supernovae) Survey: The SNe survey uses type Ia SNe as "standard candles" to measure absolute distances. Patches of the sky are monitored to discover new supernovae and measure their light curves and spectra. Measuring the distance to and redshift of the SNe provides another means of measuring the evolution of dark energy over time, providing a cross-check with the high latitude surveys.

3) HLIS (High Latitude Imaging Survey): The HLIS will measure the shapes and distances of a very large number of galaxies and galaxy clusters. The shapes of very distant galaxies are distorted by the bending of light as it passes more nearby mass concentrations. These distortions are measured and used to infer the three-dimensional mass distribution in the Universe. This survey will determine both the evolution of dark energy over time as well as provide another independent measurement of the growth of large structure of the universe.

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Figure 3: Is the Universe expanding more slowly than previously thought? A University of Arizona-led team of astronomers found that the type of supernovae commonly used to measure distances in the universe fall into distinct populations not recognized before. The findings have implications for our understanding of how fast the universe has been expanding since the Big Bang (image credit: NASA)


Some background on Dark Energy and Dark Matter

In the early 1990s, one thing was fairly certain about the expansion of the universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it. 5)

Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.

What Is Dark Energy? More is unknown than is known. We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the universe.

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Figure 4: Changes in the Rate of Expansion over Time: This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart (image credit: NASA/STScI/Ann Feild)

One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.

A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need? There are candidate theories, but none are compelling. So the mystery continues.

The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.

What Is Dark Matter? By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter. What is dark matter?

We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.

However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as MACHOs (Massive Compact Halo Objects). But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).

The Expanding Universe

When astronomer Edwin Hubble discovered nearly 100 years ago that the universe was uniformly expanding in all directions, the finding was a big surprise. Then, in the mid-1990s, another shocker occurred: astronomers found that the expansion rate was accelerating perhaps due to a repulsive property called "dark energy." Now, the latest measurements of our runaway universe suggest that it is expanding faster than astronomers thought. The consequences could be very significant for our understanding of the shadowy contents of our unruly universe. It may mean that dark energy is shoving galaxies away from each other with even greater – or growing – strength. Or, the early cosmos may contain a new type of subatomic particle referred to as "dark radiation." A third possibility is that "dark matter," an invisible form of matter that makes up the bulk of our universe, possesses some weird, unexpected characteristics. Finally, Einstein's theory of gravity may be incomplete. 6)

These unnerving scenarios are based on the research of a team led by Nobel Laureate Adam Riess, who began a quest in 2005 to measure the universe's expansion rate to unprecedented accuracy with new, innovative observing techniques. The new measurement reduces the rate of expansion to an uncertainty of only 2.4 percent. That's the good news. The bad news is that it does not agree with expansion measurements derived from probing the fireball relic radiation from the big bang. So it seems like something's amiss – possibly sending cosmologists back to the drawing board.

Astronomers using NASA's Hubble Space Telescope have discovered that the universe is expanding 5 percent to 9 percent faster than expected. "This surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95 percent of everything and don't emit light, such as dark energy, dark matter, and dark radiation," said study leader and Nobel Laureate Adam Riess of the STScI (Space Telescope Science Institute) and The Johns Hopkins University, both in Baltimore, Maryland.

There are a few possible explanations for the universe's excessive speed. One possibility is that dark energy, already known to be accelerating the universe, may be shoving galaxies away from each other with even greater – or growing – strength.

Another idea is that the cosmos contained a new subatomic particle in its early history that traveled close to the speed of light. Such speedy particles are collectively referred to as "dark radiation" and include previously known particles like neutrinos. More energy from additional dark radiation could be throwing off the best efforts to predict today's expansion rate from its post-big bang trajectory.

The boost in acceleration could also mean that dark matter possesses some weird, unknown characteristics. Dark matter is the backbone of the universe upon which galaxies built themselves up into the large-scale structures seen today. - And finally, the speedier universe may be telling astronomers that Einstein's theory of gravity is incomplete.

Riess' team made the discovery by refining the universe's current expansion rate to unprecedented accuracy, reducing the uncertainty to only 2.4 percent. The team made the refinements by developing innovative techniques that improved the precision of distance measurements to faraway galaxies.

These measurements are fundamental to making more precise calculations of how fast the universe expands with time, a value called the Hubble constant. The improved Hubble constant value is 73.2 km/second per megaparsec (a megaparsec equals 3.26 million light-years). The new value means the distance between cosmic objects will double in another 9.8 billion years.

This refined calibration presents a puzzle, however, because it does not quite match the expansion rate predicted for the universe from its trajectory seen shortly after the big bang. Measurements of the afterglow from the big bang by NASA's WMAP (Wilkinson Microwave Anisotropy Probe) and the European Space Agency's Planck satellite mission yield predictions for the Hubble constant that are 5 percent and 9 percent smaller, respectively.

"We know so little about the dark parts of the universe, it's important to measure how they push and pull on space over cosmic history," said Lucas Macri of Texas A&M University in College Station, a key collaborator on the study. Added Riess: "If we know the initial amounts of stuff in the universe, such as dark energy and dark matter, and we have the physics correct, then you can go from a measurement at the time shortly after the big bang and use that understanding to predict how fast the universe should be expanding today. However, if this discrepancy holds up, it appears we may not have the right understanding, and it changes how big the Hubble constant should be today."

Comparing the universe's expansion rate with WMAP, Planck, and the Hubble Space Telescope is like building a bridge, Riess explained. On the distant shore are the cosmic microwave background observations of the early universe. On the nearby shore are the measurements made by Riess' team using Hubble.

"You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right," Riess said. "But now the ends are not quite meeting in the middle and we want to know why."

The Hubble observations were conducted by the Supernova H0 for the Equation of State (SH0ES) team, which works to refine the accuracy of the Hubble constant to a precision that allows for a better understanding of the universe's behavior.

Riess' team made the improvements by streamlining and strengthening the construction of the cosmic distance ladder, which astronomers use to measure accurate distances to galaxies near and far from Earth. The team compared those distances with the expansion of space as measured by the stretching of light from receding galaxies. They used these two values to calculate the Hubble constant.

Among the most reliable of these cosmic yardsticks for relatively shorter distances are Cepheid variables, pulsating stars that dim and fade at rates that correspond to their true brightness. Their distances, therefore, can be inferred by comparing their true brightness with their apparent brightness as seen from Earth.

The researchers calibrated this stellar yardstick using a basic tool of geometry called parallax, the same technique that surveyors use to measure distances on Earth. With Hubble's sharp-eyed WFC3 (Wide Field Camera 3), they extended the parallax measurements farther than previously possible, across the Milky Way galaxy, to reach distant Cepheids.

To calculate accurate distances to nearby galaxies, the team looked for galaxies containing both Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the same brightness and are brilliant enough to be seen from relatively longer distances. So far, Riess' team has measured about 2,400 Cepheid stars in 19 of these galaxies, representing the largest sample of such measurements outside the Milky Way. By comparing the observed brightness of both types of stars in those nearby galaxies, the astronomers could then accurately measure their true brightness and therefore calculate distances to roughly 300 Type Ia supernovae in far-flung galaxies.

Using one instrument, WFC3, to bridge the Cepheid rungs in the distance ladder, the researchers eliminated the systematic errors that are almost unavoidably introduced by comparing measurements from different telescopes. Measuring the Hubble constant with a single instrument is like measuring a hallway with a long tape measure instead of a single 12-inch ruler. By avoiding the need to pick up the ruler and lay it back down over and over again, you can prevent cumulative errors.

The SH0ES Team is still using Hubble to reduce the uncertainty in the Hubble constant even more, with a goal to reach an accuracy of 1 percent. Current telescopes such as the European Space Agency's Gaia satellite, and future telescopes such as the JWST (James Webb Space Telescope), an infrared observatory, and the WFIRST (Wide Field Infrared Space Telescope), also could help astronomers make better measurements of the expansion rate.

Before Hubble was launched in 1990, the estimates of the Hubble constant varied by a factor of two. In the late 1990s the Hubble Space Telescope Key Project on the Extragalactic Distance Scale refined the value of the Hubble constant to within an error of only 10%, accomplishing one of the telescope's key goals. The SH0ES team has reduced the uncertainty in the Hubble constant value by 76% since beginning its quest in 2005.

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Figure 5: The illustration shows the three steps astronomers used to measure the universe's expansion rate to an unprecedented accuracy, reducing the total uncertainty to 2.4 percent [image credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)] 7)

Legend to Figure 5: Astronomers made the measurements by streamlining and strengthening the construction of the cosmic distance ladder, which is used to measure accurate distances to galaxies near and far from Earth.

Beginning at left, astronomers use Hubble to measure the distances to a class of pulsating stars called Cepheid variables, employing a basic tool of geometry called parallax. This is the same technique that surveyors use to measure distances on Earth. Once astronomers calibrate the Cepheids' true brightness, they can use them as cosmic yardsticks to measure distances to galaxies much farther away than they can with the parallax technique. The rate at which Cepheids pulsate provides an additional fine-tuning to the true brightness, with slower pulses for brighter Cepheids. The astronomers compare the calibrated true brightness values with the stars' apparent brightness, as seen from Earth, to determine accurate distances.

Once the Cepheids are calibrated, astronomers move beyond our Milky Way to nearby galaxies (shown at center). They look for galaxies that contain Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the same amount of brightness. The astronomers use the Cepheids to measure the true brightness of the supernovae in each host galaxy. From these measurements, the astronomers determine the galaxies' distances.

They then look for supernovae in galaxies located even farther away from Earth. Unlike Cepheids, Type Ia supernovae are brilliant enough to be seen from relatively longer distances. The astronomers compare the true and apparent brightness of distant supernovae to measure out to the distance where the expansion of the universe can be seen (shown at right). They compare those distance measurements with how the light from the supernovae is stretched to longer wavelengths by the expansion of space. They use these two values to calculate how fast the universe expands with time, called the Hubble constant.




Development status and mission capabilities

• June 27, 2022: Many parents are familiar with the dreaded growth spurt, where their preteen gains 6 inches in height seemingly overnight and requires a whole new wardrobe. Growth spurts happen on a cosmic scale too. In the early universe, many galaxies just 2 to 3 billion years old underwent growth spurts of their own, forming stars hundreds of times faster than they do today. 8)

- While astronomers see evidence for these galactic growth spurts, many questions remain. Why did some galaxies “live fast and die young” while others ceased forming stars more gradually? Did their neighbors influence their evolution? To answer questions like these, scientists need to study a large number of galaxies.

- The Nancy Grace Roman Space Telescope, with a field of view 200 times Hubble’s in the infrared, will be able to capture images and spectra from thousands of galaxies in a single observation. Such a bounty of data will help astronomers discover hidden chapters in the universe’s history of stars.

- In the American Wild West, high noon was a time for duels and showdowns. When it comes to the history of the universe, cosmic noon featured fireworks of a different sort. Some 2 to 3 billion years after the big bang most galaxies went through a growth spurt, forming stars at a rate hundreds of times higher than we see in our own galaxy, the Milky Way, today. When it launches by May 2027, NASA’s Nancy Grace Roman Space Telescope promises to bring new insights into the heyday of star formation.

- Cosmic noon is an important time in the universe’s history because it shaped what galaxies are like today. But many questions remain unanswered. Why did star formation peak and then decline? Why did some galaxies suddenly stop forming stars while others faded out gradually? How important were local influences, like the number of galactic neighbors, in shaping this evolution?

- To answer these questions, astronomers need to study a bountiful sample of galaxies from that time period. Roman’s power will lie in its ability to capture thousands of objects of interest in a single view. With such a large survey, scientists won’t have to pick and choose their preferred targets in advance, which can lead to unintended biases.

- “With a field of view 200 times that of the Hubble Space Telescope in infrared light, Roman can change the astronomical landscape by being so efficient,” said Kate Whitaker, assistant professor of Astronomy at the University of Massachusetts in Amherst. Whitaker’s research focuses on studying the regulation of star formation and quenching in massive galaxies in the early universe.

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Figure 6: This Hubble image features four of the thousands of galaxies found within the Hubble Ultra Deep Field. All of the highlighted galaxies show evidence of vigorous star formation (blue regions filled with hot, young stars). In the insets at right, the near-infrared spectrum of each galaxy is displayed. By examining a galaxy’s spectrum, you can learn about the ages of its stars, its star-formation history, how many heavy chemical elements it contains, and more. - Upon entering operations in 2027, the Nancy Grace Roman Space Telescope will be able to collect spectra for every object in its field of view, which is 200 times larger than Hubble’s in infrared light. As a result, it will enable studies of rare galaxies from a period known as “cosmic noon,” when many galaxies went through growth spurts [image credit: SCIENCE: NASA, ESA, STScI, Casey Papovich (TAMU), IMAGE PROCESSING: Alyssa Pagan (STScI)]

- Roman’s wide field of view also will enable astronomers to put individual galaxies into context by seeing how their growth spurts, and subsequent slow-downs, vary depending on their location within the cosmic “web” – the large-scale structure of the universe.

- “You take one image, and you get everything. We will see what and where the interesting objects are,” said Casey Papovich, professor of Astronomy at Texas A&M University in College Station, Texas. Papovich’s research includes quantifying the growth and assembly of stellar mass in galaxies in the early universe.

Going Beyond Imagery

- While images can help astronomers spot galaxies of interest, much more information can be gleaned by spreading a galaxy’s light out into a spectrum . Papovich, with Vicente (Vince) Estrada-Carpenter of St. Mary’s University in Halifax, Nova Scotia, Canada, and their colleagues, has pioneered a technique for extracting the combined light from all the stars in a galaxy.

- By examining a galaxy’s spectrum you can learn about the ages of its stars, its star-formation history, how many heavy chemical elements it contains, and more. By doing this for a large number of early galaxies, astronomers can learn about the processes that drove and eventually brought an end to this period of rapid growth.

- Roman’s power can be boosted even further by observing distant galaxies whose light has been distorted by a phenomenon called gravitational lensing. The gravity of an intervening galaxy cluster can magnify and brighten the light from a more distant galaxy, allowing astronomers to study the background galaxy in more detail than would otherwise be available.

- Whitaker is already using this technique with Hubble to study the cores of young galaxies versus their outskirts. This work seeks to determine if star formation shuts off from the outside-in or inside-out – that is, from the galaxy’s outskirts to its center or vice versa.

- “Galaxy quenching – a sudden end to star formation – can be a fast process on cosmological timescales. As a result, catching one in the act is difficult because they’re so rare,” said Whitaker. “Roman will help us find those rare examples.”

- While Roman’s space-based view will provide excellent sharpness and stability, ground-based observatories also will come into play in studying cosmic noon. For example, the Atacama Large Millimeter/submillimeter Array can measure the gas and dust content of distant galaxies. And future 30-meter-class telescopes will be able to measure exquisite details in galaxy spectra due to their ability to collect lots of light.

- “Roman and ground-based observatories will complement each other. Roman will single-handedly and efficiently identify and characterize the most interesting galaxies in large fields of view. We then can go back with ground-based telescopes to study them in more detail,” explained Papovich.

- NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the Roman mission, with participation by NASA's Jet Propulsion Laboratory in Southern California, and will provide Roman’s Mission Operations Center. The Space Telescope Science Institute in Baltimore will host Roman’s Science Operations Center and lead the data processing of Roman imaging. Caltech/IPAC in Pasadena, California, will house Roman’s Science Support Center and lead the data processing of Roman spectroscopy.

• June 14, 2022: NASA’s Nancy Grace Roman Space Telescope will study wispy streams of stars that extend far beyond the apparent edges of many galaxies. Missions like the Hubble and James Webb space telescopes would have to patch together hundreds of small images to see these structures around nearby galaxies in full. Roman will do so in a single snapshot. Astronomers will use these observations to explore how galaxies grow and the nature of dark matter. 9)

- Stellar streams look like ethereal strands of hair extending outward from some galaxies, peacefully drifting through space as part of the halo – a spherical region surrounding a galaxy. But these stellar flyaways are signs of an ancient cosmic-scale drama that serve as fossil records of a galaxy’s past. Studying them transforms astronomers into galactic archaeologists.

- “Halos are mostly made from stars that were stripped away from other galaxies,” said Tjitske Starkenburg, a postdoctoral fellow at Northwestern University in Evanston, Illinois, who examined Roman’s potential in this area. “Roman’s wide, deep images will be sharp enough that we can resolve individual stars in other galaxies’ halos, making it possible to study stellar streams in a large number of galaxies for the first time.”

- The team, led by Starkenburg, will share their results at the American Astronomical Society's 240th meeting in Pasadena, California, today.

Galactic Cannibalism, Stolen Stars

- Simulations support the theory that galaxies grow in part by gobbling up smaller groups of stars. A dwarf galaxy captured into orbit by a larger one becomes distorted by gravity. Its stars drizzle out, tracing arcs and loops around the larger galaxy until they ultimately become its newest members.

- “As individual stars leak out of the dwarf galaxy and fall into the more massive one, they form long, thin streams that remain intact for billions of years,” said Sarah Pearson, a Hubble postdoctoral fellow at New York University in New York City and the lead author of a separate study about the mission’s projected observations in this area. “So stellar streams hold secrets from the past and can illuminate billions of years of evolution.”

- Astronomers have caught this cannibalistic process in the act using telescopes like ESA's (European Space Agency's) Gaia satellite, which is fine-tuned to measure the positions and motions of stars in our Milky Way galaxy. Roman will extend these observations by making similar measurements of stars in both the Milky Way and other galaxies.

- The Milky Way is home to at least 70 stellar streams, meaning it has likely eaten at least 70 dwarf galaxies or globular star clusters – groups of hundreds of thousands of gravitationally bound stars. Roman’s Milky Way images could allow astronomers to string together snapshots in time to show stars’ movement. That will help us learn about what dark matter – invisible matter that we can only detect via its gravitational effects on visible objects – is made of.

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Figure 7: This series of images shows how astronomers find stellar streams by reversing the light and dark, similar to negative images. Color images of each of the nearby galaxies featured are included for context. Galaxies are surrounded by enormous halos of hot gas sprinkled with sporadic stars, seen as the shadowy regions that encase each galaxy here. Roman could improve on these observations by resolving individual stars to understand each stream’s stellar populations and see stellar streams of various sizes in even more galaxies [image credits: Carlin et al. (2016), based on images from Martínez-Delgado et al. (2008, 2010)]

- One theory suggests dark matter is “cold,” or made up of heavy, sluggish particles. If so, it should clump together within galaxy halos, which would disturb stellar streams in ways Roman could see. By either detecting or ruling out these distortions, Roman could narrow down the candidates for what dark matter could be made of.

- Astronomers are also looking forward to studying stellar streams in several of the Milky Way’s neighboring galaxies. They aren’t well studied in other galaxies because they’re so faint and far away. They’re also so vast that they can wrap around an entire galaxy. It takes an unrivaled panoramic view like Roman’s to capture images that are both large and detailed enough to see them.

- Especially elusive stellar streams that formed when the Milky Way siphoned stars from globular star clusters have been detected before, but they’ve never been found in other galaxies. They’re fainter because they contain fewer stars, which makes them much more difficult to spot in other, more distant galaxies.

- Roman may detect them in several of our neighboring galaxies for the first time ever. The mission’s wide, sharp, deep vision should even reveal individual stars in these enormous, dim structures. In a previous study, Pearson led the development of an algorithm to systematically search for stellar streams originating from globular clusters in neighboring galaxies.

- Starkenburg’s new study adds to the picture by predicting that Roman should be able to detect dozens of streams in other galaxies that originated from dwarf galaxies, offering unprecedented insight into the way galaxies grow.

- “It’s exciting to learn more about our Milky Way, but if we truly want to understand galaxy formation and dark matter we need a larger sample size,” Starkenburg said. “Studying stellar streams in other galaxies with Roman will help us see the bigger picture.”

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

• March 22, 2022: A team of scientists has predicted the science return from one of NASA's Nancy Grace Roman Space Telescope's groundbreaking planned surveys, which will analyze millions of galaxies strewn across space and time. The mission's enormous, deep panoramas will provide the best opportunity yet to discern between the leading theories about what's speeding up the universe's expansion. 10)

- Roman will explore this mystery using multiple methods, including spectroscopy – the study of the color information in light. This technique will allow scientists to precisely measure how fast the universe expanded in different cosmic eras and trace how the universe has evolved.

Figure 8: This video dissolves between six cubes to show the simulated distribution of galaxies at redshifts 9, 7, 5, 3, 2, and 1, with the corresponding cosmic ages shown. As the universe expands, the density of galaxies within each cube decreases, from more than half a million in the first cube to about 80 in the last. Each cube is about 100 million light-years across. Galaxies assembled along vast strands of gas separated by large voids, a foam-like structure echoed in the present-day universe on large cosmic scales [image credits: NASA’s Goddard Space Flight Center/F. Reddy and Z. Zhai, Y. Wang (IPAC) and A. Benson (Carnegie Observatories)]

- “Our study forecasts the science Roman’s spectroscopy survey will enable and shows how various adjustments could optimize its design,” said Yun Wang, a senior research scientist at Caltech/IPAC in Pasadena, California, and the lead author of the study. As the Roman Science Support Center, IPAC will be responsible for the mission’s spectroscopic science data processing, while the Space Telescope Science Institute in Baltimore will be responsible for imaging science data processing, generating catalogs, and support for cosmology data processing pipelines. “While this survey is designed to explore cosmic acceleration, it will also offer clues about many other tantalizing mysteries. It will help us understand the first generation of galaxies, allow us to map dark matter, and even reveal information about structures that are much closer to home, right in our local group of galaxies.”

- The Roman Space Telescope, planned for launch by May 2027, will provide such an enormous view of the universe that it will help scientists study cosmic mysteries in an unprecedented way. Each image will contain precise measurements of so many celestial objects that it will enable statistical studies that aren’t practical using telescopes with narrower views.

- In current plans, Roman’s spectroscopy survey will cover nearly 2,000 square degrees, or about 5% of the sky, in just over seven months. The team’s results showed that the survey should reveal precise distances for 10 million galaxies from when the universe was between about 3-6 billion years old, since light that reaches the telescope began its journey when the universe was much younger. These measurements will allow astronomers to map the web-like large-scale structure of the cosmos. The survey will also unveil the distances for 2 million galaxies from even earlier in the universe’s history, when it was only between 2-3 billion years old – unexplored territory in large-scale cosmic structure.

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Figure 9: The sequence and layout of the Roman Space Telescope's High Latitude Spectroscopic Survey tiling pattern (image credit: NASA's Goddard Space Flight Center)

- The team’s results are published in The Astrophysical Journal. 11)

Reading the Rainbow

- Nearly all the information we receive from space comes from light. Roman will use light to capture images, but it will also study light by breaking it down into individual colors. The detailed wavelength patterns, called spectra, reveal information about the object that emitted the light, including how fast it’s moving away from us. Astronomers call this phenomenon “redshift” because when an object recedes, all of the light waves we receive from it are stretched out and shifted toward redder wavelengths.

- In the 1920s, astronomers Georges Lemaître and Edwin Hubble used redshifts to make the startling discovery that, with very few exceptions, galaxies are racing away from us and each other at different speeds depending on their distance. By determining how quickly galaxies are receding from us, carried by the relentless expansion of space, astronomers can find out how far away they are – the more a galaxy’s spectrum is redshifted, the farther away it is.

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Figure 10: This graphic illustrates how cosmological redshift works and how it offers information about the universe’s evolution. The universe is expanding, and that expansion stretches light traveling through space. The more it has stretched, the greater the redshift and the greater the distance the light has traveled. As a result, we need telescopes with infrared detectors to see light from the first, most distant galaxies [image credits: NASA, ESA, Leah Hustak (STScI)]

- Roman’s spectroscopy survey will create a 3D map of the universe by measuring accurate distances and positions of millions of galaxies. Learning how galaxy distribution varies with distance, and therefore time, will give us a window into how quickly the universe expanded in different cosmic eras.

- This study will also connect galaxy distances with the echoes of sound waves from just after the big bang. These sound waves, called baryon acoustic oscillations (BAO), have grown with time due to the expansion of space and left their imprint on the cosmos by influencing galaxy distribution. For any modern galaxy, we are more likely to find another galaxy about 500 million light-years away than we are to find one slightly nearer or farther.

- Looking farther out into the universe, to earlier cosmic times, means that this preferred physical distance between galaxies – the vestige of BAO ripples – decreases. This provides a measurement of the universe’s expansion history. Galaxy redshifts also encode information about their motion due to the gravity of their neighbors, called redshift space distortions, which helps astronomers trace the growth history of large-scale structure. Learning about the way the cosmos has expanded and how structure has grown within it over time will allow scientists to explore the nature of cosmic acceleration and test Einstein’s theory of gravity over the age of the universe.

Dark Energy Versus Modified Gravity

- As the universe expands, the gravity of the matter within it should slow that expansion down. Astronomers were surprised to learn that the expansion of the universe is speeding up because it means that something about our picture of the cosmos is either wrong or incomplete. The mystery could be explained by adding a new energy component to the universe, which scientists have dubbed dark energy, or it could indicate that Einstein’s theory of gravity – the general theory of relativity – needs a modification.

- Changing the equations that describe something as fundamental as gravity may seem extreme, but it’s been done before. Isaac Newton’s law of gravity couldn’t explain some of the things astronomers observed, such as a small but mysterious motion in Mercury’s orbit.

- Astronomers ultimately realized that Einstein’s general theory of relativity perfectly accounted for problems that had surfaced, like Mercury’s orbital shift. Switching from Newton’s description of gravity to Einstein’s involved transforming modern physics by changing the way we view space and time – interconnected, instead of separate and constant.

- Cosmic acceleration could be a sign that Einstein’s theory of gravity still isn’t quite right. General relativity is extremely well tested on physical scales about the size of our solar system, but less so as we move to larger, cosmological scales. The team simulated Roman’s performance and demonstrated that the mission’s enormous, deep 3D images of the universe will provide one of the best opportunities yet to discern between the leading theories that attempt to explain cosmic acceleration.

- “We can look forward to new physics in either case – whether we learn that cosmic acceleration is caused by dark energy or we find that we have to modify Einstein’s theory of gravity,” Wang said. “Roman will test both theories at the same time.”

Figure 11: These six cubes show the simulated distribution of galaxies at redshifts 9, 8, 5, 3, 2, and 1, with the corresponding cosmic ages shown. As the universe expands, the density of galaxies within each cube decreases, from more than half a million at top left to about 80 at lower right. Each cube is about 100 million light-years across. Galaxies assembled along vast strands of gas separated by large voids, a foam-like structure echoed in the present-day universe on large cosmic scales[video credits: NASA’s Goddard Space Flight Center/F. Reddy and Z. Zhai, Y. Wang (IPAC) and A. Benson (Carnegie Observatories)]

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

• March 16, 2022: A team of scientists found NASA's Nancy Grace Roman Space Telescope will be able to measure a specific kind of space dust littered throughout dozens of nearby planetary systems’ habitable zones, or the regions around stars where temperatures are mild enough that liquid water could pool on worlds’ surfaces. Finding out how much of this material these systems contain would help astronomers learn more about how rocky planets form and guide the search for habitable worlds by future missions. 12)

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Figure 12: The bright haze in the sky in this photo comes from zodiacal dust, tiny bits of debris produced mainly by asteroids and comets. This dust scatters sunlight so effectively that, seen from afar, it’s the second brightest thing in our solar system after only the Sun. Exozodiacal dust – the same kind of debris in other planetary systems – proves an interesting target of study, but also presents a significant barrier for finding exoplanets. Roman will measure exozodiacal dust to guide future planet-hunting endeavors and learn about planet formation (image credits: Ruslan Merzlyakov/astrorms, used with permission)

- In our own solar system, zodiacal dust – small rocky grains largely left behind by colliding asteroids and crumbling comets – spans from near the Sun to the asteroid belt between Mars and Jupiter. Seen from a distance, it’s the brightest thing in the solar system after the Sun. In other planetary systems it’s called exozodiacal dust and creates a haze that obscures our view of planets because it scatters light from the host star.

- “If we don’t find much of this dust around a particular star, that means future missions will be able to see potential planets relatively easily,” said Ewan Douglas, an assistant professor of astronomy at the University of Arizona in Tucson and the lead author of a paper describing the results. “But if we do find this kind of dust, we can study it and learn all kinds of interesting things about its sources, like comets and asteroids in these systems, and the influence of unseen planets on its brightness and distribution. It’s a win-win for science!”

- Searching for exozodiacal dust is just one example of the promising potential scientific uses from Roman’s Coronagraph Instrument that could follow its 18-month technology demonstration phase. The team’s results are published in the Publications of the Astronomical Society of the Pacific. 13)

Figure 13: This animation zooms out from our solar system and shows how the sunlight scattered by zodiacal dust is brighter than the planets when viewed from afar. The same kind of dust in other planetary systems, called exozodiacal dust, creates a similar haze that makes it challenging to detect orbiting worlds (image credit: NASA's Goddard Space Flight Center)

Hints of Unseen Planets

- By studying exozodiacal dust, astronomers can find clues to what other planetary systems are like. The amount of debris hints at comet activity, since a greater number of comets should produce more dust. Seeing the dust’s distribution pattern could offer hints about orbiting planets, which could sculpt the debris with their gravity and carve paths through the material.

- “No one knows much about exozodiacal dust because it’s so close to its host star that it’s usually lost in the glare, making it notoriously difficult to observe,” said Bertrand Mennesson, Roman's deputy project scientist at NASA’s Jet Propulsion Laboratory in Southern California and a co-author of the paper. “We’re not sure what Roman will find in these other planetary systems, but we’re excited to finally have an observatory that’s equipped to explore this aspect of their habitable zones.”

- Roman could use its Coronagraph Instrument to block out a host star’s light and make sensitive measurements of the light reflected by the system’s dust in the same kind of light our eyes can see. Ground-based telescopes struggle with such observations because they must look through Earth’s turbulent atmosphere. “It’s very hard to block a twinkling star,” Douglas said.

- “The Roman Coronagraph is equipped with special sensors and deformable mirrors that will actively measure and subtract starlight in real time,” said John Debes, an astronomer at the Space Telescope Science Institute in Baltimore and a co-author of the paper. “This will help provide a very high level of contrast, a hundred times better than Hubble’s passive coronagraph offers, which we need to spot warm dust that orbits close to the host star.”

A Pathfinder for Future Missions

- While other observatories, such as the Hubble Space Telescope, have observed cold debris disks far from their host stars – farther from their stars than Neptune is from the Sun – no one has been able to photograph warm dust in the habitable zone region. While previous NASA projects have made preliminary measurements of exozodiacal dust in habitable zones, Roman’s images will be much more sensitive, thanks to its advanced high-contrast Coronagraph Instrument and its stable location in space. Orbiting a million miles from Earth around the Lagrange Point 2 (L2), instead of in low-Earth orbit like Hubble, means our planet won’t present such a challenging environment from which to make these observations.

- Imaging warm debris closer to host stars is important because it’s made up of different material than outer dust disks. Closer to the host star, rocky grains dominate the dust; farther away, it is largely composed of icy grains. The debris in each region is created by different processes, so studying the chemistry of exozodiacal dust offers information astronomers can’t get by observing the outer regions around other stars.

- “By prospecting for this dust, we could learn about the processes that shape planetary systems while providing important information for future missions that aim to image habitable-zone planets,” Debes said. “By finding out how much exozodiacal dust is in the way of possible planets in nearby systems, we can tell how large future telescopes will need to be to see through it. Observations from the Roman Coronagraph could offer a crucial steppingstone in the search for Earth analogs.”

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

• January 10, 2022: A team of astrophysicists has created a simulated image that shows how the Nancy Grace Roman Space Telescope could conduct a mega-exposure similar to but far larger than Hubble’s celebrated Ultra Deep Field Image. This Hubble observation transformed our view of the early universe, revealing galaxies that formed just a few hundred million years after the big bang. 14)

- “Roman has the unique ability to image very large areas of the sky, which allows us to see the environments around galaxies in the early universe,” said Nicole Drakos, a postdoctoral scholar at the University of California Santa Cruz, who led the study. “Our study helps demonstrate what a Roman ultra-deep field could tell us about the universe, while providing a tool for the scientific community to extract the most value from such a program.”

- By capturing the Hubble Ultra Deep Field image, astronomers pulled aside the cosmic curtains to reveal that a tiny, seemingly empty slice of the sky was actually teeming with thousands of galaxies, each containing billions of stars. The Hubble team harnessed the power of a long exposure time – hundreds of hours between 2002 and 2012 – which allowed the telescope to collect more light than it could in a single, short observation. The resulting image helped us see more than 13 billion years back in time.

- Hubble’s Ultra Deep Field offers an incredible window to the early universe, but an extremely narrow one, covering less than one ten millionth of the whole sky. The new simulation showcases Roman’s power to perform a similar observation on a much larger scale, revealing millions of galaxies instead of thousands. While a Roman ultra-deep field would be just as sharp as Hubble’s and peer equally far back in time, it could reveal an area 300 times larger, offering a much broader view of cosmic ecosystems.

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Figure 14: This synthetic image visualizes what a Roman ultra-deep field could look like. The 18 squares at the top of this image outline the area Roman can see in a single observation, known as its footprint. The inset at the lower-right zooms into one of the squares of Roman's footprint, and the inset at the lower-left zooms in even further. The image, which contains more than 10 million galaxies, was constructed from a simulation that produced a realistic distribution of the galaxies in the universe. Roman could peer across more than 13 billion years of cosmic history, reaching back to when the universe was only about half a billion years old. Such distant galaxies are extremely faint, so Roman would have to stare at one spot in space for several days to collect enough light from them. The mission’s wide field of view will provide an incredible amount of data, helping astronomers find rare objects in the epoch of reionization. The large area Roman will observe will also show differences in galaxy properties based on their surrounding environment, allowing astronomers to better understand how early galaxies formed (image credits: Nicole Drakos, Bruno Villasenor, Brant Robertson, Ryan Hausen, Mark Dickinson, Henry Ferguson, Steven Furlanetto, Jenny Greene, Piero Madau, Alice Shapley, Daniel Stark, Risa Wechsler)

- “The Hubble Ultra Deep Field gave us a glimpse of the universe’s youth, but it was too small to reveal much information about what the cosmos was really like back then as a whole,” said Brant Robertson, an astronomy professor at the University of California Santa Cruz and a co-author of the study. “It’s like looking at a single piece of a 10,000-piece puzzle. Roman could give us 100 connected puzzle pieces, offering a much better picture of what the early universe was like and opening up new scientific opportunities.”

- To generate their simulated Roman ultra-deep field image, Drakos and co-authors created a synthetic catalog of galaxies, complete with detailed information about each one. By doing so, the team essentially created a mock universe, basing their synthetic galaxies on dark matter simulations and observation-based models. They made the galaxy catalog publicly available so other scientists can use it to prepare for future Roman observations. The team also created an interactive website where users can zoom and pan across the full-resolution image.

- The team’s results will be published in The Astrophysical Journal.

Figure 15: This video demonstrates how Roman could expand on Hubble’s iconic Ultra Deep Field image. While a similar Roman observation would be just as sharp as Hubble’s and see equally far back in time, it could reveal an area 300 times larger, offering a much broader view of cosmic ecosystems (video credits: NASA's Goddard Space Flight Center)

- Drakos and co-authors show that a Roman ultra-deep field program could reveal more than a million galaxies scattered throughout cosmic history, from very young and small galaxies just beginning to form stars to the modern era, which features many massive, often relatively inactive galaxies. Scientists would be able to probe how galaxies transition from forming lots of new stars to this quieter stage, when star formation is complete.

- The possible causes of this metamorphosis are currently poorly understood, but Roman’s wide viewing power could offer clues about how a galaxy’s environment, such as its location in relation to other galaxies or galaxy clusters, affects its star formation.

- Galaxies in which star formation has ended, known as quiescent galaxies, are increasingly difficult to find the farther back in time astronomers look.

- “We’re not sure whether we haven’t detected very distant quiescent galaxies because they don’t exist, or simply because they’re so difficult to find,” Drakos said.

- Drakos and co-authors showed that Roman’s ability to image large patches of the distant universe and reveal both rare and faint objects could help astronomers find as many as 100,000 quiescent galaxies, likely including some of the farthest ones ever discovered. Astronomers could also use Roman ultra-deep field observations to determine whether galaxies transition from star-forming to quiescent differently in different cosmic eras.

The end of the cosmic “dark ages”

- The team’s work shows that Roman could illuminate our understanding of a long-ago cosmic event called reionization. Shortly after the big bang, the universe was filled with a hot sea of plasma – charged particles – that formed a dense, ionized fluid. As the universe cooled, the particles were able to stick together to form hydrogen atoms, which resulted in a neutral hydrogen fog. This marked an era called the cosmic “dark ages” since this fog prevented shorter wavelengths of light, which may have been emitted from young, forming galaxies or quasars from traveling very far.

- But then the neutral hydrogen atoms broke apart, returning to charged particles in an epoch of reionization. The fog lifted, transforming the universe from being mostly opaque to the brilliant starscape we see today. Findings from NASA’s Spitzer Space Telescope hint that the first galaxies released extremely high amounts of ionizing radiation – ultraviolet light, X-rays, and gamma rays – which could have disrupted the hydrogen fog.

- A Roman ultra-deep field program could advance our understanding of the epoch of reionization by revealing wide images containing more than 10,000 galaxies from this relatively brief cosmic age, which happened sometime between when the universe was around 600 million to 900 million years old, and a detailed view of the environments around these galaxies. This could help scientists understand what caused reionization, when exactly it happened, and whether its occurrence was uniform or patchy.

- Roman also has the power to reveal how galaxies and galaxy clusters – which form some of the largest structures in the universe – evolved over time. Scientists think galaxies were born within vast spherical clumps of dark matter called halos. Observations indicate that each galaxy’s luminosity, or absolute brightness, is linked to the mass of the dark matter halo it resides in. By creating an ultra-deep field image, Roman could help astronomers better understand this connection. This has implications for not only galaxy formation but also the standard cosmological model – the theoretical model of how the universe evolves – which includes a dark matter clumping parameter.

- “Roman could shine a light on so many cosmic mysteries in just a few hundred hours of observing time,” said Bruno Villasenor, a graduate student at the University of California Santa Cruz and a co-author of the study. “It’s amazing to think that no one knew for sure whether other galaxies existed until about a hundred years ago. Now, Roman offers us the opportunity to observe thousands of the first galaxies that appeared in the very early universe!”

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

• November 9, 2021: A team of scientists has forecast the scientific impact of the Nancy Grace Roman Space Telescope’s High Latitude Wide Area Survey on critical questions in cosmology. This observation program will consist of both imaging, which reveals the locations, shapes, sizes, and colors of objects like distant galaxies, and spectroscopy, which involves measuring the intensity of light from those objects at different wavelengths, across the same enormous swath of the universe. Scientists will be able to harness the power of a variety of cross-checking techniques using this rich data set, which promises an unprecedented look into some of cosmology’s most vexing problems. 15)

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Figure 16: Illustration of Roman's High Latitude Wide Area Survey (image credit: NASA's Goddard Space Flight Center)

- When it begins work in 2027, Roman will yield results that would be impossible to achieve using existing telescopes. Its impact will be further enhanced by teaming up with other new facilities like the Vera C. Rubin Observatory, a novel wide-field telescope now being built on the summit of Cerro Pachón in Chile. Scheduled to begin full operations by 2024, Rubin’s planned 10-year survey extends across Roman’s five-year primary mission.

- “By predicting Roman’s science return, we hope to help the scientific community develop the best strategy for observing the cosmos,” said Tim Eifler, an assistant professor at the University of Arizona in Tucson. “We eagerly await the images and data the mission will send back to help us better understand some of the biggest mysteries in the universe.”

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Figure 17: This illustration compares the relative sizes of the areas of sky covered by two surveys: Roman’s High Latitude Wide Area Survey, outlined in blue, and the largest mosaic led by Hubble, the Cosmological Evolution Survey (COSMOS), shown in red. In current plans, the Roman survey will be more than 1,000 times broader than Hubble’s. Roman will also explore more distant realms of space than most other telescopes have probed in previous efforts to study why the expansion of the universe is speeding up (image credit: NASA's Goddard Space Flight Center)

- The team’s results are described in two papers led by Eifler and published in the October edition of the Monthly Notices of the Royal Astronomical Society. The study is part of an effort by a broader team of world-leading scientists to prepare to analyze Roman’s cosmological data.

- “Our study was only possible because of all the expertise, from theorists to observers, that is present in the larger team,” Eifler said.

A multitalented observatory

- The Roman mission owes its multifaceted approach to its combination of imaging and spectroscopy across an enormous field of view, which enables two main cosmological techniques: galaxy clustering and weak gravitational lensing. The first measures the exact positions of hundreds of millions of faint galaxies. Weak lensing measures how the images of galaxies have been distorted by the gravity of intervening matter. With its wide, deep view, Roman will allow scientists to study the structure and evolution of the universe and to explore the concept of cosmic acceleration as never before.

- Learning about how the universe evolved to its present state will offer clues about what’s speeding up the universe’s expansion. In addition to weak lensing and galaxy clustering, Roman will study this mystery in several ways, including surveying the sky for a special type of exploding star called a type Ia supernova. The mission will also probe cosmic acceleration by measuring the masses and redshifts of galaxy clusters, the largest structures in the universe. The number and size of these structures depend on how the speed of the universe’s expansion changes. 16)

- “Using several different methods to study the cause behind cosmic acceleration will help astronomers greatly reduce the uncertainty that has plagued expansion measurements,” said Hironao Miyatake, an associate professor at Nagoya University in Japan and a co-author of the papers. “Each method will cross-check the others, which is one reason Roman will be able to provide extremely precise results.”

- Combining so many observational methods will allow astronomers to investigate additional mysteries, too, including determining the amount of dark matter – invisible matter that is detectable only through its gravitational effects – and tracking the growth of black holes in the early universe that form the seeds of massive galaxies.

- “Roman is designed specifically to solve mysteries such as cosmic acceleration, but its enormous view of the universe will reveal a treasure trove of data that could help explain other puzzles as well,” said Elisabeth Krause, an assistant professor at the University of Arizona and a co-author of the papers. “The mission could even help answer questions no one has thought to ask yet.”

Teaming up with Rubin

- Roman isn’t the only observatory designed to probe cosmic acceleration. In one paper, the team explored how Roman will work hand-in-hand with another telescope: the Rubin Observatory. Named for American astronomer Vera Rubin, who showed that galaxies are mostly made of dark matter, the Rubin Observatory will use its 8.4-meter (27.4-foot) primary mirror to conduct a truly gigantic survey of the sky, covering about 44% of the sky over 10 years.

- “Roman’s observations will begin, in terms of wavelength, where Rubin’s observations end,” Eifler said. “Roman plans to view a smaller area of the sky, but it will see much deeper and generate clearer pictures since it will be located above Earth’s atmosphere.”

- The current observing strategy for Roman’s High Latitude Wide Area Survey will enable observations of about 5% of the sky – 2,000 square degrees – over the course of about a year. However, the team illustrated how changing the survey’s design could yield compelling results. The survey could be extended, for example, to cover more of the same area that Rubin will observe. Or it could observe galaxies using a single broad filter, instead of imaging with four separate ones, allowing faster observations while still retaining the survey’s depth.

- “It is exciting to consider the benefits we would gain from merging the two telescopes’ observations,” Krause said. “Roman will gain from Rubin’s larger observing field, and Rubin will gain enormously from having some deeper observations with Roman’s better image quality. The missions will greatly enhance each other.”

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

• October 1, 2021: NASA has awarded a contract to provide capabilities for the Roman Space Telescope Science Support Center to the California Institute of Technology (Caltech) in Pasadena, California. The Nancy Grace Roman Space Telescope is designed to answer essential questions in areas of research including dark energy, exoplanets, and infrared astrophysics. 17)

- The total value of this cost-plus-no-fee contract is $49 million. The period of performance is from Oct. 1, 2021 through Sept. 30, 2027.

- Caltech will provide critical capabilities to support the Roman Space Telescope Project at the Science Support Center. These include providing science operations system engineering, ground system development, operations, science research support, and scientific community engagement and public outreach support for the Roman project.

- Caltech will be part of a mission team led by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Other mission team participants include the Space Telescope Science Institute (STScI) in Baltimore, Maryland, NASA’s Jet Propulsion Laboratory (JPL) in Southern California, international partners, and other support contractors.

• September 30, 2021: NASA’s Roman Space Telescope has passed its critical design review, but the impact of the pandemic will delay its launch by several months and increase its cost. 18)

- NASA announced Sept. 29 that Roman completed its critical design review, confirming that all the design and engineering work for the space telescope is complete. The review allows the mission to proceed into full-scale assembly and testing of the spacecraft.

- “After seeing our extensive hardware testing and sophisticated modeling, an independent review panel has confirmed that the observatory we have designed will work,” said Julie McEnery, senior project scientist for Roman at NASA’s Goddard Space Flight Center, in a statement. “We know what it will look like and what it will be capable of. Now that the groundwork is laid, the team is thrilled to continue building and testing the observatory they’ve envisaged.”

- Roman, originally known as the Wide Field Infrared Survey Telescope (WFIRST), is the next flagship astrophysics mission for the agency after the James Webb Space Telescope. It features a 2.4 meter mirror and instruments astronomers plan to use to study topics ranging from exoplanets to dark energy.

- At the time of the mission’s confirmation in March 2020, just as the pandemic was taking hold in the United States, Roman had a launch readiness date of no later than October 2026, although mission officials were hoping for a launch as soon as a year earlier. In the announcement of the critical design review, though, NASA said the mission is now scheduled to launch no later than May 2027.

- That delay is because of lingering effects of the pandemic, which slowed work on the spacecraft and affected supply chains. “COVID has had a significant impact on Roman,” said Paul Hertz, director of NASA’s astrophysics division, at a Sept. 28 meeting of the Astronomy and Astrophysics Advisory Committee (AAAC).

- He said the pandemic created “inefficiencies” in work on the telescope as well as “huge supply chain impacts” that has pushed back the delivery its components. That resulted in a revision of the cost and schedule of the telescope, including the seven-month delay in the launch commitment date.

- As for the cost, Hertz only said there had been an “appropriate addition of cost that matches that matches that seven-month slip in that launch date.” He did not give a specific figure, and a NASA spokesperson did not respond to a question about that revised cost Sept. 29. However, NASA’s Office of Inspector General reported in March that Roman’s costs would grow $400 million due to the pandemic. Roman had a total lifecycle cost of $3.9 billion prior to the pandemic.

JWST sticking to schedule, name

- NASA officials also offered an update on the status of the James Webb Space Telescope at the AAAC meeting, revealing that the $8.8 billion spacecraft is on its way to the launch site in French Guiana.

- “We are in transit to Kourou, having left the continental United States now,” Eric Smith, NASA program scientist for JWST, said at the AAAC meeting Sept. 29. The spacecraft is on a large container ship like those Arianespace uses for transporting launch vehicle hardware to French Guiana. “It’s business as usual for them.”

- NASA had not previously disclosed that the telescope had left the Northrop Grumman assembly facility in Southern California. At the meeting the previous day, Hertz said only that JWST was now in its shipping container.

- Project officials previously said they were sensitive about providing details about the shipment of JWST because of security concerns. Smith did not say at the meeting when JWST left port or its estimated arrival date in French Guiana.

- The shipment, he said, keeps JWST on track for a Dec. 18 launch on an Ariane 5. A team is already in Kourou preparing payload processing facilities there to host JWST. That launch date will depend on exactly when the next Ariane 5 launch, currently scheduled for Oct. 23, takes place, he added, but noted the JWST schedule has 13 days of margin once it is at the launch site.

- Smith also said that NASA has no plans to change the name of the spacecraft. A petition circulated in the astronomy community earlier this year called on NASA to change the name of JWST, named after the agency’s administrator during most of the 1960s. That petition cited allegations James Webb oversaw policies earlier in his career at the State Department to purge the department of LGBT employees, as well as one case involving a NASA employee while he led the agency.

- Smith, asked about the status of a historical review NASA undertook to investigate those claims, offered a previously unreleased statement attributed to NASA Administrator Bill Nelson that the agency found no evidence to support renaming the telescope.

- “We have found no evidence at this time that warrants changing the name of the James Webb Space Telescope,” Nelson said in the one-sentence statement later provided by the agency. A NASA spokesperson added the agency didn’t plan to release additional details about that review, such as a report, because the historical review found no evidence to support the allegations.

• September 21, 2021: When NASA’s Nancy Grace Roman Space Telescope launches in the mid-2020s, it will revolutionize astronomy by providing a panoramic field of view at least 100 times greater than Hubble's at similar image sharpness, or resolution. The Roman Space Telescope will survey the sky up to thousands of times faster than can be done with Hubble. This combination of wide field, high resolution, and an efficient survey approach promises new understandings in many areas, particularly in how galaxies form and evolve over cosmic time. How did the largest structures in the universe assemble? How did our Milky Way galaxy come to be in its current form? These are among the questions that Roman will help answer. 19)

- Galaxies are conglomerations of stars, gas, dust, and dark matter. The largest can span hundreds of thousands of light-years. Many gather together in clusters containing hundreds of galaxies, while others are relatively isolated.

- How galaxies change over time depends on many factors: for example, their history of star formation, how rapidly they formed stars over time, and how each generation of stars influenced the next through supernova explosions and stellar winds. To tease out these details, astronomers need to study large numbers of galaxies.

- “Roman will give us the ability to see faint objects and to view galaxies over long intervals of cosmic time. That will allow us to study how galaxies assembled and transformed,” said Swara Ravindranath, an astronomer at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

- While wide-field imaging will be important for galaxy studies, just as important are Roman’s spectroscopic capabilities. A spectrograph takes light from an object and spreads it into a rainbow of colors known as a spectrum. From this range of colors, astronomers can glean many details otherwise unavailable, like an object’s distance or composition. Roman’s ability to provide a spectrum of every object within the field of view, combined with Roman imaging, will enable astronomers to learn more about the universe than from either imaging or spectroscopy alone.

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Figure 18: This portion of the Hubble GOODS-South field contains hundreds of visible galaxies. A representative sample of those galaxies on the right half of the image also have their spectra overlayed in a representation of slitless spectroscopy. By using slitless spectroscopy, a spectrum is obtained that contains both spatial and wavelength information. For example, the inset highlights a spiral galaxy that shines brightly in the emission line of hydrogen-alpha (Hα) as well as in broad starlight (the horizontal strip of light). Its spiral shape is traced by the Hα portion of the spectrum. By combining imaging and spectroscopy, astronomers can learn much more than from each technique alone [image credit: NASA, ESA; image processing: Joseph DePasquale (STScI); acknowledgement: University of Geneva, Pascal Oesch (University of Geneva), Mireia Montes (UNSW)]

Revealing When and Where Stars Were Born

- Galaxies don’t form stars at a constant rate. They speed up and slow down—forming more or fewer stars—under the influence of a variety of factors, from collisions and mergers to supernova shock waves and galaxy-scale winds powered by supermassive black holes.

- By studying a galaxy’s spectrum in detail, astronomers can explore the history of star formation. “Using Roman we can estimate how fast galaxies are making stars and find the most prolific galaxies that are producing stars at an enormous rate. More importantly, we can find out not only what’s happening in a galaxy at the moment we observe it, but what its history has been,” stated Lee Armus, an astronomer at IPAC/Caltech in Pasadena, California.

- Some precocious galaxies birthed stars very rapidly for a short time, only to cease forming stars surprisingly early in the universe’s history, undergoing a rapid transition from lively to “dead.”

- “We know galaxies shut off star formation, but we don’t know why. With Roman’s wide field of view, we stand a better chance of catching these galaxies in the act,” said Kate Whitaker, an astronomer at the University of Massachusetts in Amherst.

Growing the Cosmic Web

- Even as galaxies themselves have grown over time, they also have gathered together in groups to form intricate structures billions of light-years across. Galaxies tend to collect into bubbles, sheets, and filaments, creating a vast cosmic web. By combining high-resolution imaging, which yields a galaxy’s position on the sky, with spectroscopy, which provides a distance, astronomers can map this web in three dimensions and learn about the universe’s large-scale structure.

- The expansion of the universe stretches light from distant galaxies to longer, redder wavelengths—a phenomenon called redshift. The more distant a galaxy is, the greater its redshift. Roman’s infrared detectors are ideal for capturing light from those galaxies. More distant galaxies are also fainter and harder to spot. Combining this with the fact that that some galaxy types are rare, you have to search a larger area of the sky with a more sensitive observatory to find the objects that often have the most interesting stories to tell.

- “Right now, with telescopes like Hubble we can sample tens of high-redshift galaxies. With Roman, we’ll be able to sample thousands,” explained Russell Ryan, an astronomer at STScI.

Seeking the Unknown

- While astronomers can anticipate many of the discoveries of the Roman Space Telescope, perhaps most exciting is the possibility of finding things that no one could have predicted. Typical high-resolution observations from space-based observatories like Hubble, target specific objects for detailed investigation. Roman’s survey approach will cast a wide net, thereby opening up a new “discovery space.”

- “Roman will excel in unknown unknowns. It will certainly find rare, exotic things that we don’t expect,” said Ryan.

- “Roman’s combined imaging and spectroscopy surveys will gather the ‘gold nuggets’ that we never would have mined otherwise,” added Ravindranath.

- NASA’s Goddard Space Flight Center in Greenbelt, Maryland, will provide Roman’s Mission Operations Center. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, will host Roman’s Science Operations Center and lead the data processing of Roman imaging. Caltech/IPAC in Pasadena, California, will house Roman’s Science Support Center and lead the data processing of Roman spectroscopy.

• May 26, 2021: NASA’s upcoming Nancy Grace Roman Space Telescope will see thousands of exploding stars called supernovae across vast stretches of time and space. Using these observations, astronomers aim to shine a light on several cosmic mysteries, providing a window onto the universe’s distant past and hazy present. 20)

- Roman’s supernova survey will help clear up clashing measurements of how fast the universe is currently expanding, and even provide a new way to probe the distribution of dark matter, which is detectable only through its gravitational effects. One of the mission’s primary science goals involves using supernovae to help pin down the nature of dark energy – the unexplained cosmic pressure that’s speeding up the expansion of the universe.

Space’s biggest mystery

- “Dark energy makes up the majority of the cosmos, but we don’t actually know what it is,” said Jason Rhodes, a senior research scientist at NASA’s Jet Propulsion Laboratory in Southern California. “By narrowing down possible explanations, Roman could revolutionize our understanding of the universe – and dark energy is just one of the many topics the mission will explore!”

- Roman will use multiple methods to investigate dark energy. One involves surveying the sky for a special type of exploding star, called a type Ia supernova.

Figure 19: NASA’s upcoming Nancy Grace Roman Space Telescope will see thousands of exploding stars called supernovae across vast stretches of time and space. Using these observations, astronomers aim to shine a light on several cosmic mysteries, providing a window onto the universe’s distant past and hazy present (video credits: NASA's Goddard Space Flight Center/CI Labs)

- Many supernovae occur when massive stars run out of fuel, rapidly collapse under their own weight, and then explode because of strong shock waves that propel out of their interiors. These supernovae occur about once every 50 years in our Milky Way galaxy. But evidence shows that type Ia supernovae originate from some binary star systems that contain at least one white dwarf – the small, hot core remnant of a Sun-like star. Type Ia supernovae are much rarer, happening roughly once every 500 years in the Milky Way.

- In some cases, the dwarf may siphon material from its companion. This ultimately triggers a runaway reaction that detonates the thief once it reaches a specific point where it has gained so much mass that it becomes unstable. Astronomers have also found evidence supporting another scenario, involving two white dwarfs that spiral toward each other until they merge. If their combined mass is high enough that it leads to instability, they, too, may produce a type Ia supernova.

- These explosions peak at a similar, known intrinsic brightness, making type Ia supernovae so-called standard candles – objects or events that emit a specific amount of light, allowing scientists to find their distance with a straightforward formula. Because of this, astronomers can determine how far away the supernovae are by simply measuring how bright they appear.

- Astronomers will also use Roman to study the light of these supernovae to find out how quickly they appear to be moving away from us. By comparing how fast they’re receding at different distances, scientists will trace cosmic expansion over time. This will help us understand whether and how dark energy has changed throughout the history of the universe.

- “In the late 1990s, scientists discovered that the expansion of the universe was speeding up using dozens of type Ia supernovae,” said Daniel Scolnic, an assistant professor of physics at Duke University in Durham, North Carolina, who is helping design Roman’s supernova survey. “Roman will find them by the thousands, and much farther away than the majority of those we’ve seen so far.”

- Previous type Ia supernova surveys have concentrated on the relatively nearby universe, largely due to instrument limitations. Roman’s infrared vision, gigantic field of view, and exquisite sensitivity will dramatically extend the search, pulling the cosmic curtains far enough aside to allow astronomers to spot thousands of distant type Ia supernovae.

- The mission will study dark energy’s influence in detail over more than half of the universe’s history, when it was between about four and 12 billion years old. Exploring this relatively unprobed region will help scientists add crucial pieces to the dark energy puzzle.

- “Type Ia supernovae are among the most important cosmological probes we have, but they’re hard to see when they’re far away,” Scolnic said. “We need extremely precise measurements and an incredibly stable instrument, which is exactly what Roman will provide.”

Hubble constant hubbub

- In addition to providing a cross-check with the mission’s other dark energy surveys, Roman’s type Ia supernova observations could help astronomers examine another mystery. Discrepancies keep popping up in measurements of the Hubble constant, which describes how fast the universe is currently expanding.

- Predictions based on early universe data, from about 380,000 years after the big bang, indicate that the cosmos should currently expand at about 42 miles per second (67 km/s) for every megaparsec of distance (a megaparsec is about 3.26 million light-years). But measurements of the modern universe indicate faster expansion, between roughly 43 to 47 miles per second (70 to 76 km/s) per megaparsec.

- Roman will help by exploring different potential sources of these discrepancies. Some methods to determine how fast the universe is now expanding rely on type Ia supernovae. While these explosions are remarkably similar, which is why they’re valuable tools for gauging distances, small variations do exist. Roman’s extensive survey could improve their use as standard candles by helping us understand what causes the variations.

- The mission should reveal how the properties of type Ia supernovae change with age, since it will view them across such a vast sweep of cosmic history. Roman will also spot these explosions in various locations in their host galaxies, which could offer clues to how a supernova’s environment alters its explosion.

Illuminating dark matter

- In a 2020 paper, a team led by Zhongxu Zhai, a postdoctoral research associate at Caltech/IPAC in Pasadena, California, showed that astronomers will be able to glean even more cosmic information from Roman’s supernova observations.

- “Roman will have to look through enormous stretches of the universe to see distant supernovae,” said Yun Wang, a senior research scientist at Caltech/IPAC and a co-author of the study. “A lot can happen to light on such long journeys across space. We’ve shown that we can learn a lot about the structure of the universe by analyzing how light from type Ia supernovae has been bent as it traveled past intervening matter.”

- Anything with mass warps the fabric of space-time. Light travels in a straight line, but if space-time is bent – which happens near massive objects – light follows the curve. When we look at distant type Ia supernovae, the warped space-time around intervening matter – such as individual galaxies or clumps of dark matter – can magnify the light from the more distant explosion.

- By studying this magnified light, scientists will have a new way to probe how dark matter is clustered throughout the universe. Learning more about the matter that makes up the cosmos will help scientists refine their theoretical model of how the universe evolves.

- By charting dark energy’s behavior across cosmic history, homing in on how the universe is expanding today, and providing more information on mysterious dark matter, the Roman mission will deliver an avalanche of data to astronomers seeking to solve these and other longstanding problems. With its ability to help solve so many cosmic mysteries, Roman will be one of the most important tools for studying the universe we’ve ever built.

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and science teams comprising scientists from various research institutions.

• April 13, 2021: NASA’s Nancy Grace Roman Space Telescope will provide an unprecedented window into the infrared universe when it launches in the mid-2020s. One of the mission’s planned surveys will use a quirk of gravity to reveal thousands of new planets beyond our solar system. The same survey will also provide the best opportunity yet to definitively detect solitary small black holes for the first time. Formed when a star with more than 20 solar masses exhausts the nuclear fuel in its core and collapses under its own weight, these objects are known as stellar-mass black holes. 21)

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Figure 20: This illustration shows the concept of gravitational microlensing with a black hole. When a black hole passes nearly in front of a more distant star, it can lens light from the star (image credit: NASA's Goddard Space Flight Center Conceptual Image Lab)

- Black holes have such powerful gravity that not even light can escape their clutches. Since they’re invisible, we can only find black holes indirectly, by seeing how they affect their surroundings. The supermassive black holes found at the centers of galaxies, which contain millions of times the mass of the Sun, disrupt the orbits of nearby stars and occasionally tear them apart with visible consequences.

- But astronomers think the vast majority of stellar-mass black holes, which are much lighter, have nothing around them that can tip us off to their presence. Roman will find planets throughout our galaxy by observing how their gravity distorts distant starlight, and because stellar-mass black holes produce the same effects, the mission should be able to find them too.

Figure 21: This animation illustrates the concept of gravitational microlensing with a black hole. When the black hole appears to pass nearly in front of a background star, the light rays of the star become bent as they travel through the warped space-time around the black hole. It becomes a virtual magnifying glass, amplifying the brightness of the distant background star. Unlike when a less massive star or planet is the lensing object, black holes warp space-time so much that it noticeably alters the distant star’s apparent location in the sky (video credits: NASA's Goddard Space Flight Center/Conceptual Image Lab)

- “Astronomers have identified about 20 stellar-mass black holes so far in the Milky Way, but all of them have a companion that we can see,” said Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore. “Many scientists, myself included, have spent years trying to find black holes on their own using other telescopes. It’s exciting that with Roman, it will finally be possible.”

Making a black hole

- Stars seem like eternal beacons, but each is born with a limited supply of fuel. Stars spend the majority of their lives turning hydrogen in their centers into helium, which creates an enormous amount of energy. This process, called nuclear fusion, is like a controlled explosion – a finely balanced game of tug-of-war between outward pressure and gravity.

- But as a star’s fuel runs low and fusion slows, gravity takes over and the star’s core contracts. This inward pressure heats up the core and sparks a new round of fusion, which produces so much energy that the star’s outer layers expand. The star swells in size, its surface cools, and it becomes a red giant or supergiant.

- The type of stellar corpse that’s ultimately left behind depends on the star’s mass. When a Sun-like star runs out of fuel, it eventually ejects its outer layers, and only a small, hot core called a white dwarf remains. The white dwarf will fade out over time, like the dying embers of a once-roaring fire. Our Sun has about five billion years of fuel remaining.

- More massive stars run hotter, so they use up their fuel faster. Above about eight times the mass of the Sun, most stars are doomed to die in cataclysmic explosions called supernovae before becoming black holes. At the highest masses, stars may skip the explosion and collapse directly into black holes.

- The cores of these massive stars collapse until their protons and electrons crush together to form neutrons. If the leftover core weighs less than about three solar masses, the collapse stops there, leaving behind a neutron star. For larger leftover cores, even the neutrons cannot support the pressure and the collapse continues to form a black hole.

- Millions of massive stars have undergone this process and now lurk throughout the galaxy as black holes. Astronomers think there should be about 100 million stellar-mass black holes in our galaxy, but we’ve only been able to find them when they noticeably affect their surroundings. Astronomers can infer the presence of a black hole when hot, glowing accretion disks form around them, or when they spot stars orbiting a massive but invisible object.

- “Roman will revolutionize our search for black holes because it will help us find them even when there’s nothing nearby,” Sahu said. “The galaxy should be littered with these objects.”

Seeing the invisible

- Roman will primarily use a technique called gravitational microlensing to discover planets beyond our solar system. When a massive object, such as a star, crosses in front of a more distant star from our vantage point, light from the farther star will bend as it travels through the curved space-time around the nearer one.

- The result is that the closer star acts as a natural lens, magnifying light from the background star. Planets orbiting the lens star can produce a similar effect on a smaller scale.

- In addition to causing a background star to brighten, a more massive lensing object can warp space-time so much that it noticeably alters the distant star’s apparent location in the sky. This change in position, called astrometric microlensing, is extremely small – only about one milliarcsecond. That’s like distinguishing movement as small as about the width of a quarter on top of the Empire State Building in New York all the way from Los Angeles. Using Roman’s exquisite spatial resolution to detect such a tiny apparent movement – the telltale sign of a massive black hole – astronomers will be able to constrain the black hole’s mass, distance, and motion through the galaxy.

- Microlensing signals are so rare that astronomers need to monitor hundreds of millions of stars for long periods to catch them. Observatories must be able to track the position and brightness of the background star extremely precisely – something that can only be done from above Earth’s atmosphere. Roman’s location in space and enormous field of view will provide us with the best opportunity yet to probe our galaxy’s black hole population.

- “The stellar-mass black holes we’ve discovered in binary systems have strange properties compared to what we expect,” Sahu said. “They’re all about 10 times more massive than the Sun, but we think they should span a much wider range of between three and 80 solar masses. By conducting a census of these objects, Roman will help us understand more about stars’ death throes.”

• March 31, 2021: NASA’s Nancy Grace Roman Space Telescope will create enormous cosmic panoramas, helping us answer questions about the evolution of our universe. Astronomers also expect the mission to find thousands of planets using two different techniques as it surveys a wide range of stars in the Milky Way. 22)

- Roman will locate these potential new worlds, or exoplanets, by tracking the amount of light coming from distant stars over time. In a technique called gravitational microlensing, a spike in light signals that a planet may be present. On the other hand, if the light from a star dims periodically, it could be because there is a planet crossing the face of a star as it completes an orbit. This technique is called the transit method. By employing these two methods to find new worlds, astronomers will capture an unprecedented view of the composition and arrangement of planetary systems across our galaxy.

- Scheduled for launch in the mid-2020s, Roman will be one of NASA’s most prolific planet hunters.

- The mission’s large field of view, exquisite resolution, and incredible stability will provide a unique observational platform for discovering the tiny changes in light required to find other worlds via microlensing. This detection method takes advantage of the gravitational light-bending effects of massive objects predicted by Einstein's general theory of relativity.

- It occurs when a foreground star, the lens, randomly aligns with a distant background star, the source, as seen from Earth. As the stars drift along in their orbits around the galaxy, the alignment shifts over days to weeks, changing the apparent brightness of the source star. The precise pattern of these changes provides astronomers with clues about the nature of the lensing star in the foreground, including the presence of planets around it.

- Many of the stars Roman will already be looking at for the microlensing survey may harbor transiting planets.

- “Microlensing events are rare and occur quickly, so you need to look at a lot of stars repeatedly and precisely measure brightness changes to detect them,” said astrophysicist Benjamin Montet, a Scientia Lecturer at the University of New South Wales in Sydney. “Those are exactly the same things you need to do to find transiting planets, so by creating a robust microlensing survey, Roman will produce a nice transit survey as well.”

- In a 2017 paper, Montet and his colleagues showed that Roman – formerly known as WFIRST ­– could catch more than 100,000 planets passing in front of, or transiting, their host stars. Periodic dimming as a planet repeatedly crosses in front of its star provides strong evidence of its presence, something astronomers typically have to confirm through follow-up observations.

Figure 22: This animation shows a planet crossing in front of, or transiting, its host star and the corresponding light curve astronomers would see. Using this technique, scientists anticipate Roman could find 100,000 new worlds [video credits: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)]

- The transit approach to finding exoplanets has been wildly successful for NASA's Kepler and K2 missions, which have discovered about 2,800 confirmed planets to date, and is currently used by NASA’s Transiting Exoplanet Survey Satellite (TESS). Since Roman will find planets orbiting more distant, fainter stars, scientists will often have to rely on the mission’s expansive data set to verify the planets. For example, Roman might see secondary eclipses – small brightness dips when a planetary candidate passes behind its host star, which could help confirm its presence.

- The twin detection methods of microlensing and transits complement each other, allowing Roman to find a diverse array of planets. The transit method works best for planets orbiting very close to their star. Microlensing, on the other hand, can detect planets orbiting far from their host stars. This technique can also find so-called rogue planets, which are not gravitationally bound to a star at all. These worlds can range from rocky planets smaller than Mars to gas giants.

- Roughly three quarters of the transiting planets Roman will find are expected to be gas giants like Jupiter and Saturn, or ice giants like Uranus and Neptune. Most of the remainder will likely be planets that are between four and eight times as massive as Earth, known as mini-Neptunes. These worlds are particularly interesting since there are no planets like them in our solar system.

- Some of the transiting worlds Roman captures are expected to lie within their star’s habitable zone, or the range of orbital distances where a planet may host liquid water on its surface. The location of this region varies depending on how large and hot the host star is – the smaller and cooler the star, the closer in its habitable zone will be. Roman’s sensitivity to infrared light makes it a powerful tool for finding planets around these dimmer orange stars.

- Roman will also look farther out from Earth than previous planet-hunting missions. Kepler’s original survey monitored stars at an average distance of around 2,000 light-years. It viewed a modest region of the sky, totaling about 115 square degrees. TESS scans nearly the entire sky, however it aims to find worlds that are closer to Earth, with typical distances of around 150 light-years. Roman will use both the microlensing and transit detection methods to find planets up to 26,000 light-years away.

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Figure 23: This graphic highlights the search areas of three planet-hunting missions: the upcoming Nancy Grace Roman Space Telescope, the Transiting Exoplanet Survey Satellite (TESS), and the retired Kepler Space Telescope. Astronomers expect Roman to discover roughly 100,000 transiting planets, worlds that periodically dim the light of their stars as they cross in front of them. While other missions, including Kepler's extended K2 survey (not pictured in this graphic), have unveiled relatively nearby planets, Roman will reveal a wealth of worlds much farther from home (image credit: NASA's Goddard Space Flight Center)

- Combining the results from Roman’s microlensing and transiting planet searches will help provide a more complete planet census by revealing worlds with a wide range of sizes and orbits. The mission will offer the first opportunity to find large numbers of transiting planets located thousands of light-years away, helping astronomers learn more about the demographics of planets in different regions of the galaxy.

- “The fact that we’ll be able to detect thousands of transiting planets just by looking at microlensing data that’s already been taken is exciting,” said study co-author Jennifer Yee, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts. “It’s free science.”

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado, L3Harris Technologies in Melbourne, Florida, and Teledyne Scientific & Imaging in Thousand Oaks, California.

• March 3, 2021: NASA’s Nancy Grace Roman Space Telescope will be able to explore even more cosmic questions, thanks to a new near-infrared filter. The upgrade will allow the observatory to see longer wavelengths of light, opening up exciting new opportunities for discoveries from the edge of our solar system to the farthest reaches of space. 23)

- “It’s incredible that we can make such an impactful change to the mission after all of the primary components have already passed their critical design reviews,” said Julie McEnery, the Roman Space Telescope senior project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Using the new filter, we will be able to see the full infrared range the telescope is capable of viewing, so we’re maximizing the science Roman can do.”

- With the new filter, Roman’s wavelength coverage of visible and infrared light will span 0.5 to 2.3 µm – a 20% increase over the mission’s original design. This range will also enable more collaboration with NASA’s other big observatories, each of which has its own way of viewing the cosmos. The Hubble Space Telescope can see from 0.2 to 1.7 µm, which allows it to observe the universe in ultraviolet to near-infrared light. The James Webb Space Telescope, launching in October, will see from 0.6 to 28 µm, enabling it to see near-infrared, mid-infrared, and a small amount of visible light. Roman’s improved range of wavelengths, along with its much larger field of view, will reveal more interesting targets for Hubble and Webb to follow up on for detailed observations.

Figure 24: Watch this video to learn more about the Nancy Grace Roman Space Telescope's new near-infrared filter and the benefits it will provide (image credit: NASA/GSFC)

- Expanding Roman’s capabilities to include much of the near-infrared K band, which extends from 2.0 to 2.4 µm, will help us peer farther across space, probe deeper into dusty regions, and view more types of objects. Roman’s sweeping cosmic surveys will unveil countless celestial bodies and phenomena that would otherwise be difficult or impossible to find.

- “A seemingly small change in wavelength range has an enormous effect,” said George Helou, director of IPAC at Caltech in Pasadena, California, and one of the advocates for the modification. “Roman will see things that are 100 times fainter than the best ground-based K-band surveys can see because of the advantages of space for infrared astronomy. It’s impossible to foretell all of the mysteries Roman will help solve using this filter.”

Treasures in our cosmic backyard

- While the mission is optimized to explore dark energy and exoplanets – planets beyond our solar system – its enormous field of view will capture troves of other cosmic wonders too.

- Roman will excel at detecting the myriad small, dark bodies located in the outskirts of our solar system, beyond Neptune’s orbit. Using its improved vision, the mission will now be able to search these bodies for water ice.

- This region, known as the Kuiper belt, contains the remnants of a primordial disk of icy bodies that were left over from the formation of the solar system. Many of these cosmic fossils are largely unchanged since they formed billions of years ago. Studying them provides a window into the solar system’s early days.

- Most of the Kuiper belt’s original inhabitants are no longer there. Many were thrown out into interstellar space as the solar system took shape. Others were eventually sent toward the inner solar system, becoming comets. Occasionally their new paths crossed Earth’s orbit.

- Scientists think ancient comet impacts delivered at least some of Earth’s water, but they’re not sure how much. A census of the water ice on bodies in the outer solar system could offer valuable clues.

Lifting veils of dust

- Though it’s a bit counterintuitive, our Milky Way galaxy can be one of the most difficult galaxies to study. When we peer through the plane of the Milky Way, many objects are shrouded from view by clouds of dust and gas that drift in between stars.

- Dust scatters and absorbs visible light because the particles are the same size or even larger than the light’s wavelength. Since infrared light travels in longer waves, it can pass more easily through clouds of dust.

- Viewing space in infrared light allows astronomers to pierce hazy regions, revealing things they wouldn’t be able to see otherwise. With Roman’s new filter, the observatory will now be able to peer through dust clouds up to three times thicker than it could as originally designed, which will help us study the structure of the Milky Way.

- The mission will spot stars that lie in and beyond our galaxy’s central hub, which is densely packed with stars and debris. By estimating how far away the stars are, scientists will be able to piece together a better picture of our home galaxy.

- Roman’s expanded view will also help us learn even more about brown dwarfs – objects that are not massive enough to undergo nuclear fusion in their cores like stars. The mission will find these “failed stars” near the heart of the galaxy, where catastrophic events like supernovae occur more often.

- Astronomers think this location may affect how stars and planets form since exploding stars seed their surroundings with new elements when they die. Using the new filter, the mission will be able to characterize brown dwarfs by probing their composition. This could help us identify differences between objects near the heart of the galaxy and out in the spiral arms.

Gazing across the expanse of space

- If we want to view the most far-flung objects in space, we need an infrared telescope. As light travels through the expanding universe, it stretches into longer wavelengths. The longer it travels before reaching us, the more extended its wavelengths become. UV light stretches to visible light wavelengths, and then visible light extends to infrared.

- By extending Roman’s view even further into the infrared, the mission will be able to see back to when the universe was less than 300 million years old, or about 2% of its current age of 13.8 billion years. Exploring such distant regions of space could help us understand when stars and galaxies first began forming.

- The origin of galaxies is still a mystery because the first objects that formed are extremely faint and spread sparsely across the sky. Roman’s new filter, coupled with the telescope’s wide field of view and its sensitive camera, could help us find enough first-generation galaxies to understand the population as a whole. Then astronomers can select prime targets for missions like the James Webb Space Telescope to zoom in for more detailed follow-up observations.

- The new filter could also provide another way to pin down the Hubble constant, a number that describes how fast the universe is expanding. It has recently sparked debate among astronomers because different results have emerged different measurements.

- Astronomers often use a certain type of star called Cepheid variables to help determine the expansion rate. These stars brighten and dim periodically, and in the early 1900s American astronomer Henrietta Leavitt noticed a relationship between a Cepheid’s luminosity – that is, its average intrinsic brightness – and the cycle’s length.

- When astronomers detect Cepheids in remote galaxies, they can determine accurate distances by comparing the actual, intrinsic brightness of the stars to their apparent brightness from Earth. Then astronomers can measure how fast the universe is expanding by seeing how fast galaxies at different distances are moving away.

- Another type of star, called RR Lyrae variables, have a similar relationship between their actual brightness and the amount of time it takes to brighten, dim, and brighten again. They’re fainter than Cepheids, and their period-luminosity relationship can’t easily be determined in most wavelengths of light, but Roman will be able to study them using its new filter. Observing RR Lyrae and Cepheid stars in infrared light to determine distances to other galaxies may help clear up recently revealed discrepancies in our measurements of the universe’s expansion rate.

- “Enhancing Roman's vision further into the infrared provides astronomers with a powerful new tool to explore our universe,” said McEnery. “Using the new filter we will make discoveries over a vast area, from distant galaxies all the way to our local neighborhood.”

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado, L3Harris Technologies in Melbourne, Florida, and Teledyne Scientific & Imaging in Thousand Oaks, California.

• January 25, 2021: When it launches in the mid-2020s, NASA’s Nancy Grace Roman Space Telescope will explore an expansive range of infrared astrophysics topics. One eagerly anticipated survey will use a gravitational effect called microlensing to reveal thousands of worlds that are similar to the planets in our solar system. Now, a new study shows that the same survey will also unveil more extreme planets and planet-like bodies in the heart of the Milky Way galaxy, thanks to their gravitational tug on the stars they orbit. 24)

- “We were thrilled to discover that Roman will be able to offer even more information about the planets throughout our galaxy than originally planned,” said Shota Miyazaki, a graduate student at Osaka University in Japan who led the study. “It will be very exciting to learn more about a new, unstudied batch of worlds.”

- Roman will primarily use the gravitational microlensing detection method to discover exoplanets – planets beyond our solar system. When a massive object, such as a star, crosses in front of a more distant star from our vantage point, light from the farther star will bend as it travels through the curved space-time around the nearer one.

- The result is that the closer star acts as a natural lens, magnifying light from the background star. Planets orbiting the lens star can produce a similar effect on a smaller scale, so astronomers aim to detect them by analyzing light from the farther star.

- Since this method is sensitive to planets as small as Mars with a wide range of orbits, scientists expect Roman’s microlensing survey to unveil analogs of nearly every planet in our solar system. Miyazaki and his colleagues have shown that the survey also has the power to reveal more exotic worlds – giant planets in tiny orbits, known as hot Jupiters, and so-called “failed stars,” known as brown dwarfs, which are not massive enough to power themselves by fusion the way stars do.

- This new study shows that Roman will be able to detect these objects orbiting the more distant stars in microlensing events, in addition to finding planets orbiting the nearer (lensing) stars. - The team’s findings are published in The Astronomical Journal.

- Astronomers see a microlensing event as a temporary brightening of the distant star, which peaks when the stars are nearly perfectly aligned. Miyazaki and his team found that in some cases, scientists will also be able to detect a periodic, slight variation in the lensed starlight caused by the motion of planets orbiting the farther star during a microlensing event.

- As a planet moves around its host star, it exerts a tiny gravitational tug that shifts the star’s position a bit. This can pull the distant star closer and farther from a perfect alignment. Since the nearer star acts as a natural lens, it’s like the distant star’s light will be pulled slightly in and out of focus by the orbiting planet. By picking out little shudders in the starlight, astronomers will be able to infer the presence of planets.

Figure 25: This animation demonstrates the xallarap effect. As a planet moves around its host star, it exerts a tiny gravitational tug that shifts the star’s position a bit. This can pull the distant star closer and farther from a perfect alignment. Since the nearer star acts as a natural lens, it’s like the distant star’s light will be pulled slightly in and out of focus by the orbiting planet. By picking out little shudders in the starlight, astronomers will be able to infer the presence of planets (video credit: NASA's Goddard Space Flight Center)

- “It’s called the xallarap effect, which is parallax spelled backward. Parallax relies on motion of the observer – Earth moving around the Sun – to produce a change in the alignment between the distant source star, the closer lens star and the observer. Xallarap works the opposite way, modifying the alignment due to the motion of the source,” said David Bennett, who leads the gravitational microlensing group at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

- While microlensing is generally best suited to finding worlds farther from their star than Venus is from the Sun, the xallarap effect works best with very massive planets in small orbits, since they make their host star move the most. Revealing more distant planets will also allow us to probe a different population of worlds.

Mining the core of the galaxy

- Most of the first few hundred exoplanets discovered in our galaxy had masses hundreds of times greater than Earth’s. Unlike the giant planets in our solar system, which take 12 to 165 years to orbit the Sun, these newfound worlds whirl around their host stars in as little as a few days.

- These planets, now known as hot Jupiters due to their giant size and the intense heat from their host stars, weren’t expected from existing planetary formation models and forced astronomers to rethink them. Now there are several theories that attempt to explain why hot Jupiters exist, but we still aren’t sure which – if any – is correct. Roman’s observations should reveal new clues.

- Even more massive than hot Jupiters, brown dwarfs range from about 4,000 to 25,000 times Earth’s mass. They’re too heavy to be characterized as planets, but not quite massive enough to undergo nuclear fusion in their cores like stars.

- Other planet-hunting missions have primarily searched for new worlds relatively nearby, up to a few thousand light-years away. Close proximity makes more detailed studies possible. However, astronomers think that studying bodies close to our galaxy’s core may yield new insight into how planetary systems evolve. Miyazaki and his team estimate that Roman will find around 10 hot Jupiters and 30 brown dwarfs nearer to the center of the galaxy using the xallarap effect.

- The center of the galaxy is populated mainly with stars that formed around 10 billion years ago. Studying planets around such old stars could help us understand whether hot Jupiters form so close to their stars, or are born farther away and migrate inward over time. Astronomers will be able to see if hot Jupiters can maintain such small orbits for long periods of time by seeing how frequently they’re found around ancient stars.

- Unlike stars in the galaxy’s disk, which typically roam the Milky Way at comfortable distances from one another, stars near the core are packed much closer together. Roman could reveal whether having so many stars so close to each other affects orbiting planets. If a star passes close to a planetary system, its gravity could pull planets out of their usual orbits.

- Supernovae are also more common near the center of the galaxy. These catastrophic events are so intense that they can forge new elements, which are spewed into the surrounding area as the exploding stars die. Astronomers think this might affect planet formation. Finding worlds in this region could help us understand more about the factors that influence the planet-building process.

- Roman will open up a window into the distant past by looking at older stars and planets. The mission will also help us explore whether brown dwarfs form as easily near the center of the galaxy as they do closer to Earth by comparing how frequently they’re found in each region.

- By tallying up very old hot Jupiters and brown dwarfs using the xallarap effect and finding more familiar worlds using microlensing, Roman will bring us another step closer to understanding our place in the cosmos.

- “We’ve found a lot of planetary systems that seem strange compared with ours, but it’s still not clear whether they’re the oddballs or we are,” said Samson Johnson, a graduate student at Ohio State University in Columbus and a co-author of the paper. “Roman will help us figure it out, while helping answer other big questions in astrophysics.”

• January 11, 2021: One of the Hubble Space Telescope’s most iconic images is the Hubble Ultra Deep Field, which unveiled myriad galaxies across the universe, stretching back to within a few hundred million years of the Big Bang. Hubble peered at a single patch of seemingly empty sky for hundreds of hours beginning in September 2003, and astronomers first unveiled this galaxy tapestry in 2004, with more observations in subsequent years. 25)

- NASA’s upcoming Nancy Grace Roman Space Telescope will be able to photograph an area of the sky at least 100 times larger than Hubble with the same crisp sharpness. Among the many observations that will be enabled by this wide view of the cosmos, astronomers are considering the possibility and scientific potential of a Roman Space Telescope “ultra-deep field.” Such an observation could reveal new insights into subjects ranging from star formation during the universe’s youth to the way galaxies cluster together in space.

- Roman will enable new science in all areas of astrophysics, from the solar system to the edge of the observable universe. Much of Roman’s observing time will be dedicated to surveys over wide swaths of the sky. However, some observing time will also be available for the general astronomical community to request other projects. A Roman ultra deep field could greatly benefit the scientific community, say astronomers.

- “As a community science concept, there could be exciting science returns from ultra-deep field observations by Roman. We would like to engage the astronomical community to think about ways in which they could take advantage of Roman’s capabilities,” said Anton Koekemoer of the Space Telescope Science Institute in Baltimore, Maryland. Koekemoer presented the Roman ultra-deep field idea at the 237th meeting of the American Astronomical Society, on behalf of a group of astronomers spanning more than 30 institutions.

- As an example, a Roman ultra-deep field could be similar to the Hubble Ultra Deep Field – looking in a single direction for a few hundred hours to build up an extremely detailed image of very faint, distant objects. Yet while Hubble snagged thousands of galaxies this way, Roman would collect millions. As a result, it would enable new science and vastly improve our understanding of the universe.

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Figure 26: This composite image illustrates the possibility of a Roman Space Telescope “ultra deep field” observation. In a deep field, astronomers collect light from a patch of sky for an extended period of time to reveal the faintest and most distant objects. This view centers on the Hubble Ultra Deep Field (outlined in blue), which represents the deepest portrait of the universe ever achieved by humankind, at visible, ultraviolet and near-infrared wavelengths. Two insets reveal stunning details of the galaxies within the field. Beyond the Hubble Ultra Deep Field, additional observations obtained over the past two decades have filled in the surrounding space. These wider Hubble observations reveal over 265,000 galaxies, but are much shallower than the Hubble Ultra Deep Field in terms of the most distant galaxies observed. These Hubble images are overlaid on an even wider view using ground-based data from the Digitized Sky Survey. An orange outline shows the field of view of NASA’s upcoming Nancy Grace Roman Space Telescope. Roman’s 18 detectors will be able to observe an area of sky at least 100 times larger than the Hubble Ultra Deep Field at one time, with the same crisp sharpness as Hubble (image credits: NASA, ESA, and A. Koekemoer (STScI); Acknowledgement: Digitized Sky Survey)

Structure and history of the universe

- Perhaps most exciting is the possibility of studying the very early universe, which corresponds to the most distant galaxies. Those galaxies are also the rarest: for example, only a handful are seen in the Hubble Ultra Deep Field.

- Thanks to Roman’s wide field of view and near-infrared data of similar quality to Hubble’s, it could discover many hundreds, or possibly thousands, of these youngest, most distant galaxies, interspersed among the millions of other galaxies. That would let astronomers measure how they group together in space as well as their ages and how their stars have formed.

- “Roman would also yield powerful synergies with current and future telescopes on the ground and in space, including NASA’s James Webb Space Telescope and others,” said Koekemoer.

- Moving forward in cosmic time, Roman would pick up additional galaxies that existed about 800 million to 1 billion years after the big bang. At that time, galaxies were just beginning to group together into clusters under the influence of dark matter. While researchers have simulated this process of forming large-scale structures, a Roman ultra-deep field would provide real world examples to test those simulations.

Figure 27: In 2003, Hubble captured its iconic Ultra Deep Field image, which changed our understanding of the universe. With 100 times more coverage, imagine what we could learn if the Nancy Grace Roman Space Telescope did the same (video credit: NASA's Goddard Space Flight Center)

Star formation over cosmic time

- The early universe also experienced a firestorm of star formation. Stars were being born at rates hundreds of times faster than what we see today. In particular, astronomers are eager to study “cosmic dawn” and “cosmic noon,” which together cover a time 500 million to 3 billion years after the big bang when most star formation was happening, as well as when supermassive black holes were most active.

- “Because Roman’s field of view is so large, it will be game changing. We would be able to sample not just one environment in a narrow field of view, but instead a variety of environments captured by Roman’s wide-eyed view. This will give us a better sense of where and when star formation was happening,” explained Sangeeta Malhotra of NASA Goddard Space Flight Center in Greenbelt, Maryland. Malhotra is a co-investigator on the Roman science investigation teams working on cosmic dawn, and has led programs that do deep spectroscopy with Hubble, to learn about distant, young galaxies.

- Astronomers are eager to measure star formation rates in this distant epoch, which could influence a variety of factors such as the amount of heavy elements observed. Rates of star formation might depend on whether or not a galaxy lies within a large cluster. Roman will be capable of taking faint spectra that will show distinct “fingerprints” of these elements, and give accurate distances (called redshifts) of galaxies.

- “Population experts might ask, what differences are there between people who live in big cities, versus those in suburbia, or rural areas? Similarly, as astronomers we can ask, do the most active star forming galaxies live in very clustered regions, or just at the edges of clusters, or do they live in isolation?” Malhotra said.

Big data and machine learning

- One of the greatest challenges of the Roman mission will be learning how to analyze the abundance of scientific information in the public datasets that it will produce. In a sense, Roman will create new opportunities not only in terms of sky coverage, but also in data mining.

- A Roman ultra-deep field would contain information on millions of galaxies – far too many to be studied by researchers one at a time. Machine learning – a form of artificial intelligence – will be needed to process the massive database. While this is a challenge, it also offers an opportunity. “You could explore completely new questions that you couldn’t previously address,” stated Koekemoer.

- “The discovery potential enabled by the huge datasets from the Roman mission could lead to breakthroughs in our understanding of the universe, beyond what we might currently envision,” Koekemoer added. “That could be Roman’s lasting legacy for the scientific community: not only in answering the science questions we think we can address, but also new questions that we have yet to think of.”

• November 18, 2020: NASA’s Nancy Grace Roman Space Telescope will detect vestiges of sound waves that once rippled through the primordial cosmic sea. According to new simulations, Roman’s observations could extend these measurements into an unprobed epoch between the universe’s infancy and the present day. Studying the echoes from this era will help us trace the evolution of the universe and solve pressing cosmic conundrums. 26)

- Sound waves from the nascent universe, called baryon acoustic oscillations (BAOs), left their imprint on the cosmos by influencing galaxy distribution. Researchers have explored this imprint back to when the universe was three billion years old, or roughly 20% of its current age of 13.8 billion years – the same epoch Roman's BAO studies are optimized to investigate. Now a team of scientists has demonstrated that the mission could peer even farther back in time to explore impressions left by BAOs.

- “This isn’t something you can study in a lab, so we created mock universes and ran simulations,” said Siddharth Satpathy, who led the study. Now a machine-learning engineer at Cisco in San Francisco, he conducted this research while earning a doctorate in computational astrophysics from Carnegie Mellon University in Pittsburgh. “We were excited to find that Roman will be powerful enough to study BAO remnants in the universe’s youth,” he added. 27)

Figure 28: This animation explains how BAOs arose in the early universe and how astronomers can study the faint imprint they made on galaxy distribution to probe dark energy’s effects over time. In the beginning, the cosmos was filled with a hot, dense fluid called plasma. Tiny variations in density excited sound waves that rippled through the fluid. When the universe was about 400,000 years old, the waves froze where they were. Slightly more galaxies formed along the ripples. These frozen ripples stretched as the universe expanded, increasing the distance between galaxies. Astronomers can study this preferred distance between galaxies in different cosmic ages to understand the expansion history of the universe (video credit: NASA's Goddard Space Flight Center, Lars Leonard)

Hot plasma soup

- For most of its first half-million years, the universe looked extremely different than it does today. Instead of being speckled with stars and galaxies, the cosmos was filled with a sea of plasma – charged particles – that formed a dense, almost uniform fluid.

- There were tiny fluctuations of about one part in 100,000. What few variations there were took the form of slightly denser kernels of matter, like a single ounce of cinnamon sprinkled into about 13,000 cups of cookie dough. Since the clumps had more mass, their gravity attracted additional material.

- It was so hot that particles couldn’t stick together when they collided – they just bounced off each other. Alternating between the pull of gravity and this repelling effect created waves of pressure – sound – that propagated through the plasma.

- Over time, the universe cooled and particles combined to form neutral atoms. Because the particles stopped repelling each other, the waves ceased. Their traces, however, still linger, etched on the cosmos.

Frozen echoes

- When atoms formed, the ripples essentially froze in place, carrying within them a bit more matter than the average across the universe. With the repulsive pressure of the plasma gone, gravity became the dominant force.

- Over the course of hundreds of millions of years, clumps from the plasma that once filled the universe slurped up more material to become stars. Their mutual gravity pulled stars together into groups, ultimately forming the galaxies we see today. And slightly more galaxies formed along the ripples than elsewhere.

- While the waves no longer propagated, the frozen ripples stretched as the universe expanded, increasing the distance between galaxies. By looking at how galaxies are spread out in different cosmic epochs, we can explore how the universe has expanded over time.

- “BAOs have left their mark on the cosmos, but we haven’t fully examined their traces,” said co-author Rupert Croft, a professor of physics at Carnegie Mellon University. “By studying BAO impressions in an unprobed region, we can excavate cosmic fossils, which will allow us to unearth new information about the forces that have shaped the universe.”

- Scientists have noticed a pattern in the way galaxies cluster together from measurements of the nearby universe. For any galaxy today, we are more likely to find another galaxy about 500 million light-years away than slightly nearer or farther. But looking farther out into space, to earlier cosmic times, means that this distance – the vestige of the frozen BAO ripples – will decrease.

- Roman will extend previous research by mapping the expansion of the universe to unprecedented detail. Satpathy and his team showed that Roman’s surveys will be able to probe for BAO remnants five times farther than originally planned, back to when the universe was only about 600 million years old – just 4% of its current age.

- Learning more about the way the cosmos has expanded over time will allow scientists to explore dark energy – a mysterious pressure that is accelerating the expansion of the universe. Roman is optimized to survey the cosmos for BAO impressions across the middle of its current age because that’s when scientists think dark energy transitioned from being a minor contributor to the contents of the universe to the most dominant force.

- But some theories hypothesize a bout of dark energy activity when the universe was much younger. Peering farther into the universe’s past will help add pieces to the puzzle.

- “We haven’t extensively explored BAO imprints from when the universe was very young because we need an enormous sample of galaxies to do so,” said Jason Rhodes, a senior research scientist at NASA’s Jet Propulsion Lab in Pasadena, California. “That’s where Roman comes in. The mission has such a wide field of view that observations like this will become possible.”

- Roman’s High Latitude Spectroscopic Survey will measure accurate distances and positions for millions of galaxies. Scientists plan to analyze how their distribution varies with distance by creating a 3D map of the universe, which will help us decipher how dark energy has shaped the cosmos over time.

- Two additional Roman surveys will also study dark energy, and each technique will cross-check the others. The mission will provide important data to help scientists investigate, and possibly even foresee, the universe’s fate.

- The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Pasadena, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions.

• September 8, 2020: L3Harris Technologies of Melbourne, Florida, has finished figuring, polishing and coating the primary mirror for NASA’s Nancy Grace Roman Space Telescope. 28)

- Roman’s primary mirror will collect and focus light from exoplanets, stars, galaxies and supernovae for the telescope, ultimately feeding scientific instruments. Roman will allow scientists to study the cosmos in a complementary way to the Hubble Space Telescope, using 100 x larger field of view than Hubble, to study far more objects in the sky.

- L3Harris engineers applied advanced technology to create a lightweight primary mirror. The same diameter as Hubble’s main mirror (2.4 meters) Roman’s primary mirror is one-fourth the weight of Hubble's which is a key benefit for all space missions.

- The primary mirror has undergone testing in L3Harris’s thermal vacuum chambers designed to simulate the cold,harsh space environment, and an optical test verified the performance of the mirror. L3Harris engineers and technicians will simulate zero gravity by offloading the weight of the mirror through specialty support equipment specifically developed for this purpose.

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Figure 29: L3Harris' Bonnie Patterson stands with the completed primary mirror for the Nancy Grace Roman Space Telescope (photo credit: L3Harris)

• July 24, 2020: When it launches in the mid-2020s, NASA’s Nancy Grace Roman Space Telescope will create enormous panoramic pictures of space in unprecedented detail. The mission’s wide field of view will enable scientists to conduct sweeping cosmic surveys, yielding a wealth of new information about the universe. 29)

- The Roman mission’s ground system, which will make data from the spacecraft available to scientists and the public, has just successfully completed its preliminary design review. The plan for science operations has met all of the design, schedule, and budget requirements, and will now proceed to the next phase: building the newly designed data system.

- “This is an exciting milestone for the mission,” said Ken Carpenter, the Roman ground system project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We are on track to complete the data system in time for launch, and we look forward to the ground-breaking science it will enable.”

- Roman will have the same resolution as the Hubble Space Telescope but capture a field of view nearly 100 times larger. Scientists expect the spacecraft to collect more data than any of NASA’s other astrophysics missions.

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Figure 30: This infographic showcases the difference in data volume between the Nancy Grace Roman and Hubble space telescopes. Each day, Roman will send over 500 times more data back to Earth than Hubble (image credits: NASA’s Goddard Space Flight Center)

- Using Hubble’s observations, astronomers have revolutionized our view of the universe and unleashed a flood of discoveries. Hubble has gathered 172 terabytes of data since its launch in 1990. If all of this data were printed as text and the pages were placed on top of each other, the stack would reach about 5,000 miles (8,000 km) high. That’s far enough to reach about 15 times higher than Hubble’s orbit, or about 2% of the distance to the Moon.

- Roman will gather data about 500 times faster than Hubble, adding up to 20,000 terabytes (20 petabytes) over the course of its five-year primary mission. If this data were printed, the stack of papers would tower 330 miles (530 km) high after a single day. By the end of Roman’s primary mission, the stack would extend well beyond the Moon. Untold cosmic treasures will be brought to light by Roman’s rich observations.

- Such a vast volume of information will require NASA to rely on new processing and archival techniques. Scientists will access and analyze Roman’s data using cloud-based remote services and more sophisticated tools than those used by previous missions.

- All of Roman’s data will be publicly available within days of the observations – a first for a NASA astrophysics flagship mission. This is significant because Roman’s colossal images will often contain far more than the primary target of observation.

- Since scientists everywhere will have rapid access to the data, they will be able to quickly discover short-lived phenomena, such as supernova explosions. Detecting these phenomena quickly will allow other telescopes to perform follow-up observations.

Pinpointing planets

- One of the science areas that will benefit from the mission’s vast data is the microlensing survey. Gravitational lensing is an observational effect that occurs because the presence of mass warps the fabric of space-time. The effect is extreme around very massive objects, like black holes and entire galaxies. But even relatively small objects like stars and planets cause a detectable degree of warping, called microlensing.

- Any time two stars align closely from our vantage point, light from the more distant star curves as it travels through the warped space-time around the nearer star. The nearer star acts like a natural cosmic lens, focusing and intensifying light from the background star.

- Scientists see this as a spike in brightness. Planets orbiting the foreground star may also modify the lensed light, acting as their own tiny lenses. These small signatures drive the design of Roman’s microlensing survey.

- “With such a large number of stars and frequent observations, Roman’s microlensing survey will see thousands of planetary events,” said Rachel Akeson, task lead for the Roman Science Support Center at IPAC/Caltech in Pasadena, California. “Each one will have a unique signature which we can use to determine the planet’s mass and distance from its star.”

- Roman’s microlensing survey will also detect hundreds of other bizarre and interesting cosmic objects. Roman will discover starless planets that roam the galaxy as rogue worlds; brown dwarfs, which are too massive to be characterized as planets but not massive enough to ignite as stars; and stellar corpses, including neutron stars and black holes, which are left behind when stars exhaust their fuel.

- Microlensing events are extremely rare and require extensive observations. Roman will monitor hundreds of millions of stars every 15 minutes for months at a time – something no other space telescope can do, generating an unprecedented stream of new information.

Gazing beyond our galaxy

- While the microlensing survey will look toward the heart of our galaxy, where stars are most densely concentrated, Roman’s cosmological surveys will peer far beyond our stars to study hundreds of millions of other galaxies. These observations will help illuminate two of the biggest cosmic puzzles: dark matter and dark energy.

- Visible matter accounts for only about five percent of the contents of the universe. Nearly 27 percent of the universe comes in the form of dark matter, which doesn't emit or absorb light. Dark matter is only detectable through its gravitational effects on visible matter.

- Roman will help us figure out what dark matter is made of by exploring the structure and distribution of regular matter and dark matter across space and time. This investigation can only be done effectively using precise measurements from many galaxies.

- The remaining approximately 68 percent of the universe is made up of dark energy. This mysterious cosmic pressure is causing the expansion of the universe to accelerate, but so far we don’t know much more about it.

Figure 31: This video of the Eagle Nebula showcases the superb resolution and wide field of view of NASA’s upcoming Nancy Grace Roman Space Telescope. It begins with a Hubble image of the famous Pillars of Creation superimposed on a ground-based image. The view then zooms out to show the full field of view of Roman’s Wide Field Instrument. Roman’s images will have the resolution of Hubble while covering an area about 100 times larger in a single pointing [video credit: L. Hustak (STScI)]

- Roman will study dark energy through multiple observational strategies, including surveys of galaxy clusters and supernovae. Scientists will create a 3D map of the universe to help us understand how the universe grew over time under the influence of dark energy.

- Since Roman will have such a large field of view, it will dramatically reduce the amount of time needed to gather data. The Cosmic Assembly Near-infrared Deep Extragalactic Survey (CANDELS) is one of the largest projects ever done with Hubble, designed to study the development of galaxies over time. While it took Hubble nearly 21 days, Roman would complete a similar survey in less than half an hour – 1,000 times faster than Hubble. Using Roman, scientists will be able to extend these observations in ways that would be impractical with other telescopes.

- “With its incredibly fast survey speeds, Roman will observe planets by the thousands, galaxies by the millions, and stars by the billions,” said Karoline Gilbert, mission scientist for the Roman Science Operations Center at the Space Telescope Science Institute in Baltimore. “These vast datasets will allow us to address cosmic mysteries that hint at new fundamental physics.”

- The Nancy Grace Roman Space Telescope is managed at Goddard, with participation by NASA's Jet Propulsion Laboratory and Caltech/IPAC in Pasadena, the STScI (Space Telescope Science Institute) in Baltimore, and a science team comprising scientists from research institutions across the United States.

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Figure 32: This simulated image illustrates the wide range of science enabled by Roman's extremely wide field of view and exquisite resolution. The purple squares, which all contain background imagery simulated using data from Hubble’s Cosmic Assembly Near-infrared Deep Extragalactic Survey (CANDELS) program, outline the area Roman can capture in a single observation. An orange square shows the field of view of Hubble’s Wide Field Camera 3 for comparison. While the CANDELS program took Hubble nearly 21 days to survey in near-infrared light, Roman’s large field of view and higher efficiency would allow it to survey the same area in less than half an hour. Top left: This view illustrates a region of the large nearby spiral galaxy M83. Top right: A hypothetical distant dwarf galaxy appears in this magnified view, demonstrating Roman’s ability to detect small, faint galaxies at large distances. Bottom left: This magnified view illustrates how Roman will be able to resolve bright stars even in the dense cores of globular star clusters. Bottom right: A zoom of the CANDELS-based background shows the density of high-redshift galaxies Roman will detect (image credit: Benjamin Williams, David Weinberg, Anil Seth, Eric Bell, Dave Sand, Dominic Benford, and the WINGS Science Investigation Team)

• March 30, 2020: NASA’s Wide Field Infrared Survey Telescope (WFIRST) will search for exoplanets, planets outside our solar system, toward the center of our Milky Way galaxy, where most stars are. Studying the properties of exoplanet worlds will help us understand what planetary systems throughout the galaxy are like and how planets form and evolve. 30)

- Combining WFIRST’s findings with results from NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions will complete the first planet census that is sensitive to a wide range of planet masses and orbits, bringing us a step closer to discovering habitable Earth-like worlds beyond our own.

- To date, astronomers have found most planets when they pass in front of their host star in events called transits, which temporarily dim the star's light. WFIRST data can spot transits too, but the mission will primarily watch for the opposite effect — little surges of radiance produced by a light-bending phenomenon called microlensing. These events are much less common than transits because they rely on the chance alignment of two widely separated and unrelated stars drifting through space.

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Figure 33: WFIRST will make its microlensing observations in the direction of the center of the Milky Way galaxy. The higher density of stars will yield more microlensing events, including those that reveal exoplanets (image credit: NASA's Goddard Space Flight Center/CI Lab)

Figure 34: This animation illustrates two ways a gravitational microlensing event could look to an observer. At top is the way it could appear to a telescope able to resolve the features. The source star appears to move and distort as its light is warped by the closer lensing star and its planet. At bottom is a light curve showing the intensity of light from the event. As the two stars reach best alignment, the signal reaches its peak. The planet orbiting the lensing star is detectable as a brief change in brightness (video credit: NASA/GSFC Conceptual Image Lab)

- "Microlensing signals from small planets are rare and brief, but they’re stronger than the signals from other methods,” said David Bennett, who leads the gravitational microlensing group at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Since it’s a one-in-a-million event, the key to WFIRST finding low-mass planets is to search hundreds of millions of stars.”

- In addition, microlensing is better at finding planets in and beyond the habitable zone — the orbital distances where planets might have liquid water on their surfaces.

- Microlensing 101: This effect occurs when light passes near a massive object. Anything with mass warps the fabric of space-time, sort of like the dent a bowling ball makes when set on a trampoline. Light travels in a straight line, but if space-time is bent — which happens near something massive, like a star — light follows the curve.

- Any time two stars align closely from our vantage point, light from the more distant star curves as it travels through the warped space-time of the nearer star. This phenomenon, one of the predictions of Einstein’s general theory of relativity, was famously confirmed by British physicist Sir Arthur Eddington during a total solar eclipse in 1919. If the alignment is especially close, the nearer star acts like a natural cosmic lens, focusing and intensifying light from the background star.

- Planets orbiting the foreground star may also modify the lensed light, acting as their own tiny lenses. The distortion they create allows astronomers to measure the planet’s mass and distance from its host star. This is how WFIRST will use microlensing to discover new worlds.

Familiar and exotic worlds

- “Trying to interpret planet populations today is like trying to interpret a picture with half of it covered,” said Matthew Penny, an assistant professor of physics and astronomy at Louisiana State University in Baton Rouge who led a study to predict WFIRST’s microlensing survey capabilities. “To fully understand how planetary systems form we need to find planets of all masses at all distances. No one technique can do this, but WFIRST’s microlensing survey, combined with the results from Kepler and TESS, will reveal far more of the picture.”

- More than 4,000 confirmed exoplanets have been discovered so far, but only 86 were found via microlensing. The techniques commonly used to find other worlds are biased toward planets that tend to be very different from those in our solar system. The transit method, for example, is best at finding sub-Neptune-sized planets that have orbits much smaller than Mercury’s. For a solar system like our own, transit studies could miss every planet.

- WFIRST’s microlensing survey will help us find analogs to every planet in our solar system except Mercury, whose small orbit and low mass combine to put it beyond the mission’s reach. WFIRST will find planets that are the mass of Earth and even smaller — perhaps even large moons, like Jupiter’s moon Ganymede.

- WFIRST will find planets in other poorly studied categories, too. Microlensing is best suited to finding worlds from the habitable zone of their star and farther out. This includes ice giants, like Uranus and Neptune in our solar system, and even rogue planets — worlds freely roaming the galaxy unbound to any stars.

- While ice giants are a minority in our solar system, a 2016 study indicated that they may be the most common kind of planet throughout the galaxy. WFIRST will put that theory to the test and help us get a better understanding of which planetary characteristics are most prevalent.