Core Survey by NASA’s Roman Mission Will Unveil Universe’s Dark Side

The largest survey planned by NASA’s upcoming Nancy Grace Roman Space Telescope will reveal hundreds of millions of galaxies scattered across the cosmos. After Roman launches this fall, scientists will use these shimmering beacons to study the dark foundations of the universe: dark matter and dark energy.

“We set out to create the ultimate large-scale infrared survey, and I think we accomplished that,” said Ryan Hickox, a professor at Dartmouth College in Hanover, New Hampshire, and co-chair of the committee that shaped the survey design. “We will use Roman’s huge, deep 3D images to explore the fundamental nature of the universe, including its dark side.”

Roman’s high-latitude wide area survey is one of the mission’s three primary observational programs. It will cover more than 5,000 square degrees (about 12% of the sky) in just under a year and a half. Roman will look away from the dusty plane of our galaxy, the Milky Way (that’s what the “high latitude” part of the survey’s name means), looking up and out of the galaxy rather than through it to get the clearest view of the distant cosmos.

“This study is going to be a spectacular map of the cosmos, the first time we have obtained Hubble-quality imaging of a large part of the sky,” said David Weinberg, a professor of astronomy at Ohio State University in Columbus, who played a major role in designing the survey. “Even a single score with Roman requires an entire wall of 4K TVs to display at full resolution. Displaying the entire survey at high latitude simultaneously would require half a million 4K TVs, enough to cover 200 football fields or the cliff of El Capitan.”

The survey will combine the powers of imaging and spectroscopy to unveil a goldmine of galaxies scattered across cosmic time. Astronomers will use data from the survey to explore invisible dark matter, detectable only via its gravitational effects on other objects, and the nature of dark energy – a pressure that appears to accelerate the expansion of the universe.

“Cosmic acceleration is the greatest mystery of cosmology and perhaps all of physics,” Weinberg said. “Somehow, when we reach scales of billions of light years, gravity pushes rather than pulls. Studying the vast Roman area will provide vital new clues to help us solve this mystery, because it allows us to measure the history of cosmic structure and the rate of early expansion with much more precision than we can today.”

Weigh the shadows

Anything with mass distorts spacetime, the underlying fabric of the universe. Extremely massive objects like galaxy clusters distort space-time so much that they distort the appearance of background objects – a phenomenon called gravitational lensing.

“It’s like looking through a cosmic mirror,” Hickox said. “It can spread or duplicate distant galaxies, or if the alignment is perfect, it can magnify them like a natural telescope.”

Roman’s view will be large and sharp enough to study this small-scale lensing effect and see how clumps of dark matter distort the appearance of distant galaxies. Astronomers will create a detailed map of the large-scale distribution of matter – visible and invisible – in the universe and further fill in the gaps in our understanding of dark matter. Studying how structures grow over time will also help astronomers explore the strength of dark energy at different cosmic stages.

“The data analysis standards required to measure weak gravitational lensing are such that the astronomical community as a whole will benefit from very high-quality data across the entire study area, which will undoubtedly lead to unexpected discoveries,” said Olivier Doré, principal investigator at NASA’s Jet Propulsion Laboratory in Southern California, who leads a team focused on Roman cosmological imaging with the High-Latitude Wide-Area Survey. “This investigation will accomplish much more than just revealing dark energy! »

While NASA’s Hubble and James Webb space telescopes also study gravitational lensing, Roman’s breakthrough lies in its large field of view.

“Weak lensing distorts the shapes of galaxies in ways too subtle to be visible in a single galaxy – it’s invisible until you do a statistical analysis,” Hickox said. “Roman will see more than a billion galaxies in this study, and we estimate that about 600 million of them will be detailed enough for Roman to study these effects. Roman will therefore trace the growth of the structure of the universe in 3D from shortly after the big bang to the present, mapping dark matter more precisely than ever before.”

Probing dark energy

Roman’s large-scale study will also collect spectra from around 20 million galaxies. Analyzing spectra helps show how the universe has expanded over different cosmic epochs, because when an object recedes, all the light waves we receive from it are stretched and shifted toward redder wavelengths – a phenomenon called redshift.

By determining how fast galaxies are moving away from us, driven by the relentless expansion of space, astronomers can determine how far away they are: the more redshifted a galaxy’s spectrum is, the further away it is. Astronomers will use this phenomenon to create a 3D map of all galaxies measured in the study area out to about 11.5 billion light years away.

This will reveal the frozen echoes of ancient sound waves that once rippled across the primordial cosmic sea. For most of the universe’s first half-million years, the cosmos was a dense, nearly uniform sea of ​​plasma (charged particles).

Rare, tiny clusters attracted more matter toward them by gravitation. But it was too hot for the material to stick, so it bounced back. This pushing and pulling created waves of pressure – sound – that propagated through the plasma.

Over time, the universe cooled and the waves stopped, essentially freezing the ripples (called baryonic acoustic oscillations) in place. Since ripples were places where more matter was collected, slightly more galaxies formed along them than elsewhere. As the universe expanded over billions of years, these structures also expanded.

These rings act as a ruler for the universe. Today, their width is about 500 million light years. Roman will precisely measure their size across cosmic time, revealing the possible evolution of dark energy.

Recent results from other telescopes suggest that dark energy may change in intensity over cosmic time. “Roman will be able to perform high-precision tests that should tell us whether or not these indices constitute real deviations from our current standard model,” said Risa Wechsler, director of KIPAC (Kavli Institute for Particle Astrophysics and Cosmology) at Stanford University in California and co-chair of the committee that shaped the survey design. “Roman’s imaging study combined with his redshift survey gives us new insights into the evolution of the universe – both how it expands and how structures develop over time – that will help us understand what dark energy and gravity do with unprecedented precision.”

In total, Roman will help us understand the effects of dark energy 10 times more precisely than current measurements, helping to discern the leading theories that try to explain why the expansion of the universe is accelerating.

Because of the way Roman will study the universe, it will reveal everything from small rocky objects in our outer solar system and individual stars in nearby galaxies to galaxy mergers and black holes at the cosmic frontier more than 13 billion years ago.

“Roman is exciting because it covers a very large area with image quality only available in space,” Wechsler said. “This allows for a wide range of scientific research, from things we can anticipate studying to discoveries we haven’t yet thought of.”

The Nancy Grace Roman Space Telescope is operated at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation from NASA’s Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Baltimore Space Telescope Science Institute; and a scientific team composed of scientists from various research institutions. Key industry partners include BAE Systems Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.

By Ashley Balzer
NASA Goddard Space Flight CenterGreenbelt, Maryland.

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