NASA’s Roman Telescope Will Observe Thousands of Newfound Cosmic Voids
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Our universe is filled with galaxies, in all directions as far as our instruments can see. Some researchers estimate that there are as many as two trillion galaxies in the observable universe. At first glance, these galaxies might appear to be randomly scattered throughout space, but this is not the case. Careful mapping has shown that they are distributed on the surface of giant cosmic “bubbles” reaching several hundred million light years. Inside these bubbles, few galaxies are found, which is why these regions are called cosmic voids. NASA’s Nancy Grace Roman Space Telescope will allow us to measure these voids with new precision, which may tell us about the history of the expansion of the universe.
“Roman’s ability to observe large areas of the sky at great depths, spotting an abundance of faint and distant galaxies, will revolutionize the study of cosmic voids,” said Giovanni Verza of the Flatiron Institute and New York University, lead author of a paper published in The Astrophysical Journal.
Cosmic recipe
The cosmos is made up of three key elements: normal matter, dark matter and dark energy. The gravity of normal and dark matter attempts to slow the expansion of the universe, while dark energy opposes gravity to speed up the expansion of the universe. The nature of dark matter and dark energy is currently unknown. Scientists try to understand them by studying their effects on things we can observe, such as the distribution of galaxies in space.
“Because they are relatively empty of matter, voids are regions of space dominated by dark energy. By studying voids, we should be able to impose powerful constraints on the nature of dark energy,” said co-author Alice Pisani of the CNRS (French National Center for Scientific Research) in France and Princeton University in New Jersey.
To determine how Roman might study the voids, the researchers considered a potential design for the Roman High-Latitude Wide-Area Survey, one of three major community surveys that Roman will conduct. The High-Latitude Wide-Area Survey will look far from the plane of our galaxy (hence the term high latitude in galactic coordinates). The team found that this study should be able to detect and measure tens of thousands of cosmic voids, some as small as 20 million light years. Such a large number of voids will allow scientists to use statistical methods to determine how observed shapes are influenced by key components of the universe.
To determine the actual 3D shapes of the voids, astronomers will use two types of Roman data: the position of galaxies in the sky and their cosmological redshift, the latter of which is determined using spectroscopic data. To convert redshift into physical distance, astronomers make assumptions about the components of the universe, including the strength of dark energy and how it may have changed over time.
Pisani likened this to trying to deduce a cake recipe (i.e. the composition of the universe) from the final dessert you are served. “You try to put in the right ingredients – the right amount of matter, the right amount of dark energy – and then you check to see if your cake looks the way it should. If it doesn’t, that means you put in the wrong ingredients.”
In this case, the appearance of the “cake” is the shape obtained by statistically stacking all the voids detected by Roman on top of each other. On average, voids should have a spherical shape because there is no “preferred” location or direction in the universe (i.e. the universe is both homogeneous and isotropic on large scales). This means that, if stacking is done correctly, the resulting shape will be perfectly round (or spherically symmetrical). If not, then you will have to adjust your cosmic recipe.
Roman power
The researchers pointed out that to study cosmic voids in large numbers, an observatory must be able to survey a large volume of the universe, because the voids themselves can be tens or hundreds of millions of light years across. The spectroscopic data needed to study the voids will come from part of the Roman High-Latitude Wide-Area Survey that will cover about 2,400 square degrees of the sky, or 12,000 full moons. It will also be able to see fainter and more distant objects, producing a greater density of galaxies than complementary missions like ESA’s (European Space Agency) Euclid.
“Voids are defined by the fact that they contain so few galaxies. So to detect voids you need to be able to observe fairly sparse and faint galaxies. With Roman, we can better observe the galaxies that populate the voids, which will ultimately give us a better understanding of the cosmological parameters like dark energy that sculpt the voids,” said co-author Giulia Degni of the University of Rome Tre and INFN (National Institute of nuclear physics) of Rome.
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 Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.
By Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland.
cpulliam@stsci.edu
