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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 across space, but they’re not. Careful mapping has shown that they are distributed across the surfaces of giant cosmic “bubbles” up to several hundred million light-years across. Inside these bubbles, few galaxies are found, so those regions are called cosmic voids. NASA’s Nancy Grace Roman Space Telescope will allow us to measure these voids with new precision, which can tell us about the history of the universe’s expansion.
“Roman’s ability to observe wide areas of the sky to 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 on a paper published in The Astrophysical Journal.
Cosmic Recipe
The cosmos is made of three key components: normal matter, dark matter, and dark energy. The gravity of normal and dark matter tries to slow the expansion of the universe, while dark energy opposes gravity to speed up the universe’s expansion. The nature of both dark matter and dark energy are currently unknown. Scientists are trying to understand them by studying their effects on things we can observe, such as the distribution of galaxies across space.
“Since they’re relatively empty of matter, voids are regions of space that are dominated by dark energy. By studying voids, we should be able to put powerful constraints on the nature of dark energy,” said co-author Alice Pisani of CNRS (the French National Centre for Scientific Research) in France and Princeton University in New Jersey.
To determine how Roman might study voids, the researchers considered one potential design of the Roman High-Latitude Wide-Area Survey, one of three core community surveys that Roman will conduct. The High-Latitude Wide-Area Survey will look away from the plane of our galaxy (hence the term high latitude in galactic coordinates). The team found that this survey should be able to detect and measure tens of thousands of cosmic voids, some as small as just 20 million light-years across. Such large numbers of voids will allow scientists to use statistical methods to determine how their observed shapes are influenced by the key components of the universe.
To determine the actual, 3D shapes of the voids, astronomers will use two types of data from Roman — the positions of galaxies in the sky and their cosmological redshift, the latter of which is determined using spectroscopic data. To convert redshift to a physical distance, astronomers make assumptions about the components of the universe, including the strength of dark energy and how it might have evolved over time.
Pisani compared it to trying to infer a cake recipe (i.e., the universe’s makeup) from the final dessert served to you. “You try to put in the right ingredients — the right amount of matter, the right amount of dark energy — and then you check whether your cake looks as 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 found by statistically stacking all of the voids detected by Roman on top of each other. On average, voids are expected to 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 the stacking is done correctly, the resulting shape will be perfectly round (or spherically symmetric). If not, then you have to adjust your cosmic recipe.
Power of Roman
The researchers emphasized that to study cosmic voids in large numbers, an observatory must be able to probe a large volume of the universe, because the voids themselves can be tens or hundreds of millions of light-years across. The spectroscopic data necessary to study voids will come from a portion of the Roman High-Latitude Wide-Area Survey that will cover on the order of 2,400 square degrees of the sky, or 12,000 full moons. It will also be able to see fainter and more distant objects, yielding a greater density of galaxies than complementary missions like ESA’s (European Space Agency’s) Euclid.
“Voids are defined by the fact that they contain so few galaxies. So to detect voids, you have to be able to observe galaxies that are quite sparse and faint. With Roman, we can better look at the galaxies that populate voids, which ultimately will give us greater understanding of the cosmological parameters like dark energy that are sculpting voids,” said co-author Giulia Degni of Roma Tre University and INFN (the National Institute of Nuclear Physics) in Rome.
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 in Southern California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute in Baltimore; and a science team comprising scientists from various research institutions. The primary industrial partners are 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, Md.
cpulliam@stsci.edu
