Universe Today
The cosmos is populated with many puzzling, gigantic, and awe-inspiring objects. Supermassive black holes billions of times more massive than the Sun reside in the center of massive galaxies. Huge stars explode in cataclysmic collisions whose light reaches us from more than 10 billion light-years away. Enormous galaxies collide and merge, leading to tremendous bursts of star formation.
But one of the most puzzling things cosmologists can see are cosmic voids. They're a feature of the Large-Scale Structure of the Universe (LSSU).
Confronting the LSSU means confronting the enormity of the cosmos, and how tiny Earth is. We're just one small planet in a galaxy that probably contains over 100 billion planets, maybe several hundred billion. That's an enormous number of planets, and its one of the reasons people think there must be life elsewhere in our galaxy.
But the Milky Way is just one galaxy in the Local Group of galaxies. The Local Group is part of the Virgo Cluster, which contains as many as 2,000 galaxies. The Virgo Cluster is part of the Virgo Supercluster, which contains at least 100 galaxy clusters. And the Virgo Supercluster is part of the Laniakea Supercluster, which is part of the Pisces–Cetus Supercluster Complex. It's impossible to count the galaxies in Pisces-Cetus, but it contains hundreds of galaxy clusters and groups.
All of these galaxies, groups, clusters, and superclusters exist in filaments along dark matter concentrations in the cosmos.
*The large-scale structure of the Universe consists of vast galaxy filaments made up of galaxies, groups, clusters, superclusters, and supercluster complexes. These filaments span billions of light years. The empty space between them are called cosmic voids. Image Credit: By NASA, ESA, and E. Hallman (University of Colorado, Boulder) - http://www.nasa.gov/mission_pages/hubble/science/hst_img_20080520.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7332828*
Between all of these filaments are voids, massive bubbles in the cosmos that are lined with galaxies. They can be hundreds of millions of light-years across. They're not exactly empty; they do contain the odd galaxy. But overall they have less than 10% of the matter concentrations found in the rest of the cosmos.
These massive voids are important in studying Dark Energy (DE), the mysterious force that's driving the Universe to expand. Voids expand faster than more matter-dense regions in the Universe. Cosmologists can measure how the sizes of voids change over time, and how galaxies move around on their edges, and then test those observations against their theoretical models of DE.
*This image shows the voids and the superclusters within about 500 million light-years of the Milky Way. Image Credit: By Base image is from Azcolvin429, cropped by Zeryphex, improved by Astronom5109 - This file was derived from: 7 Local Superclusters.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=58212354*
Since there's much less matter there, and less gravity as a result, voids are more pure regions where it's easier to isolate the effects of DE from the effects of gravity. Overall, the statitistics from the growth of voids can help explain how DE's strength is changing over time.
Studying DE is one of the Nancy Grace Roman Space Telescope's goals. It will use baryon acoustic oscillation, weak gravitational lensing, and observations of Type 1a supernova to study DE. It has an extremely wide field of view and observes in infrared. By using these three independent methods at once, it will map cosmic structure in detail, including cosmic voids.
New research in The Astrophysical Journal predicts how well the Nancy Grace Roman Space Telescope will detect voids and how those observations will constrain statistics on cosmic voids. It's titled "Cosmology with Voids from the Nancy Grace Roman Space Telescope." The lead author is Giovanni Verza, from the Flatiron Institute and New York University.
"Cosmic voids, the underdense regions in the galaxy distribution, provide tight constraints on cosmological parameters," the authors write in their paper. For the first time, the telescope will provide "a cosmic void sample of exceptional quality down to a few megaparsecs." According to the authors, the Roman's observations open a new window into cosmic void science.
"Their sensitivity to dark energy properties is expected, since voids are the first regions to be dominated by dark energy," Verza and his colleagues write. That means that the Roman's measurements of the constraints of voids will place important constraints on DE.
The Roman will perform three different surveys of the sky. One of them is the High-Latitude Wide-Area Survey, which will use weak gravitational lensing to probe cosmic expansion.
*This infographic describes the High-Latitude Wide-Area Survey that will be conducted by NASA’s Nancy Grace Roman Space Telescope. Image Credit: NASA's Goddard Space Flight Center*
The researchers used simulations to predict how the Roman's High-Latitude Wide-Area Survey will perform. The High-Latitude in its name means it will look away from the Milky Way's galactic plane. Verza and his colleagues say that during this survey, the Roman will detect tens of thousands of cosmic voids, and that some of them will be only about 20 million light-years across. "We detect 82,551 voids in the 2000 squared degree galaxy lightcone," they explain in their research.
The Survey will use the telescope's Wide-Field Instrument to capture spectra of galaxies on the edges of the voids. That will determine the cosmic redshifts, and when combined with their positions in the sky, will uncover their 3D shapes of the voids.
“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 will also work in conjunction with the ESA's Euclid mission. Euclid's wide survey will cover about 14,000 square degrees on the sky and will capture an extremely broad view. The Roman will capture 2,000 square degrees on the sky, but will be much deeper than Euclid's. Euclid observes in optical and infrared light, while the Roman observes only in infrared. In that sense, the pair will complement each other well, since they're sensitive to different populations of galaxies at different distances. The Vera Rubin Observatory will also be part of the collaboration, with its survey areas overlapping those of the other two telescopes.
While we still don't know exactly what it is, mapping the effect of Dark Energy on the Large-Scale Structure of the Universe will help constrain our understanding of its effect on the cosmos. If the Roman can find and measure more than 82,000 cosmic voids, as this simulation suggests, then we're about to learn a lot more about how DE has driven the accelerating expansion of the Universe.
"With a first, comprehensive analysis of a Roman-like mock, this work paves the way to using Roman voids to independently constrain cosmological parameters with tight precision," the researchers conclude.
