The cosmos is populated with many mysterious, gigantic and impressive objects. Supermassive black holes, billions of times more massive than the Sun, are found at the center of massive galaxies. Giant stars explode in catastrophic collisions, their light reaching us from distances of more than 10 billion light-years. Giant galaxies collide and merge, leading to massive bursts of star formation.
But one of the most puzzling things cosmologists can see are cosmic voids. They are a feature of the Large-Scale Structure of the Universe (LSSU).
Confronting the LSSU means coming to terms with the enormity of the cosmos and the tinyness of the Earth. We are just a small planet in a galaxy that probably contains over 100 billion planets, perhaps several hundred billion. That’s a huge number of planets and one of the reasons people believe 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 up to 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 is impossible to count the galaxies in Pisces-Cetus, but it contains hundreds of clusters and groups of galaxies.
All of these galaxies, groups, clusters and superclusters exist in filaments along the dark matter concentrations in the cosmos.
*The large-scale structure of the Universe consists of huge galaxy filaments, consisting of galaxies, groups, clusters, superclusters and supercluster complexes. These filaments extend over billions of light years. The empty space between them is called the cosmic void. Image source: 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 cavities, massive bubbles in the cosmos lined with galaxies. They can be hundreds of millions of light years across. They’re not exactly empty; They actually contain one galaxy or another. But overall they have less than 10% of the concentrations of matter found in the rest of the cosmos.
These massive cavities are important for the study of dark energy (DE), the mysterious force that drives the expansion of the universe. Voids are expanding faster than regions with higher density of matter in the universe. Cosmologists can measure how the size of voids changes over time and how galaxies move at their edges, and then test these observations against their theoretical DE models.
*This image shows the voids and superclusters within about 500 million light-years of the Milky Way. Image source: By The base image is from Azcolvin429, cropped by Zeryphex, enhanced 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*
Because there is much less matter there and therefore less gravity, cavities are cleaner regions where it is easier to isolate the effects of DE from the effects of gravity. Overall, the void growth statistics can explain how the strength of DE changes over time.
The study of DE is one of the goals of the Nancy Grace Roman Space Telescope. Baryon acoustic oscillations, weak gravitational lensing and observations of type 1a supernovae are used to study DE. It has an extremely wide field of view and observes in the infrared range. By using these three independent methods simultaneously, the cosmic structure, including cosmic voids, is mapped in detail.
New research in the Astrophysical Journal predicts how well the Nancy Grace Roman Space Telescope will detect voids and how these observations will constrain statistics on cosmic voids. It is titled “Cosmology with Cavities 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, represent strict constraints on cosmological parameters,” the authors write in their paper. For the first time, the telescope will provide “a sample of the cosmic void of exceptional quality down to a few megaparsecs.” According to the authors, Roman’s observations open a new window into the science of cosmic emptiness.
“Their sensitivity to dark energy properties is expected since cavities are the first regions dominated by dark energy,” Verza and his colleagues write. This means that the Roman measurements of cavity limitations will impose important constraints on DE.
The Roman will take three different photographs of the sky. One of these is the High-Latitude Wide-Area Survey, which uses weak gravitational lensing to study cosmic expansion.
*This infographic describes the High-Latitude Wide-Area Survey conducted by NASA’s Nancy Grace Roman Space Telescope. Photo credit: NASA Goddard Space Flight Center*
The researchers used simulations to predict how the novel’s High-Latitude Wide-Area Survey will work. The high latitude in its name means that it faces away from the galactic plane of the Milky Way. Verza and his colleagues say that during this survey, the Roman will discover tens of thousands of cosmic cavities, some of which will be only about 20 million light-years across. “We discover 82,551 cavities in the galaxy’s 2000 square degree light cone,” they explain in their research.
The survey will use the telescope’s Wide Field instrument to collect spectra of galaxies at the edges of the cavities. This will determine the cosmic redshifts and, combined with their positions in the sky, reveal their 3D shapes of the voids.
“Voids are characterized by containing so few galaxies. So to detect voids, you need 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 will ultimately give us a better understanding of the cosmological parameters such as dark energy that shape voids,” said co-author Giulia Degni from the University of Roma III and INFN (the National Institute for Nuclear Physics) in Rome.
The Nancy Grace Roman Space Telescope will also collaborate with ESA’s Euclid mission. Euclid’s wide survey will cover approximately 14,000 square degrees of the sky and capture an extremely wide view. The Roman star will occupy 2,000 square degrees in the sky, but will be much deeper than Euclid’s. Euclid observed in optical and infrared light, while the Roman observed only in infrared light. In this sense, the pair will complement each other well, as they are sensitive to different populations of galaxies at different distances. The Vera Rubin Observatory will also be part of the collaboration as its study areas overlap with those of the other two telescopes.
Although 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 narrow our understanding of its effect on the cosmos. If the Romans can find and measure more than 82,000 cosmic voids, as this simulation suggests, then we will learn much more about how DE fueled the accelerated expansion of the universe.
“With a first, comprehensive analysis of a Roman model, this work paves the way for the use of Roman cavities to constrain cosmological parameters independently and with the highest precision,” the researchers conclude.
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