Galaxies are unevenly distributed throughout the Universe. There are places where they are practically absent, and these are called voids. Scientists still have only a superficial understanding of them, but a new space telescope named after Nancy Grace Roman will allow them to be properly examined.
Cosmic voids
Our Universe is filled with galaxies stretching in all directions as far as our instruments can see. Some researchers believe that there are about 2 trillion galaxies in the observable Universe. At first glance, it may seem that these galaxies are arranged randomly in space, but this is not the case. Careful mapping has shown that they are distributed across the surface of giant cosmic “bubbles” with diameters of up to several hundred million light-years. There are few galaxies inside these bubbles, which is why these areas are called cosmic voids. NASA’s Nancy Grace Roman Space Telescope will allow us to measure these voids with new precision, giving us information about the history of the expansion of the Universe.
“Roman’s ability to observe large areas of the sky at great depth, detecting many faint and distant galaxies, revolutionizes the study of cosmic voids,” said Giovanni Verza of the Flatiron Institute and New York University, lead author of the article published in The Astrophysical Journal.
Dark energy and the study of cosmic voids
Space consists of three main components: ordinary matter, dark matter, and dark energy. The gravity of ordinary and dark matter attempts to slow down the expansion of the Universe, while dark energy counteracts gravity, accelerating the expansion of the Universe. The nature of both dark matter and dark energy is currently unknown. Scientists are trying to understand them by studying their effects on what we can observe, such as the distribution of galaxies in space.
“Since they are relatively empty, voids are areas of space where dark energy dominates. By studying voids, we can place powerful constraints on the nature of dark energy,” said study co-author Alice Pisani from CNRS (French National Center for Scientific Research) in France and Princeton University in New Jersey.
To determine how Roman could explore voids, scientists considered one possible variant of the Roman High-Latitude Wide-Area Survey, one of three major collaborative studies conducted by Roman. The High-Latitude Wide-Area Survey will point in the opposite direction from the plane of our galaxy (hence the term “high latitude” in galactic coordinates). The team found that this study should be able to identify and measure tens of thousands of cosmic voids, some of which are only 20 million light-years across. Such a large number of voids will allow scientists to use statistical methods to determine how key components of the Universe affect their observed shapes.
Recipe for “space cake”
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 redshifts, 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 evolved over time.
Pisani compared it to trying to derive a cake recipe (i.e., the composition of the Universe) from the final dessert you were served. “You try to add 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, it means you added the wrong ingredients.”
In this case, the appearance of the “cake” is a shape obtained by statistically superimposing all the voids discovered by Roman. On average, the voids should be spherical in shape, since there is no “preferred” place or direction in the Universe (i.e., the Universe is homogeneous and isotropic on large scales). This means that if the superposition is done correctly, the resulting shape will be perfectly round (or spherically symmetric). If not, then you will have to adjust your cosmic recipe.
Coverage of the sky by Roman telescope
The researchers emphasized that in order to study cosmic voids in large quantities, the observatory needs to be capable of exploring a large volume of the Universe, since the voids themselves can be tens or hundreds of millions of light-years in size.
The spectroscopic data needed to study voids will be obtained from the Roman High-Altitude Wide-Field Survey, which will cover approximately 2,400 square degrees of the sky, or 12,000 full moons. It will also be able to see fainter and more distant objects, providing a higher density of galaxies than complementary missions such as the European Space Agency’s (ESA) Euclid.
According to phys.org
