Stare into the void

Stare into the void

Title: Measurements of cosmic expansion and growth rate of void structure in Sloan Digital Sky Survey between redshift 0.07 and 1.0

Author: Alex Woodfinden, Seshadri Nadathur, Will J. Percival, Slađana Radinović, Elena Massara, Hans A. Winther

First author’s institution: Waterloo Center for Astrophysics, University of Waterloo

Status: Sent to ArXiv [12 May 2022]

Pitch: BAOs are cool

Cosmologists love Baryon acoustic oscillations (BAO), and here’s why you should too. When the universe was just a baby made of hot plasma, there was a battle between gravity and radiation. Small over-densities in the hot plasma struggled to collapse under gravity, while tightly coupled radiation provided pressure that resisted this collapse. The result of this battle was oscillations of matter that spread across the plasma, almost like ripples in a pond. As the universe began to cool, radiation no longer provided support for gravitational collapse, and excess densities of matter found in these ripples could eventually collapse into galaxy clusters. These ripple-like patterns of galaxy clusters are known as BAOs, and they are extremely important to cosmologists because their size and properties depend on the details of the composition of our universe. Accurate measurements of these BAOs can provide limitations on important cosmological parameters such as the amount of baryonic matter, dark matter, and dark energy in the universe. In addition, the size of BAOs monitors a wide range of redshifts can be used to measure how the rate of expansion of the universe changes over cosmic time. Given all we can learn from studying BAO, it is no wonder why cosmologists have used them as cosmological probes for decades. Have I sold you on BAOs yet?

Pitch 2: So is Cosmic Voids

If so, I have another cosmological probe to sell you on: cosmic voids. If BAO represents overcrowded regions in the universe, it is no surprise that there are also regions of extremely low density. These cosmic voids are around a tenth of the average density of the universe, and they are massive, make up about 90% of our universe. And, as it turns out, these voids are as exciting to test parameters of cosmology as BAOs are! Vacuum properties are sensitive to everything from dark energy and modified gravityto structural growth and galaxy formation. Who knew you could get anything out of studying nothing!

Measurements of the growth rate of the structure from cosmic voids
Figure 1: Measurements of the structure’s growth rate (ƒσ₈) compared to the values ​​measured by the cosmic microwave background (blue strip). The red markings represent the measurements from the vacuum-galaxy correlation, the gray markings represent measurements from BAOs and the green markings represent the vacuum galaxy correlation measurements with different analysis techniques. Picture 6 in the newspaper

But most important for today’s discussion is the information coded within void-galaxy cross-correlation function. Although it sounds like a mouthful, this feature really only describes the properties, such as density and strange velocities, of galaxies that surround voids. This statistical property is particularly important because it can characterize both red-shift-space distortions (RSD) and Alcock-Paczyński (AP) effect. Astronomers use the speed of an object to determine its redshift, and RSDs are simply prejudices in our redshift measurement due to another strange speed component. Because special velocities depend on local gravitational interactions, precise characterization of RSDs can tell a lot about the properties of matter in our universe, such as the growth rate of the structure. The AP effect, on the other hand, is a distortion of the shape of a distribution of galaxy clusters. If we assume that galaxy clusters are distributed as a sphere around voids, the AP effect can make the distribution of clusters appear flattened or elongated if astronomers make incorrect assumptions about the geometry of the universe. Analysis of these distortions gives astronomers measurements of what is called the Alcock-Paczyński distance ratio, which is the relationship between upcoming angular diameter distance and that Hubble distance. This is particularly exciting for the study of cosmic voids, as they should be able to limit this distance relationship even more precisely than BAO can. All in all, cosmic voids are a new study of the properties of our universe. Not sold yet? Well, let’s let the results of today’s newspaper speak then!

The proof is in the cosmic void

The authors of today’s essay examined the cross-correlation between galaxies and cosmic voids in order to unravel the richness of cosmological information packed within them. After careful and rigorous treatment of both selection and systematic errors that can occur in the analysis of cosmic voids, the authors found exciting results. First, the authors found that only cosmic voids can limit the value of the structure’s growth rate as precisely as BAO can (Figure 1). Although this is not a new result, this precision points to the fact that voids are an excellent tool to use in combination with BAOs to get better measurements of cosmological parameters than any of them can do alone. But perhaps even more convincing is the fact that the author’s results also confirm that cosmic voids beat BAOs when it comes to measuring the AP distance ratio (Figure 2). Such results demonstrate the importance of voids as cosmological probes and support a powerful new path for cosmologists to explore to unlock the mysteries of our universe. And, like the cherry on top of it all, there are new galaxy studies such as DESI and Euclid coming in the near future, which will examine much larger volumes of the universe over a larger redshift interval. Watch out for the world, precision cosmology is about to become even more accurate!

Measurements of the AP distance ratio from the paper
Figure 2: Measurements of the AP distance ratio Dₘ / Dhrs. Red dots represent measurements from void-galaxy correlations, and gray dots represent those from BAO. The green and blue bands are expected values ​​from CMB (blue) and CMB in combination with other cosmological probes (green). Picture 7 in the newspaper

Astrobite edited by Roel Lefever

Selected image credit: FORS Team, 8.2-meter VLT Antu, ESO

About Kayla Kornoelje

I’m a freshman at the University of Chicago, studying cosmology and the cosmic microwave background. Outside of research, I love writing sci-fi, drawing, petting my cat and hanging out on Twitter @kayla_kornoelje!

#Stare #void

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