Cosmic voids are regions in the universe inhabited by just a handful of galaxies. Studying them helps us understand how galaxies form and evolve, even providing a clearer picture of how the larger structures of the universe come to be.
Studying something that is mostly empty is not easy and it’s very difficult to gain new insights into cosmic voids. However, an international group of researchers managed to get a better understanding of them by cleverly using observations of voids in the Cosmic Microwave Background (CMB) conducted by the European Space Agency's Planck mission. They stacked 774 of these voids together and analyzed them as one to study any subtleties that might be present. This allowed them to work out that the gas within cosmic voids might be hotter than previously thought. The study is published in Physical Review D.
While the researchers are being cautious about the study, due to the limitations of both the model and observations, they still provided potential explanations for what might be heating up the gas. The most interesting possibility is that this intergalactic reservoir of gas is being heated by powerful supermassive black hole jets.
The team was able to estimate how hot this gas was thanks to the Sunyaev-Zel'dovich effect. This happens when the low-energy photons of the CMB hit high-energy electrons in the hot gas, promptly stealing some energy and thus slightly changing what the CMB looks like. Such an approach has been used to work out where some of the missing matter in the universe might have been hiding.
It is important to keep clear in our minds how elements of the universe are distributed. Sure, if you look at it on large scales the universe is the same everywhere, but galaxies and clusters of galaxies are actually organized into filaments that we called the cosmic web. As time goes by, under the action of gravity, the dense regions become denser and the voids become emptier.
The team states that their analysis will benefit from the advent of new telescopes and surveys that will provide new ways to trace the density of these voids and increase the robustness of the cosmic void catalog. The team also shared their full analysis online so that others can reproduce their results.