On the largest scales, the universe is arranged in a web-like pattern: galaxies are pulled together into clusters, which are connected by filaments and separated by voids. These clusters and filaments contain dark matter, as well as ordinary matter such as gas and galaxies.
We call this the “cosmic web,” and we can see it by mapping the locations and densities of galaxies from large surveys done with optical telescopes.
We believe that the cosmic web is also permeated with magnetic fields, which are created by energetic particles in motion and in turn control the movement of these particles. Our theories predict that when gravity pulls a filament together, it will cause shock waves that strengthen the magnetic field and create a glow that can be seen with a radio telescope.
In new research published in The progress of scienceFor the first time, we have observed these shock waves around pairs of galaxy clusters and the filaments connecting them.
Previously, we have only ever observed these radio shock waves directly from collisions between galaxy clusters. However, we believe they exist around small groups of galaxies, as well as in cosmic filaments.
There are still gaps in our knowledge of these magnetic fields, such as how strong they are, how they have evolved, and what their role is in the formation of this cosmic web.
Discovering and studying this glow can not only confirm our theories of how the large-scale structure of the universe formed, but help answer questions about cosmic magnetic fields and their significance.
Digging in the noise
We expect this radio glow to be both very faint and spread over large areas, meaning that detecting it directly is very challenging.
Also, the galaxies themselves are much brighter and can hide these faint cosmic signals. To make it even more difficult, the noise from our telescopes is usually many times larger than the expected radio glow.
For these reasons, rather than direct When observing these radio shock waves, we had to get creative by using a technique known as stacking. This is when you average images of many objects that are too faint to see individually, which reduces the noise, or rather improves the average signal over the noise.
So what did we stack? We found more than 600,000 pairs of galaxy clusters that are close to each other in space and are likely connected by filaments. We then adjusted our images of them so that any radio signal from the clusters or the area between them – where we expect the shock waves to be – will add up.
We first used this method in a paper published in 2021 using data from two radio telescopes: the Murchison Widefield Array in Western Australia and the Owens Valley Radio Observatory Long Wavelength Array in New Mexico. These were chosen not only because they covered almost the entire sky, but also because they operated at low radio frequencies where this signal is expected to be brighter.
In the first project, we made an exciting discovery: we found a glow between the pairs of clusters! However, because there was a average of many clusters, all of which contain many galaxies, it was difficult to say for sure that the signal came from the cosmic magnetic fields, rather than other sources such as galaxies.
A “shocking” revelation
Normally, the magnetic fields in clusters are jumbled up due to turbulence. However, these shock waves force the magnetic fields into order, which means that the radio glow they emit is strongly polarized.
We decided to try the stacking experiment on maps of polarized radio light. This has the advantage of helping to determine what is causing the signal.
Signals from ordinary galaxies are only 5% polarized or less, while signals from shock waves can be 30% polarized or more.
In our new work, we used radio data from the Global Magneto Ionic Medium Survey as well as the Planck satellite to repeat the experiment. These surveys cover almost the entire sky and have both polarized and regular radio maps.
We detected very clear rings of polarized light surrounding cluster pairs. This means that the centers of the clusters are depolarized, which is expected as they are highly turbulent environments.
However, at the edges of the clusters, the magnetic fields are ordered thanks to the shock waves, which means we see this ring of polarized light.
We also found an excess of strongly polarized light between the clusters, much more than you would expect from just galaxies. We can interpret this as light from the bumps in the connecting filaments. This is the first time that such emissions have been found in this type of environment.
We compared our results with state-of-the-art cosmological simulations, the first of their kind to predict not only the total signal of the radio emission, but also polarized signal too. Our data agreed very well with these simulations, and by combining them we are able to understand the magnetic field signal left over from the early universe.
In the future, we would like to repeat this discovery for different times throughout the history of the universe. We still don’t know the origin of these cosmic magnetic fields, but further observations like this could help us figure out where they came from and how they’ve evolved.
Tessa Vernstrom et al, Polarized accretion shocks from the cosmic web, The progress of science (2023). DOI: 10.1126/sciadv.ade7233
The progress of science