On the largest scales, the universe is ordered into 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 regular matter like 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 made with optical telescopes.
We think the cosmic web is also permeated by magnetic fields, which are created by energetic particles in motion and in turn guide the movement of those particles. Our theories predict that, as gravity draws a filament together, it will cause shockwaves that make the magnetic field stronger and create a glow that can be seen with a radio telescope.
In new research published in Science Advances, we have for the first time observed these shockwaves around pairs of galaxy clusters and the filaments that connect them.
In the past, we have only ever observed these radio shockwaves 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 have they evolved, and what their role is in the formation of this cosmic web.
Detecting and studying this glow could not only confirm our theories for how the large-scale structure of the universe has formed, but help answer questions about cosmic magnetic fields and their significance.
Digging into the noise
We expect this radio glow to be both very faint and spread over large areas, which means it is very challenging to detect it directly.
What’s more, 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 directly observing these radio shockwaves, we had to get creative, using a technique known as stacking. This is when you average together images of many objects too faint to see individually, which decreases the noise, or rather enhances the average signal above the noise.
