Recent studies have successfully observed magnetic shockwaves in the cosmic web by examining radio emissions between galaxy clusters.
This achievement, confirmed by comparing polarized light patterns with advanced simulations, opens new avenues for understanding cosmic magnetic fields and their role in the Universe’s structure.
Understanding the Cosmic Web
On the grandest scales, the Universe forms a vast, web-like structure. Galaxies group into clusters, connected by thin, thread-like filaments, with large empty voids in between. Both the clusters and filaments are composed of dark matter as well as regular matter, such as gas and galaxies.
This intricate structure is known as the “cosmic web.” Astronomers can observe it by mapping the positions and densities of galaxies through extensive surveys conducted with optical telescopes.
Scientists believe the cosmic web is also threaded with magnetic fields. These fields are generated by energetic particles in motion, which the fields themselves help direct. Theories suggest that as gravity pulls a filament tighter, shockwaves form, strengthening the magnetic fields and producing a faint glow detectable by radio telescopes.
Breakthrough in Cosmic Observations
In 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 also help answer questions about cosmic magnetic fields and their significance.
Challenges of Detection
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.
Advanced Techniques in Radio Astronomy
So what did we stack? We found more than 600,000 pairs of galaxy clusters that are near each other in space, and so are likely to be connected by filaments. We then aligned our images of them so that any radio signal from the clusters or the region between them – where we expect the shockwaves to be – would add together.
We first used this method in a paper published in 2021 with 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 nearly all the 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 it was an average of many clusters, all containing many galaxies, it was difficult to say for sure the signal was coming from the cosmic magnetic fields, rather than other sources like 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 the radio glow they emit is highly 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 regular galaxies are only 5% polarized or less, while signals from shockwaves can be 30% polarized or more.
In our recent 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.
Findings and Future Directions
We detected very clear rings of polarized light surrounding cluster pairs. This means the centers of the clusters are depolarized, which is expected as they are very turbulent environments.
However, on the edges of the clusters, the magnetic fields are put in order thanks to the shockwaves, meaning we see this ring of polarized light.
We also found an excess of highly polarized light between the clusters, much more than you would expect from just galaxies. We can interpret this as light from the shocks in the connecting filaments. This is the first time such emissions have been found in this kind of environment.
We compared our results with state-of-the-art cosmological simulations, the first of their kind to predict not just the total signal of the radio emission but the polarized signal as well. 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 detection for different times over the history of the Universe. We still do not know the origin of these cosmic magnetic fields, but further observations like this can help us to figure out where they came from and how they have evolved.
Written by:
- Tessa Vernstrom – Senior research fellow, The University of Western Australia
- Christopher Riseley – Research Fellow, Università di Bologna
Adapted from an article originally published in The Conversation.
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2 Comments
Dark Matter is a mathematical conjecture. When an article like this assumes that exists, it also explains why “filaments” (undefined) are discussed without mentioning anything about plasma, Birkeland Currents , or anything electric. This shows the problems of cosmological “explanations” which are self-censored from mentioning alternative theories to explain observations.
Good