For the first time, astronomers have seen shock waves rippling along the strands of the cosmic web, the vast tangle of galaxies, gas and dark matter that fills the visible universe.
Combining hundreds of thousands of radio telescope images revealed a faint glow emitted by a shock wave that sends charged particles flying through magnetic fields that run along the cosmic web. Detecting these shock waves could give astronomers a better look at these large-scale magnetic fields, whose properties and origins are largely mysterious, the researchers report in Science Advances on 17 February .
Finally, astronomers “can confirm what until now was only predicted by simulations — that these shock waves exist,” says astrophysicist Markus Bruggen of the University of Hamburg in Germany, who was not involved in the new study.
On the grandest scale, our universe is like Swiss cheese. Galaxies are not evenly distributed in space, but rather are gathered together in huge clusters, connected by flexible filaments of rarefied gas, galaxies, and dark matter, and separated by not-quite-empty voids.
Under the influence of gravity, galaxy clusters merge, the filaments collide, and gas from the voids falls onto the filaments and clusters. In the space web simulation, all this action successively sets off huge shock waves within and along the filaments.
Filaments make up most of the cosmic web, but they are much harder to see than galaxies. Although scientists have previously observed shock waves around galaxy clusters, shocks in filaments have “never really been seen,” says astronomer Reinout van Weren of Leiden University in the Netherlands, who was not involved in the study. “But they should be basically all over the space network.”
Shock waves around the filaments accelerate charged particles through the magnetic fields that fill the cosmic web. When this happens, the particles emit light with wavelengths that radio telescopes can detect, although the signals are very weak.
A single shock wave in a filament “would look like nothing, it would look like noise,” says radio astronomer Tessa Wernstrom of the International Center for Radio Astronomy Research in Crawley, Australia.
Instead of looking for individual shock waves, Wernstrom and her colleagues combined radio images of more than 600,000 pairs of galaxy clusters close enough to be strung together to create a single “composite” image. This amplified the weak signals and showed that there is, on average, a weak radio glow from the filaments between the clusters.
“When you can dig below the noise and still actually get results, that’s exciting to me personally,” says Wernstrom.
A weak signal is highly polarized, meaning that the radio waves mostly align with each other. Highly polarized light is unusual in space, but expected from radio light emitted by shock waves, van Wieren says. “So I think this is really, really good evidence that shocks are probably really there.”
In this computer simulation, gas falling onto the cosmic web (blue) is heated and expanded, creating shock waves that travel through the hot, expanded gas (red) and throughout the vast network of galaxy clusters and filaments that fill our universe. These shock waves interact with magnetic fields (green) in the cosmic web, creating radio signals that astronomers can observe.
This discovery goes beyond confirming the predictions of space network simulations. Polarized radio emission also offers a rare look at the magnetic fields that permeate the cosmic web, if only indirectly.
“These shocks,” Bruggen says, “can indeed show that there are large-scale magnetic fields that create [щось] like a shell around these threads.”
He, van Wieren and Wernstrom note that the question of how cosmic magnetic fields arose in the first place is still open. The role that these fields play in the formation of the cosmic web is equally mysterious.
“It’s one of the four fundamental forces of nature, isn’t it? Magnetism,” says Wernstrom. “But at least on such a large scale, we don’t really know how important it is.”
