On the largest scales possible, the universe resembles a web of light spun by an inconceivably large spider. Now, astronomers have detected very faint light from these cosmic web filaments in the deep, dark, distant universe.
Most of us probably picture the universe as a teeming mass of galaxies and stars scattered more or less randomly across space, but zoom out far enough and it actually takes on the structure of a gigantic web. Galaxies tend to cluster together at the junctions of this web, connected by filaments of gas stretching millions of light-years long. The gaps between these filaments are mind-bogglingly empty voids in space.
In 2015, astronomers from Caltech used the Keck Cosmic Web Imager (KCWI) instrument to capture images of the cosmic web in the distant universe. However, the only way the team was able to see the very faint filaments was thanks to the light of a nearby quasar. In a new study, the team has managed to directly image light from the filaments themselves.
“Before this latest finding, we saw the filamentary structures under the equivalent of a lamppost,” said Christopher Martin, lead author of the study. “Now we can see them without a lamp.”
In the new study, the team managed to image fainter structures using new methods to filter out background light, such as that from our own atmosphere, solar system and galaxy.
“We look at two different patches of sky, A and B,” explained Martin. “The filament structures will be at distinct distances in the two directions in the patches, so you can take the background light from image B and subtract it from A, and vice versa, leaving just the structures. I ran detailed simulations of this in 2019 to convince myself that this method would work.”
With the help of this technique, KCWI was able to capture light from the strongest emission line – hydrogen gas – in the light spectrum from these filaments in a region of the cosmic web located 10 to 12 billion light-years away. This particular section of filament was lurking in a darker corner of the universe, farther from galaxy clusters, quasars or other light sources, where it would normally remain hidden.
Since the expansion of the universe shifts light towards the red end of the spectrum, light signatures from more distant objects appear more red than those closer to us. With this in mind, the team was able to take two-dimensional images from KCWI, separate them based on the wavelengths of light visible and stack them to produce a 3D image of a distant chunk of the cosmic web. Other studies have recently imaged the faint glow of cosmic web filaments using Hubble's Ultra Deep field survey.
Astronomers’ ability to resolve these filaments will only improve, too. KCWI has recently been upgraded with a new component that sees the longer-wavelength parts of the spectrum – the redder bits – which will essentially allow it to look farther back in space and time.
“With KCRM, the newly deployed red channel of KCWI, we can see even farther into the past,” said Mateusz Matuszewski, senior instrument scientist on the project. “We are very excited about what this new tool will help us learn about the more distant filaments and the era when the first stars and black holes formed.”
The research was published in the journal Nature Astronomy. A 3D image of a chunk of cosmic web filament can be seen in the video below.
Source: Caltech