The cosmic web exists
Astronomers have confirmed it by viewing gas billions of light years away. Richard A Lovett reports.
Faintly glowing wisps of gas surrounding galaxies 12 billion light-years away have given astronomers their first chance to confirm the existence of a structure known as the cosmic web.
The web is a cobweb of gas filaments, which the standard model of cosmology predicts would have formed in the aftermath of the Big Bang. Where these giant filaments cross, the theory goes, is where galaxies form.
Hints that such filaments might exist, says Erika Hamden, an astrophysicist at the University of Arizona, Tucson, had previously shown up in the spectra of distant galaxies, which contained absorption bands indicating that the light from them had passed through large hydrogen clouds en route to Earth.
“So you can tell there’s a bunch of hydrogen between you and the galaxies,” she says.
But whether that meant the light had passed through one or more of the cosmic web’s filaments, or something else, was unclear.
In a paper published in the journal Science, however, a team led by Hideki Umehata, an astronomer at the RIKEN Cluster for Pioneering Research, Japan, used one of the Earth’s largest telescopes to look at a massive protogalaxy known as SSA22, 12 billion light-years away in the direction of the constellation Aquarius.
That’s far enough away that the light from it had been traveling toward us for most of the history of the Universe...meaning that Umehata’s team was peering back in time to only 1.8 billion years after the Big Bang.
Umehata’s team then ran the light through a spectroscope to look for the glow of hydrogen illuminated by high-energy radiation from nearby galaxies.
These galaxies “kind of act like flashlights,” explains Hamden, whose own paper commenting on Umehata’s appears in the same issue.
Detection was made possible, she adds, by a spectrographic instrument called the Multi Unit Spectroscopic Explorer (MUSE): a high-resolution spectrograph that allows astronomers to look for spectroscopic signatures of things like hydrogen, over a wide field of view.
“That is the only reason it could be done,” Hamden says, “because these structures are so big that you need a big field of view [to see them].”
It’s an important result, she adds, because the cosmic web is one of the key predictions of the standard model of cosmology. Finding a piece of it “is an indicator that we’re on the right track”.
It’s also important, she says, because astronomers have long believed that most of the normal matter in the Universe isn’t in galaxies. Proving that the web exists is therefore an important step in figuring out where it does lie.
The next step, according to Umehata, is to look for more filaments.
So far, he says, “We just opened a small window. The cosmic web should be much larger.”
Also, he adds, it would be useful to search for elements other than hydrogen, such as helium or carbon, and to look for older filaments closer to the Earth.
“It would be exciting to see the evolution of filaments, and galaxy formation within filaments, across cosmic time.”