Filament of cosmic web set aglow

All of the universe’s matter, cosmologists believe, forms a web of dark matter and gas that was spun shortly after the Big Bang and has been stretching out ever since. Now astronomers say they have glimpsed a brightly lit strand of this web.

But the researchers found far more cold gas than they expected based on computer simulations of the cosmic web, suggesting the web’s structure could differ from theoretical predictions.

Most of the universe’s mass seems to reside in slender interconnected filaments, which are primarily made of dark matter with smaller amounts of gas. Cosmologists’ theories suggest that gas collects at filament intersections and becomes dense enough to form luminous galaxies. But because the filaments’ dark matter is invisible and their sparse gas emits little light, the threads connecting galaxies have remained mostly hidden.

Astronomers have recently begun to spot filaments backlit from a distance by extremely bright galactic nuclei called quasars. But researchers could gain far more information about the web’s structure from a quasar shining onto a nearby filament, which would then emit its own light, says Michael Rauch, an astronomer at the Carnegie Institution for Science in Pasadena, Calif. “To see the cosmic web in emission is the holy grail.”

Seeking it, Sebastiano Cantalupo of the University of California, Santa Cruz and his colleagues tried a clever technique. They put a filter on the Keck I telescope in Mauna Kea, Hawaii, to block all wavelengths except the one at which illuminated hydrogen gas in filaments should glow. Through this filter, they took a 10-hour image of a distant quasar and its surroundings. They found a large, outstretched cloud of quasar-lit hydrogen well outside the radius of the halo of gas and dark matter that surrounds quasars. Because no other structures are known to exist so far from galaxies, Cantalupo and his colleagues conclude January 19 in Nature that the cloud is a cosmic web filament.

Making such a finding is difficult because quasars shine only in specific directions like a flashlight beam, says Cantalupo. He calls the discovery “a lucky cosmic coincidence.” He and his colleagues say that the surprisingly large amount of gas they found — more than 10 times as much as simulations predict — could force cosmologists to revise their theories.

“It’s scientifically highly exciting to find an object like this,” says Rauch. But he is not convinced the structure is a cosmic filament. He notes that Cantalupo’s imaged quasar and a second quasar the team found in the neighborhood could have merged halos in a way that looks like a web filament. Such a scenario could explain the cloud’s high gas content.

But Cantalupo’s team argues that the cloud’s wispy, asymmetrical shape and the large distance between the quasars make such an explanation unlikely. Rather, the researchers suspect that current simulations are too imprecise to estimate filaments’ gas content correctly.

The Cantalupo team’s finding may require revisions to theories, says Volker Springel, a cosmologist at Heidelberg University in Germany. Other astronomers will probably emulate the researchers’ technique; if they find similar results, “then I think it would really convince all the skeptics.”