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An arc of 3 billion light-years galaxies can challenge cosmology

A giant arc of galaxies appears to extend more than 3 billion light-years into the distant universe. If the arc turned out to be real, it would challenge a basic cosmology assumption: that on large scales, matter in the universe is evenly distributed no matter where you look.

"Cosmology as we know it would return," cosmologist Alexia Lopez said at a June 7 news conference at the American Astronomical Society's virtual meeting. "Our standard model, if not too much, falls."

Lopez, of the University of Lancashire Central in Preston, England, and his colleagues discovered the intended structure, which they simply call the Giant Arc, studying the light of about 40,000 quasars captured by the Sloan Digital Sky Survey. Quasars are the luminous nuclei of giant galaxies so distant that they appear as points of light. As it headed for Earth, some of that light is absorbed by atoms in and around galaxies in the foreground, leaving specific signatures in the light that eventually reach astronomers ’telescopes (SN: 7/12/18).

The signature of the Giant Arc is on magnesium atoms that have lost an electron, in the halos of galaxies about 9.2 billion light-years away. The quasar light absorbed by these atoms traces an almost symmetrical curve of dozens of galaxies spanning about one-fifteenth of the radius of the observable universe, Lopez reported. The structure itself is invisible in the sky to human eyes, but if you could see it, the arc would cover about 20 times the width of the full moon.

Astronomers have discovered what they say is a giant arc of galaxies (smile-shaped curve in the middle of this image) using light from distant quasars (blue dots) to trace where in the sky that light was absorbed by magnesium atoms in the halos ( dark spots) surrounding galaxies in the foreground.A. López

“This is very fundamental proof of the hypothesis that the universe is homogeneous on a large scale,” says astrophysicist Subir Sarkar of Oxford University, who studies large-scale structures in the universe but did not participate in the new work. If the Giant Bow is real, "this is a very big thing."

But researchers aren’t convinced it’s real yet. “Our eye has a tendency to pick up patterns,” Sarkar says, noting that some people have claimed to see cosmologist Stephen Hawking’s initials written in fluctuations in the background of cosmic microwaves, the oldest light in the universe.

Lopez conducted several statistical tests to find out the probabilities that galaxies would line up in a giant arc by chance. He found that there is about a 0.3 percent chance that the structure will be a statistical blow.

It sounds pretty good, but it doesn’t reach the physicists ’gold threshold of 0.00003 percent, Sarkar says. “Right now, I would say they don’t have convincing evidence yet,” he says. Further observations, from Lopez's group and others, could confirm or refute the Giant Arch.

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If real, the Giant Arc would join a growing group of large-scale structures in the universe that, taken together, would break the standard model of cosmology. This model assumes that when sufficiently large volumes of space are observed (above approximately 1 billion light-years), matter is evenly distributed. The Giant Arch appears about three times more than that theoretical threshold. It joins other structures with similar superlative names, such as the Great Wall of Sloan, the giant ray explosion ring, and the huge Quasar group.

“We can have a large-scale structure that could only be a statistical blow,” Lopez said. "That's not the problem. All of them combined is what makes the problem even bigger."

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