The biggest discovery yet?

A graduate student is challenging one of astronomy’s most foundational principles. Alexia Lopez, a doctoral candidate at the United Kingdom’s University of Central Lancaster, was working through maps generated by the Sloan Digital Sky Survey when she noticed something unusual. “It was sort of a hint of a big arc,” she told LiveScience.

There on the maps, Lopez saw what could be the largest object in the universe. Lopez told the American Astronomical Society that she and her doctoral adviser, noted astrophysicist Roger Clowes, are more than 99 percent certain the Giant Arc is real and measures  3.3 billion light-years in length. If corroborated, the discovery could have massive ramifications for astrophysics’ prediction that matter will be relatively evenly distributed across the universe.

Just like solar systems clump together to form galaxies, galaxies and galaxy structures can clump tightly together into neighborhoods that scientists call large-scale structures. Examples include the Sloan Great Wall that scientists discovered in 2003 and the South Pole Wall, discovered last year. Their discoverers say the large-scale structures are collections of galaxies and dust that span more than 1 billion light-years.

The Giant Arc dwarfs even those objects. According to Lopez and Clowes’ calculations, the crescent of galaxies spans a distance of 3.3 billion light-years and sits 9.2 billion light years away from Earth. Fifteen Giant Arcs stacked end-on-end would reach from Earth to the edge of the observable universe, Lopez said. It is too dim and distant to see with the naked eye, but it would appear behind the Boötes, or Herdsman, constellation and cover 10 degrees of the night sky—as wide as 20 full moons, she said.

The sheer size of the Giant Arc challenges one of astrophysics’ most-foundational assumptions. Astronomers who advocate a Big Bang hypothesis theorize that matter must, generally speaking, be evenly distributed around the universe. In other words, according to what’s known as the cosmological principle, the Big Bang rendered the universe homogenous, at least at a large enough scale.

Astrophysicists predict that, according to the principle, any cube of the universe spanning 1.2 billion light-years ought to have the same density as any other similar cube. Large-scale structures like the Sloan Great Wall and the South Pole Wall challenged that theory, since their discoverers say they hit that 1.2-billion-light-year threshold.

But the Giant Arc blows right through it. The cosmological principle wouldn’t predict a structure nearly three times larger than the expected 1.2 billion-light-year threshold. If the Giant Arc is real, matter in the universe may not be as evenly distributed as previously thought. “There have been a number of large-scale structures discovered over the years,” Clowes told Live Science. “They’re so large, you wonder if they’re compatible with the cosmological principle.”

Scientists around the world will begin testing Lopez and Clowes’ work to see if the Giant Arc actually exists, or just looks that way from Earth’s vantage point. Until then, one of astrophysics’ core principles hangs in the balance. “It would overturn cosmology as we know it,” Lopez said at a news conference for the virtual American Astronomical Society meeting. “Our standard model, not to put it too heavily, kind of falls through.”