Rick Robinson

Nov 22nd 2021

An Arc of Galaxies Three Billion Light-Years Long May Challenge Cosmology


In the direction of the springtime constellation Boötes lies what may be the largest structural feature in the universe: an arc of galaxies extending some 3.3 billion light years. The possible structure, now simply called the Great Arc, is more than nine billion light years distant — millions of times farther than the visible stars of Boötes, such as the bright red giant Arcturus.

Astronomers are not yet sure whether the Great Arc is a true physical structure or a mere visual coincidence. But if it is real, the Great Arc is not simply colossal; it also poses a challenge to the Cosmological Principle, a basic conception of the universe that dates back to Albert Einstein.

“Oh, Look at This”

The (possible!) discovery of this vast and problematic assemblage of galaxies grew out of an ongoing project to map the distant regions of the universe.

According to Science News, the mapping project uses light from thousands of very remote quasars — bright, compact galactic cores so far away that they appear in the telescope as starlike points of light. As the light from these quasars passes through space on its way to Earth, some of that light is absorbed by atoms in intermediate galaxies. Thus, we can determine their presence and composition even without directly imaging them.

As British PhD candidate Alexia Lopez examined the data from the survey, she began to find hints of a pattern. As Livescience recounts, Lopez described it as “sort of a hint of a big arc.” She showed the findings to her doctoral advisor. “Oh, look at this,” she exclaimed.

Her advisor, Roger Clowes, suggested running some statistical tests to determine whether the Great Arc is a true physical structure or a mere visual coincidence. Such visual effects are common, such as the constellation Boötes itself, made up of unrelated stars that merely happen to form a distinctive pattern as seen from Earth.

The statistical tests came back showing only a 0.0003 chance of the Great Arc being a mere coincidence.

Too Big to Fit?

If the Great Arc is real, it poses a major challenge to Einstein’s Cosmological Principle, which has been a key concept in cosmology for a hundred years. In a nutshell, the Cosmological Principle holds that the universe, when examined on the largest scale, looks more or less the same no matter what direction you look in. Per Sci-News, the principle holds that the largest true structures in the universe should be no more than about 1.2 billion light years across — little more than a third the size of the Great Arc.

Albert Einstein first introduced the Cosmological Principle in order to simplify mathematical calculations, but it has done so well at explaining the universe we observe that astronomers would be very reluctant to abandon it.

In the interview with Livescience, cosmologist Daniel Pomarède, co-discoverer of another anomalously large cosmic structure — the South Pole Wall, spoke of the key role the Cosmological Principle plays in our understanding of the universe. “It would be very bold to say that it will be replaced by something else,” he said. According to Sci-News, astrophysicist Subir Sarkar noted that “our eye has a tendency to pick up patterns,” wryly adding that some people have claimed to see Stephen Hawking’s initials in the cosmic background radiation, the oldest light in the universe.

Lopez agreed that the Great Arc poses a challenge. But evidence continues to grow for the existence of oversized cosmic structures such as the South Pole Wall and the Great Arc. In a Futurism article, Lopez also noted that if confirmed, her finding “would overturn cosmology as we know it.” She added that “our standard model, not to put it too heavily, kind of falls through.”

And there — pending further research — the matter stands. A powerful theory, and a growing hint of contradicting facts: business as usual on the frontiers of cosmology.

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