The giant arcs that can dwarf everything in the cosmos

In 2021, British PhD student Alexia Lopez was analyzing the light coming from distant quasars when she made a startling discovery.

She discovered a giant, nearly symmetrical arc of galaxies 9.3 billion light-years away in the constellation Boötes the Herdsman. The structure spans a whopping 3.3 billion light-years across, and is 1/15th the radius of the observable universe. If we could see it from Earth, it would be the size of 35 full moons seen across the sky.

Known as the Giant Arc, the structure calls into question some of the fundamental assumptions about the universe. According to the Standard Model of cosmology – the theory on which our understanding of the universe is based – matter should be more or less evenly distributed across space. When scientists look at the universe on very large scales, there should be no noticeable irregularities; everything should look the same in all directions.

Nevertheless, the Giant Arc is not the only example of its kind. These giant structures are now forcing scientists to rethink the theory of how the universe evolved.

Lopez was studying for her master’s degree at the University of Central Lancashire in the UK when her supervisor suggested using a new method to analyze large-scale structures in the universe. She used quasars—distant galaxies that emit an extraordinary amount of light—to look for signs of ionized magnesium, a sure sign of gas clouds surrounding a galaxy. When light passes through this ionized magnesium, certain frequencies are absorbed, leaving unique light signatures that astronomers can detect.

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“I looked at known and documented galaxy clusters, and then started plotting what those regions looked like in the Magnesium II method,” says Lopez. “One cluster I was looking at was very small, but when I plotted it in magnesium II there was this interesting tight band of magnesium absorption across the field of view. That’s how I ended up discovering it. It was a happy accident and I was just lucky that I was the one who found it.”

What Lopez’s “happy accident” uncovered was astounding. Looking toward the constellation Boötes, a cluster of between 45 and 50 gas clouds, each associated with at least one galaxy, appeared to arrange itself in an arc 3.3 billion light-years across. That’s a significant size given that the observable universe is 94 billion light-years wide.

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