The giant arcs that may dwarf everything in the cosmos

By | March 6, 2023

Jasmin Fox-Skelly writes:

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

She detected a giant, almost symmetrical arc of galaxies 9.3 billion light years away in the constellation of Boötes the Herdsman. Spanning a massive 3.3 billion light years across, the structure is a whopping 1/15th the radius of the observable Universe. If we could see it from Earth, it would be the size of 35 full moons displayed across the sky.

Known as the Giant Arc, the structure throws into question some of the basic 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 view the Universe on very large scales there should be no noticeable irregularities; everything should look the same in every direction.

Yet the Giant Arc isn’t the only example of its kind. These gargantuan structures are now forcing scientists to reassess their theory of how the Universe evolved.

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

“I looked into known and documented galaxy clusters, and then started plotting what these areas looked like in the Magnesium II method,” says Lopez. “One cluster I looked at was very small, but when I plotted it in magnesium II there was this interesting dense band of magnesium absorption across the field of view. This is how I ended up discovering it. It was a happy accident and I was just lucky that it was me that found it.”

What Lopez’ “happy accident” uncovered was astonishing. When looking towards the constellation Boötes, a cluster of between 45 to 50 gas clouds, each associated with at least one galaxy, seemed to arrange themselves in an arc 3.3 billion light years across. That is a considerable size given the observable Universe is 94 billion light years wide. [Continue reading…]

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