An arc of galaxies that spans 3.3 billion light years 鈥� about 3.5 per cent of the observable universe 鈥� presents a big problem for one of the core tenets of cosmology.
Dubbed the Giant Arc by of the University of Central Lancashire, UK, who discovered the structure, it isn鈥檛 actually visible in the night sky 鈥� if it were, it would be as long as 20 full moons side by side.
Instead, Lopez spotted the arc by studying light from about 40,000 quasars, which are distant galaxies with a brightly shining active core. She looked for signs that the light had been absorbed by passing through ionised magnesium, showing that it had passed through a gas cloud surrounding a galaxy between the quasar and Earth.
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Lopez found that in the direction of the constellation Bo枚tes, between 45 and 50 intervening galaxies or galaxy clusters are arranged in an arc 3.3 billion light years long, at a distance from Earth of about 9.2 billion light years.
鈥淚f you had 15 Giant Arcs, they would reach from here to the edge of the observable universe,鈥� says Lopez, who presented the find on 7 June at the virtual meeting of the American Astronomical Society.
The observable universe is 93 billion light years wide, making this a significant cosmological structure. According to Lopez, it is extremely unlikely (a probability of just 0.0003 per cent) that such a large structure could arise by a chance arrangement.
On the other hand, an important assumption going into the standard model of how the universe started and evolved, the cosmological principle, says that it is very unlikely such an arc should exist, because over distances of more than about 1.2 billion light years, the universe should look roughly the same everywhere.
The arc joins a handful of structures that don鈥檛 fit well with the cosmological principle. The new discovery intensifies the challenge that these present to the existing picture of how the universe evolved.
鈥淚f the cosmological principle doesn鈥檛 hold up, then our standard model of the universe kind of falls through,鈥� says Lopez. 鈥淭here are alternative theories that can maybe help explain large scale structures, but the standard model is founded on the cosmological principle being true; we have to have homogeneity and isotropy in the universe. It could have some severe implications.鈥�
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