'Giant arc' stretching 3.3 billion light-years across the cosmos shouldn't exist


One of the largest structures ever found, a newly discovered crescent of galaxies with a length of 3.3 billion light-years challenges some of astronomers' most basic ideas about the nature of the universe.

The Giant Arc is a vast collection of galaxies, galaxy clusters, and a great deal of gas and dust. It covers around a fifteenth of the observable universe and is 9.2 billion light-years away.

According to Alexia Lopez, a cosmology doctorate candidate at the University of Central Lancashire (UCLan) in the UK, its finding was "serendipitous." Lopez was utilising light from about 120,000 quasars, which are far-off luminous centres of galaxies where supermassive black holes consume matter and release energy, to map objects in the night sky.

Between us and the quasars, this light travels through matter and is absorbed by various elements, leaving behind telltale signs that can be very useful to researchers. Lopez used the marks left by magnesium in particular to calculate the distance to the gas and dust in between as well as the object's location in the night sky.

Giant Arc. Magnesium-absorbing regions are indicated by grey areas, which reflect the dispersion of galaxies and galaxy clusters. Background quasars, also referred to as spotlights, are the blue dots. (Photo courtesy of Alexia Lopez/UCLan.)

According to Lopez, the quasars function in this way, "like spotlights in a dark room, lighting this intervening stuff." Over the course of the cosmic maps, a structure started to take shape. Lopez said that there was "kind of a hint of a vast arc." I can still picture myself approaching Roger Clowes and saying, "Oh, look at this."

Her PhD advisor at UCLan, Clowes, recommended additional research to make sure it wasn't an error or a data trick. The researchers conducted two independent statistical analyses and found that there was less than a 0.0003% chance that the Giant Arc was a hoax. At the 238th online meeting of the American Astronomical Society, they presented their research.

The Giant Arc's structure is seen in grey, with blue overlays of surrounding quasars. These two datasets have a shaky connection to one another. (Photo by Alexia Lopez/UCLan.

But the discovery challenges a fundamental belief about the universe and will rank among the cosmos' greatest discoveries. The cosmological principle, which states that matter is more or less evenly dispersed throughout space at the largest scales, has been a long-standing tenet of astronomy.

The Sloan Great Wall and the South Pole Wall, both of which are dwarfed by even larger cosmic structures, are smaller than the Giant Arc. Over the years, large-scale structures have been found, according to Clowes, who spoke with Live Science. They seem to defy the cosmological principle because they are so enormous.

It is possible that matter has not been spread evenly throughout the cosmos if such large things have gathered in specific regions of the universe. The cosmological principle, however, is the foundation of the current mainstream model of the cosmos, Lopez continued.

"Perhaps we need to start looking at a different set of theories or principles if we're finding it to be false."

Lopez is unaware of the specifics of those hypotheses, but she did bring up the notion of altering gravity's behaviour at the greatest scales, which has recently gained popularity among a vocal but tiny group of scientists.

Daniel Pomarède, a cosmologist at Paris-Saclay University in France and the creator of the South Pole Wall, concurred that the cosmological principle ought to set a theoretical cap on the size of cosmic objects.

According to some study, buildings should eventually reach a particular size and be unable to grow any further, Pomarède told Live Science. Instead, we continue to discover these more large formations.

However, he isn't quite ready to abandon the cosmological principle, which has been incorporated for about a century into universe models.

"To say that it will be replaced by something else would be very brave," he remarked.

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