Giant Arc Structure 1/15th The Size Of The Observable Universe Challenges What We Know About The Cosmos

A giant structure around 9.2 billion light-years from Earth and measuring around 1.3 billion light-years across is raising huge questions about our understanding of the universe. If the structure persists with continued observations and analysis, it could shake up the cosmological principle, and the idea that the universe is the same in all directions.

The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

The more we learn about the world and the cosmos, the more we have had to let go of ideas of our own importance. We discovered that the world is not the center of the universe around which all objects rotate. Then we found that the Earth rotated around the Sun, and believed that that was the center of the universe instead.

Now we know that is not the case either. Abandoning ideas of our own importance, we now use the Copernican principle and its updated astronomical twin, the cosmological principle: that we on Earth are not at the center of the universe, nor occupy a privileged region within it. Though regions of space may differ – for instance, the Great Nothing – viewed at a large enough scale, the universe is isotropic and homogenous, or the same wherever you are within it.

A violation of this principle would be huge. Not as big as if the laws of thermodynamics were broken, but still a big deal.

“The Copernican principle is a cornerstone of most of astronomy, it is assumed without question, and plays an important role in many statistical tests for the viability of cosmological models,” Albert Stebbins of Fermilab explained to Phys.org in 2008. “It is also a necessary consequence of the stronger assumption of the Cosmological Principle: namely, that not only do we not live in a special part of the universe, but there are no special parts of the universe – everything is the same everywhere (up to statistical variation)."

“It is a very handy principle, since it implies that here and now is the same as there and now, and here and then is the same as there and then. We do not have to look back in time at our current location to see how the universe was in our past – we can just look very far away, and given the large light travel time, we are looking at a distant part of the universe in the distant past. Given the Cosmological Principle, their past is the same as our past.”

Scientists expect that in an isotropic and homogenous universe, we should see no structures above a certain size. If we find structures above this size, it would mean that the universe is not uniform on large scales, and matter is not distributed smoothly. In short, unless there are other processes we are missing, it could mean that the universe is not the same in all directions.

Enter two structures, looking gigantic.

In 2021, astronomers discovered the "Giant Arc" spanning 3.3 billion light-years in the constellation of Boötes the Herdsman. This almost symmetrical arc of galaxies, about 9.2 light-years from Earth, is around 1/15th the radius of the observable universe.

“When viewed on such a large scale, we expect to see a statistically smooth distribution of matter across the Universe, based on the Cosmological Principle introduced by Einstein to make the maths easier, that the Universe is isotropic and homogeneous. It means that the night sky, when viewed on a sufficiently large scale, should look the same, regardless of the observers’ locations or the directions in which they are looking," Alexia Lopez, then a PhD candidate at the University of Lancashire and finder of the arc, explained in a statement.

"The growing number of large-scale structures over the size limit of what is considered theoretically viable is becoming harder to ignore," Lopez added. 

"According to cosmologists, the current theoretical limit is calculated to be 1.2 billion light years, which makes the Giant Arc almost three times larger. Can the standard model of cosmology account for these huge structures in the Universe as just rare flukes, or is there more to it than that?"

A few years after that discovery, along came another gigantic problem, in the form of the Big Ring. Discovered by Lopez, again, the second structure is in the same region as the Giant Arc, separated by only around 12 degrees in the sky.

The Big Ring, like the Giant Arc, was discovered by seeing it essentially backlit by quasars and spotting where magnesium is absorbed by the intervening galaxies which make it up. It is a circular structure, measuring around 1.3 billion light-years in diameter, and around 4 billion light-years in circumference, though more like a spiral corkscrew than a true circle. If you were able to see it with the naked eye, it would take around 15 Moons to cover it, even though it lies 9.2 billion light-years from Earth. 

It, along with other large structures seen by other teams, presents quite the challenge to cosmological models. 

“The Cosmological Principle assumes that the part of the universe we can see is viewed as a ‘fair sample’ of what we expect the rest of the universe to be like. We expect matter to be evenly distributed everywhere in space when we view the universe on a large scale, so there should be no noticeable irregularities above a certain size," Lopez said in a separate statement.

“Cosmologists calculate the current theoretical size limit of structures to be 1.2 billion light-years, yet both of these structures are much larger – the Giant Arc is almost three times bigger and the Big Ring’s circumference is comparable to the Giant Arc’s length."

“From current cosmological theories we didn't think structures on this scale were possible. We could expect maybe one exceedingly large structure in all our observable universe. Yet, the Big Ring and the Giant Arc are two huge structures and are even cosmological neighbours, which is extraordinarily fascinating.”

The explanation for these structures? As yet, we just don't know.

“Neither of these two ultra-large structures is easy to explain in our current understanding of the universe. And their ultra-large sizes, distinctive shapes, and cosmological proximity must surely be telling us something important – but what exactly?" Lopez added.

“One possibility is that the Big Ring could be related to Baryonic Acoustic Oscillations (BAOs). BAOs arise from oscillations in the early universe and today should appear, statistically at least, as spherical shells in the arrangement of galaxies. However, detailed analysis of the Big Ring revealed it is not really compatible with the BAO explanation: the Big Ring is too large and is not spherical.”

Other explanations may be needed, or our models need to be altered. Before that, more study on these structures – and maybe more out there – is needed to confirm their presence and gather clues on what made them so very, very large.