Understand what galaxy arcs are – 03/18/2023 – Science
In 2021, PhD student Alexia Lopez was analyzing light coming from distant quasars when she made a startling discovery.
She found a giant, almost symmetrical arc of galaxies, 9.3 billion light-years from Earth, in the constellation Boieiro. At a massive 3.3 billion light-years across, the structure covers 1/15 of the radius of the observable Universe. If we could see it from Earth, it would be the size of 35 full moons lined up in the sky.
Known as the Giant Arch, the structure questions some of our basic conceptions of 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 homogeneously distributed throughout space. When scientists observe the Universe on very large scales, they shouldn’t observe large irregularities; everything should look the same in all directions.
But the Giant Bow is not the only example of its kind. These immense structures are now forcing scientists to redefine their theory of the Universe’s evolution.
Lopez was studying for her master’s degree at the University of Central Lancashire in the UK when her advisor suggested that she use 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, which are clear indications of the existence of gas clouds around a galaxy.
When light passes through this ionized magnesium, certain frequencies are absorbed, leaving unique light “signatures” that astronomers can detect.
“I looked at known and documented clusters of galaxies and began to trace the appearance of these regions using the Magnesium II method”, says the Briton.
“One cluster I looked at was very small, but when I looked at it in Magnesium II, there was this interesting dense band of magnesium absorption across the field of view,” she says.
“That’s how I ended up finding out. It was a happy accident and I was just lucky to be the one who found it.”
Lopez’s “happy accident” brought a spectacular discovery. Observed through the constellation Boieiro, a group of about 45 to 50 gas clouds, each associated with at least one galaxy, seemed to be arranged in an arc 3.3 billion light-years long. The size is considerable, as the amplitude of the observable Universe is 94 billion light years.
Lopez’s paper states that it is extremely unlikely (only 0.0003% probability) that this large structure came about by chance.
The researcher suggests that it may have been formed for some reason in the natural physics of the Universe that we currently do not know.
Their findings directly challenge a central facet of the standard cosmological model, which is the best explanation we have for the beginning and evolution of the Universe.
This facet is known as the cosmological principle. It asserts that, on a large scale, the Universe should look approximately the same everywhere, no matter where you are or which direction you are looking. There should be no giant structures and the space should be smooth and even.
This is convenient, as it allows researchers to draw conclusions about the entire Universe based only on what we see from our particular corner. But it also makes sense that after the Big Bang, the Universe expanded outwards, throwing matter simultaneously in all directions.
This is where the problem arises. According to the standard model, structures like the Giant Arch simply would not have had time to form.
“The current idea of how structures in the Universe formed is through a process known as gravitational instability,” according to Subir Sarkar, professor of theoretical physics at the University of Oxford, UK.
About a million years after the Big Bang, when the Universe was expanding, tiny fluctuations in density caused bits of matter to clump together. Over billions of years, the force of gravity managed to cause these clusters to form stars and galaxies.
But there is a size limitation for this process. Any object larger than about 1.2 billion light years would simply not have had enough time to form.
“To form structures, you need particles to come together, for gravitational collapse to occur,” explains Sarkar. “These particles would need to move in from the outside of the structure to get there.”
“So, if its structure is 500 million light-years long, it would take light 500 million years to move from one end to the other”, continues the professor.
“But we’re talking about particles that are moving much slower than light, so it would take billions of years to create a structure that size — and the Universe has only existed for about 14 billion years.”
Arc is not alone
The Giant Arch discovered by Lopez is not the only large-scale structure discovered by astronomers.
There is also the “Great Wall” of galaxies (also called the CfA2 Great Wall), discovered in 1989 by Margaret Geller and John Huchra. The wall is about 500 million light-years long, 300 million light-years wide and 15 million light-years deep.
The Sloan Great Wall is even bigger — a cosmic structure formed by a giant wall of galaxies, discovered in 2003 by J. Richard Gott III, Mario Juric and their colleagues at Princeton University in the United States. This wall is about 1.5 billion light-years across.
And the discovery of these astronomical behemoths has accelerated even more in the last decade.
In 2014, scientists discovered the Laniakea supercluster, a collection of galaxies where our Milky Way is located. Laniakea is 520 million light-years across and its mass is equivalent to about 100 quadrillion suns.
In 2016, the BOSS Great Wall was discovered – a complex of galaxies more than a billion light-years across. BOSS is made up of 830 separate galaxies, which have been pulled together by gravity to form four superclusters. Galaxies are connected by long filaments of hot gas.
And in 2020, the 1.4 billion light-year-long South Pole Wall was added to the list.
But the current record among these structures is the Hercules-Crown Borealis Great Wall. Discovered in 2013, it covers 10 billion light years – more than 10% of the size of the observable Universe.
“We did the math and we realized, ‘wow, this is the biggest thing in the universe,'” says Jon Hakkila, professor of physics and astronomy at the University of Alabama at Huntsville, in the United States.
His concern was justified. Hakkila and Lopez performed a series of statistical tests to try to prove that the results could not have come about by chance. For Arco Gigante, the results showed a reliability level of 99.9997%.
In scientific research, the gold standard for statistical significance is known as 5-sigma, which is equal to about a 1 in 3.5 million probability that results are due to chance. The Giant Arc has reached significance of 4.5 sigma, so there is still the possibility that the structure is a random arrangement of stars.
“Our eyes are very good at seeing patterns,” explains Sarkar. “You can see letters in the clouds, but it’s not a real structure. Your mind is forming a structure based on what is actually random.”
“But I don’t think that’s the case in this situation. I think it’s a real physical chain of superclusters”, says the professor.
If other structures like the Giant Arch and the Hercules-Crown Borealis Great Wall prove to exist, astronomers will be forced to rewrite (or at least revise) the standard model of cosmology.
It is not the first time that the model will need to be adapted. In 1933, scientist Fritz Zwicky, from the California Institute of Technology (Caltech), in the United States, measured the mass of a cluster of galaxies and concluded that the number was smaller than expected.
In fact, the mass was so small that the galaxies should have separated and escaped the cluster’s gravitational pull. So something else must hold galaxy clusters together.
This “something else” is dark matter, a mysterious substance believed to make up 27% of the universe.
In 1998, the model received new adaptations to include dark energy, after two independent teams of astronomers measured the expansion of the Universe and found that it is accelerating.
In any case, we should know for sure in the coming years what will need to be done. The Space and Time Legacy Survey (LSST) is a planned 10-year study of the southern sky that could provide astronomers with an unprecedented view of the Universe.
“It takes a lot of effort to make a paradigm shift, especially if people have invested their lives and careers in it,” says Sarkar. “But in science we need to see who is ultimately right.”