In 2021, Alexia Lopez, a doctoral student, was analyzing light from distant quasars when she made a startling discovery.
It found a giant, near-symmetrical arc of galaxies, 9.3 billion light-years from Earth, in the constellation of Boyero.
At a massive 3.3 billion light-years across, the structure covers 1/15th 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 arc, the Hulk questions some of our most basic notions of the universe.
According to the Standard Model of cosmology — the theory that underlies our understanding of the universe — matter should be more or less uniformly distributed throughout space. When scientists observe the universe on very large scales, they shouldn’t notice large irregularities; Everything should look the same in all directions.
But the giant bow is not the only example of its kind. These massive structures are now forcing scientists to redefine their theory of the evolution of the universe.
Lopez was studying for her master’s degree at the University of Central Lancashire in the UK when her advisor suggested she use a new method for analyzing large-scale structures in the universe.
It used quasars — distant galaxies that emit an extraordinary amount of light — to search for signs of ionized magnesium, which are clear indications of the presence of gas clouds around the galaxy.
When light passes through this ionized magnesium, certain frequencies are absorbed, leaving unique optical “signals” that astronomers can detect.
“I looked at known and documented galaxy clusters and began tracing the appearance of these regions using the Magnesium II method,” says the Briton.
“One of the clusters I looked at was very small, but when I looked at Magnesium II, there was this interesting, dense band of magnesium uptake across the field of view,” she says.
“That’s how I ended up finding it. It was a happy accident and I was lucky to be the one who found it.”
Lopez’s “Happy Accident” was an amazing breakthrough. Observed by the Boieiro constellation, a group of about 45 to 50 gas clouds, each associated with at least one galaxy, appeared to be arranged in an arc 3.3 billion light-years across. The size is large, as the visible universe has a capacity of 94 billion light-years.
Lopez’s paper states that it is extremely unlikely (only 0.0003% probability) that this large structure arose by chance.
The researcher suggests that it may have formed for some reason in the natural physics of the universe that we currently do not know.
Their findings directly challenge a central aspect of the standard cosmological model, which is the best explanation we have for the beginning and evolution of the universe.
This aspect is known as the universal principle. She asserts that on a large scale, the universe should look roughly the same everywhere, no matter where you are or what direction you’re looking. There should be no giant structures and the space should be smooth and uniform.
This is convenient, as it allows researchers to draw conclusions about the entire universe based solely on what we see from our own vantage point. But it also makes sense that after the Big Bang, the universe expanded outward, simultaneously throwing matter out in all directions.
This is where the problem arises. According to the Standard Model, structures like the Giant Arch simply didn’t have time to form.
“The current idea of how structures form in the universe is through a process known as gravitational instability,” says 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, small fluctuations in density caused bits of matter to clump together. Over billions of years, the force of gravity has caused these clusters to form stars and galaxies.
However, there are limitations to the scale of this operation. Any object larger than about 1.2 billion light-years away simply wouldn’t have had enough time to form.
“For structures to form, you need the particles to clump together, for gravitational collapse to occur,” Sarkar explains. “These particles have to move in from outside the structure to get there.”
“So, if its structure is 500 million light years long, it will take 500 million years to pass from one end to the other,” the professor continues.
“But we’re talking about particles that move much slower than light, so it would take billions of years to create a structure of that size — and the universe has only existed for about 14 billion years.”
Sagittarius is not alone
The giant arch discovered by Lopez wasn’t the only large structure discovered by astronomers.
There is also the “Great Wall of Galaxies” (also called CfA2 Great Wall), discovered in 1989 by Margaret Geller and John Hochra. The wall is about 500 million light-years long, 300 million light-years wide, and 15 million light-years deep.
Sloan’s Great Wall is even bigger than that, it is a cosmic structure formed by a giant wall of galaxies, discovered in 2003 c. This wall is about 1.5 billion light-years across.
The discovery of these giant astronomical planets has accelerated further in the past decade.
In 2014, scientists discovered the Laniakea supercluster, a group of galaxies where our Milky Way is located. Laniakea has a diameter of 520 million light-years and a mass equivalent to about 100 quadrillion suns.
In 2016, the BOSS Great Wall was discovered – a complex of galaxies spanning more than a billion light-years. BOSS consists of 830 separate galaxies, which have been pulled together by gravity to form four giant clusters. Galaxies are connected by long filaments of hot gas.
And in 2020, the Antarctic wall, 1.4 billion light-years across, was added to the list.
But the current record holder among these structures is the Hercules-Crown Borealis Great Wall. Discovered in 2013, it spans 10 billion light-years – more than 10% of the size of the visible universe.
“We did the math and realized, ‘Wow, this is the biggest thing in the universe,'” says John Hakila, a professor of physics and astronomy at the University of Alabama in Huntsville, US.
His concern was justified. Haquila and Lopez ran a series of statistical tests to try to prove that the results couldn’t come 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 equates to a 1 in 3.5 million probability that the results are due to chance. The giant arc has reached a magnitude of 4.5 sigma, so there is still a possibility that the structure is a random collection of stars.
“Our eyes are very good at seeing patterns,” Sarkar explains. “You can see the letters in the clouds, but it’s not a real structure. Your brain is forming a structure based on what is actually random.”
“But I don’t think that’s the case in this case. I think it’s a real physical chain of giant clusters,” says the professor.
If other structures like the Giant Arch and Hercules-Crown Borealis Great Wall prove to exist, astronomers will be forced to rewrite (or at least revise) the Standard Model of cosmology.
This is not the first time the model has needed to be modified. In 1933, scientist Fritz Zwicky, of the California Institute of Technology (Caltech), in the United States, measured the mass of a group of galaxies and concluded that the number was lower than expected.
In fact, the cluster was so small that the galaxies had to separate and escape the cluster’s gravity. So there is something else that must hold the clusters of galaxies together.
This “other thing” is dark matter, a mysterious substance thought to make up 27% of the universe.
In 1998, the model received new tweaks to include dark energy, after two independent teams of astronomers measured the expansion of the universe and found that it was accelerating.
In any case, we must know for sure what to do in the coming years. The Legacy Survey of Space and Time (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 invest their lives and jobs in it,” says Sarkar. “But in science we need to know who is right in the end.”
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