The Giant Arc is a newly-discovered structure spanning 3.3 billion light years, and it's challenging our understanding of the universe.

Scientists have discovered a structure in the distant universe so immense that it is actually challenging our understanding of the universe. 

Graphic of the Giant Arc (left) and Bootes constellation from Earth (right). Image: Alexia Lopez/UCLan (left) Allexxander via Getty Images

Known as the Giant Arc, this crescent-shaped stream of galaxies stretches across 3.3 billion light years, making it about one-fifteenth the radius of the entire observable universe. 

The unusual discovery was recently announced by Alexia Lopez, a PhD student at the University of Central Lancashire who detected the Giant Arc, at the 238th virtual meeting of the American Astronomical Society. “It’s so big that it’s hard to explain with our current theories,” Lopez said during her presentation on June 7.

Lopez first spotted the structure, which is located 9.2 billion light years from Earth in the constellation Bootes, in observations captured by the Sloan Digital Sky Survey, based in New Mexico. At the time, she was researching a technique called the magnesium II (MgII) method that probes the spectral fingerprints of ionized magnesium in space to detect structures that might otherwise remain hidden.

“The Giant Arc was a serendipitous discovery,” Lopez said in an email. “I was working on understanding the MgII method and how well it worked, when this one particular piece of sky I was testing happened to show the first hints towards something exciting. That’s when I started to look more into this bit of sky and slowly, but surely, the Giant Arc was revealed.”

The gargantuan arc defies expectations set by the cosmological principle, which states that the universe should be homogeneous and isotropic on large scales. In other words, if you were to zoom out to view huge swaths of the cosmos, the distribution of matter should have a relatively smooth and even pattern, and large-scale clumps of material should rarely exceed 1.2 billion light years in length.

A schematic of the Giant Arc, which is visible at the center. Image: Alexia Lopez/UCLan

The Giant Arc is nearly three times that big, and it’s not the only structure to break that threshold, raising the possibility that these formations might pose “a potential challenge to the standard model” of cosmology, Lopez said in her presentation. It’s also one of many findings that seem to push the limits of the cosmological principle, including unexplained hints of directionality to the universe.

“The implications of the Giant Arc, combined with several other Large Scale Structure (LSS) discoveries implies that there may be a problem with our initial assumptions,” Lopez said. “We have to keep testing and probing these LSSs to understand whether they are real or just a fluke.”

Lopez and her colleagues—including her advisor Roger Clowes, an astrophysicist at the Jeremiah Horrocks Institute at the University of Central Lancashire—charted out the contours of this galaxy-studded crescent with the help of background light from extremely luminous and energetic galactic cores known as quasars. Dimmer structures located between the ancient universe and modern Earth can imprint their chemical signatures into quasar light, revealing entities that would be hard to detect with other means. 

This way, the team was able to expose the Giant Arc by capturing the absorption of ionized magnesium within the structure into the quasar’s spectrum. Lopez and her colleagues then conducted a statistical analysis of the observations, which produced a level of confidence of 4.5 sigma, meaning that the odds that the discovery is a fluke are under 0.01 percent. 

While the existence of the Giant Arc does not overturn the cosmological principle on its own, it does raise new questions about its underlying assumptions. Lopez’s team plans to pursue those mysteries with further observations of the colossal structure and its environment, which may reveal how it became so anonymously huge and what that means for our understanding of the universe we live in.    

“Our next steps are comparing the Giant Arc with other data sets,” Lopez said. “We have started doing this with the neighbourhood quasars and find a tentative association. We also wish to understand what the details of the MgII absorption itself can tell us about the environment of different parts of the Giant Arc. Further statistical tests may be required to solidify the significance of the GA.” 

On a broader level, Lopez noted that scientists “have to continue to map the sky as much as we can with more detail so that we are seeing the bigger picture of the universe.” 

“Then we can continue to compare real results with expected results from our theories (whether these are modified/alternative theories or extensions of our current theory),” she concluded.