Key Takeaways:

  • Giant Arc is a crescent of galaxies spanning 3.3 billion light-years, one of the largest structures in the universe.
  • The Giant Arc was discovered accidentally while studying light from quasars.
  • The existence of the Giant Arc challenges the idea that matter is evenly distributed throughout the universe.
  • The discovery may require us to explore new theories about gravity or the universe’s structure.
  • Finding the Giant Arc suggests there might be more massive structures out there waiting to be discovered.
A simulation of the giant arc structure located in the Bootes Constellation
The Giant Arc. Grey regions show areas that absorb magnesium, which reveals the distribution of galaxies and galaxy clusters. The blue dots show background quasars, or spotlights. (Image credit: Alexia Lopez/UCLan)

A newly discovered crescent of galaxies spanning 3.3 billion light-years is one of the largest known structures in the universe, challenging some of astronomers’ most fundamental assumptions about the universe.

Galaxies, galactic clusters, and a large amount of gas and dust make up the magnificent structure known as the Giant Arc. At 9.2 billion light-years distance, it covers about 15% of the universe that can be observed.

According to Alexia Lopez, a cosmology PhD candidate at the University of Central Lancashire (UCLan) in the United Kingdom, the discovery was “serendipitous,” she told Live Science. Using light from roughly 120,000 quasars—distant, bright galaxies where supermassive black holes are consuming matter and releasing energy—Lopez was piecing together maps of objects in the night sky.

This light is absorbed by various elements as it travels through matter between Earth and the quasars, leaving behind telltale signs that can provide scientists with crucial information. Lopez, in particular, used the marks left by magnesium to calculate the material’s position in the night sky as well as its distance to the intervening gas and dust.

In this way, the quasars act “like spotlights in a dark room, illuminating this intervening matter,” Lopez said.

Between the cosmic maps something started to take shape. According to Lopez, “it was kind of a hint of a big arc.” “I remember going to Roger [Clowes] and saying ‘Oh, look at this.'”

Giant Arc layout
A depiction of the structure of the Giant Arc shown in grey, with neighborhood quasars superimposed, shown in blue. A tentative association can be seen between these two datasets. (Image credit: Alexia Lopez/UCLan)

Her doctoral adviser at UCLan, Clowes, recommended additional testing to make sure the alignment wasn’t a coincidence or a data trick. The scientists came to the conclusion that there was less than a 0.0003% chance the Giant Arc wasn’t real after conducting two distinct statistical tests. They gave a presentation of their findings at the American Astronomical Society’s 238th virtual meeting.

However, the discovery challenges a fundamental belief about the universe and will rank among the greatest things in the universe. The cosmological principle, which holds that matter is roughly equally distributed throughout space at the largest scales, has been accepted by astronomers for a very long time.

The South Pole Wall and the Sloan Great Wall, two other massive assemblies that are dwarfed by even larger cosmic features, are smaller than the Giant Arc.

According to Clowes, “many large-scale structures have been discovered over the years,” Live Science reported. “They’re so large, you wonder if they’re compatible with the cosmological principle.”

The fact that these enormous objects have gathered in specific areas of the universe suggests that matter may not be spread equally throughout the cosmos.

However, Lopez noted that the cosmological principle is the basis of the current standard model of the universe. “If we’re finding it not to be true, maybe we need to start looking at a different set of theories or rules.”

Although Lopez brought up the possibility of altering the way gravity functions on the largest scales—a notion that has gained traction among a vocal but small group of scientists in recent years—she is unsure of the specifics of those theories.

Daniel Pomarède, a cosmographer at Paris-Saclay University in France who co-discovered the South Pole Wall, agreed that the cosmological principle should dictate a theoretical limit to the size of cosmic entities.

According to some research, structures should be limited in size and should not be able to expand any further, Pomarède told Live Science. “Instead, we keep finding these bigger and bigger structures.”

He isn’t quite ready to abandon the cosmological principle, though, as it has been incorporated into universe models for almost a century. “It would be very bold to say that it will be replaced by something else,” he said.

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