What Is The Largest Known Object In The Universe?

Large Magellanic Cloud. ESA/NASA/Hubble.

The universe is a vast void, mostly made up of an eerie, empty vacuum. In spite of all this emptiness, there are regions that contain enormous structures. For example, our Sun is a ginormous compilation of gas and dust. And even greater than our Sun are magnificent structures such as planetary nebulae, entire galaxies and galaxy clusters. 

But which structure in the universe trumps all the rest in size? It turns out that it's something mysterious, and its existence defies the laws of physics. 

The largest structure in the universe is impressively titled: Hercules-Corona Borealis Great Wall, or Great Gamma-Ray Burst Wall (GRB Wall). A gamma-ray burst is an ultra-energetic explosion of gamma-radiation – the brightest electromagnetic event in the universe. The source of gamma-ray bursts are thought to be associated with the death throes of dying stars within distant galaxies. For scientists, GRBs are beacons that indicate the region is surrounded by a dense area of dust and matter. 

Recently, scientists found a dense region of space with a curiously high number of these GRB beacons – 14 to be precise. They have likened this structure to a wall, where immense structures that contain GRBs are stacked on top of each other like bricks. Except that instead of stretching from say one side of a house to the other, they extend for light-years across the expanse of space. Their results can be found published in Astronomy & Astrophysics.

Since the wall is so far away, this makes it rather faint and difficult to map accurately. However, preliminary estimates for its size range from 5.4 billion light-years across (which is unimaginably enormous) all the way to 18 billion light-years across (which is mind-blowingly, monumentally enormous). If imagining this structure is proving difficult, then imagine the difficulty that scientists are having in figuring out how it was made. The enigma of the GRB Wall is that it just seems too big to exist. It's far too large to be either a remnant of the Big Bang or the result of coalescing matter. 

Jon Hakkila, one of the authors of the study and a professor at the College of Charleston, commented to IFLScience on the creation of such a structure: "Theoretical models describing the formation of these structures appear to be at odds with the observations." 

Even more perplexingly, the wall just seems to be getting bigger. "GRBs are detected at a relatively constant rate, which means that the size of the GRB map is slowly growing." 

There is one other object that is of a similar magnitude to this supercluster. Strictly speaking, it isn't an object. Instead of the largest region of matter, it's the biggest region of an absence of matter: the supervoid. 

The supervoid is a vast area in the universe that is approximately 1.4 billion light-years across. When compared to the GRB Wall, the supervoid pales in size. Despite its name, this region of space is not truly empty: It is sparsely sprinkled with galaxies and dark matter, but the density of matter in this region is noticeably lower than all other regions of space.

The Supervoid (expanded region). ESA Planck Collaboration.

What makes this spot in the universe so unattractive for galaxies and other matter to visit? István Szapudi, who led a team of scientists that researched the origins of the supervoid at the University of Hawaii, enlightened IFLScience about how this area of space came to be. And like all great stories, it starts with the creation of time and space: the Big Bang. 

The Big Bang was the expansion of spacetime. It was largely uniform – the same in every direction, expanding perfectly. However, not everywhere is entirely unvarying. Occasionally there are tiny imperfections on a subatomic scale. Szapudi explained how these quantum blemishes could propagate and create bigger and bigger inconsistencies in the universe further on in time.

"According to our theories, the Supervoid corresponds to an initial quantum fluctuation of the inflation field right after the big bang," said Szapudi to IFLScience. "Originally, it was a small fluctuation, but as the universe expanded, and fluctuations have grown, it became a vast area missing 10's of thousands of galaxies."

Because there is a lack of matter in this region, that also makes it colder than the surrounding regions. This is what gives the supervoid its nickname: the Cold Spot. Szapudi's results can be found in the Monthly Notices of the Royal Astronomical Society.

These two objects, almost opposites, are the titans dominating the positions of largest structure and largest void in the universe. Their unimaginable vastness is enough to humble anyone. In the meantime, astronomers and telescopes are busy scanning the skies. And who knows? Maybe they'll happen upon something even bigger.

Central Image: Collapsar. NASA

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