Mysterious 'Cold Spot' May Be The Largest Structure In The Universe

04/27/2015 08:41 am ET | Updated Apr 27, 2015

Astronomers are crowing about the discovery of what they say just might be "the largest individual structure ever identified by humanity."

It's not a galaxy or a cluster of galaxies--nor even a galaxy supercluster. It's not even a structure in the usual sense. Rather, it's a vast cosmic bubble of sorts--a roughly spherical "supervoid" some 1.8 billion light-years across.

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cold spot
The Cold Spot is in the constellation Eridanus. Graphics by Gergő Kránicz. Image credit: ESA Planck Collaboration.

What makes the bubble different from the surrounding regions of the universe? There's no barrier--it's just that the density of galaxies is significantly lower inside the supervoid than outside.

"Supervoids are not entirely empty, they're under-dense," Dr. Andras Kovacs, of Eotvos Lorand University in Budapest, Hungary, and one of the researchers behind for the discovery, told Popular Science. In fact, the Guardian reported, some 100,000 galaxies are "missing" from the supervoid.

A cosmic riddle. The discovery helps explain a mystery that dates back to 2004, when astronomers examining a map of the radiation left over after the Big Bang--the so-called cosmic microwave background (CMB)--noticed a big and unusually cold region of sky in the southern hemisphere constellation Eridanus.

It wasn't particularly surprising to discover a cold spot in the sky, according to a written statement released by the University of Hawaii. But this one--which astronomers dubbed, well, the Cold Spot--was far bigger and far colder than others.

Astronomers didn't know what to make of it.

"The Cold Spot raised a lot of eyebrows," Prof. Carlos Frenk, a cosmologist at the University of Durham in England who was not involved in the research, told The Guardian. "The real question was what was causing it and whether it was a challenge to orthodoxy.”

If the Cold Spot originated with the Big Bang, it might be a sign of some exotic physics that modern cosmological theory simply doesn't explain, according to the University of Hawaii statement. On the other hand, the Cold Spot might simply be evidence of a relatively empty region of space between us and the CMB.

It all lines up. The discovery of the supervoid suggests that the latter explanation is the likely one. As Dr. Istvan Szapudi, an astrophysicist at the University of Hawaii Institute for Astronomy in Manoa and another of the scientists involved in the research, told The Huffington Post in an email. "While we did not establish a causal relationship [between the Cold Spot and the supervoid], the fact that two very rare objects are aligned makes it very likely that they have something to do with each other."

Szapudi, Kovacs, and their collaborators made the discovery using optical data from the Pan-STARRS telescope on the Hawaiian island of Maui and infrared data from NASA's Wide Field Survey Explorer (WISE) satellite. Combing both sets of observations allowed the astronomers to estimate the distance to and position of each galaxy in that part of the sky. The supervoid is about 3 billion light-years from the Earth.

A paper describing the research was published online April 20 in the journal Monthly Notices of the Royal Astronomical Society.

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