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Astronomers Unravel the Mystery of the Universe’s Giant Cold Spot

The Cold Spot may well turn out to be the largest individual structure ever identified by humanity.
Location of the Cold Spot on CMB map. Image: NASA Blueshift

Eleven years ago, a group of astronomers discovered a mysterious mega-structure while studying a map of the cosmic microwave background (CMB)—the thermal fallout of the Big Bang.

Dubbed "the Cold Spot," the region is about four times chillier than the surrounding CMB, but the feature that really sets it apart is its hulking size. Stretching over one billion light years across, the Cold Spot may well turn out to be the largest individual structure ever identified by humanity.

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It's no wonder, then, that scientists around the world have puzzled over the origins and identity of the bizarre anomaly for over a decade, suggesting everything from parallel universes to alien physics as potential explanations for its weird properties.

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The Cold Spot mapped by PS1 and WISE data, and the Planck satellite. Image: ESA Planck Collaboration

But according to a study published today in the Monthly Notices of the Royal Astronomical Society, the Cold Spot could be explained by a so-called "supervoid" located in the foreground of the CMB. Supervoids are essentially massive empty spaces in the universe, devoid of any meaningful amount of matter.

When light passes through these vacant voids, it loses momentum and slows down. As a result of this energy deficit, the light will have a longer wavelength when it finally exits the void, which in turn corresponds to colder temperatures. This is known as the Integrated Sachs-Wolfe (ISW) effect, and is related to the accelerating expansion of the universe.

To feel out whether the Cold Spot might be caused by a supervoid, the study's authors, led by astronomer Istvan Szapudi of the University of Hawaii, conducted surveys of the region with the Pan-STARRS1 (PS1) telescope and NASA's Wide Field Survey Explorer (WISE) satellite.

Sure enough, Szapudi and his team discovered a supervoid located about three billion light years away, and stretching approximately 1.8 billion light years across. While previous teams had detected bits and pieces of this void, the new paper provides a more comprehensive look at this huge gap of nothingness in deep space.

However, it's not a total slam dunk. This theory doesn't fully account for the temperature drop in the Cold Spot, and it's difficult to predict the effects of these large voids because we know so little about them. Because of this, Szapudi and his colleagues intend to continue honing in on the region with other facilities, such as the Dark Energy Survey.

But even though it's not a complete match, it's very unlikely that the location of the supervoid relative to the Cold Spot is a total coincidence. Figuring out the exact mechanics of this mysterious dark void and its light-draining powers will no doubt be a challenge. But given how mind-boggling the findings have been so far, it's shaping up to be a fascinating process.