If The Mysterious Cold Spot Isn't A Void, Is It Evidence Of A Multiverse?

The cosmic microwave background (CMB) is the oldest light in the universe, a relic left over from the Big Bang itself. With a radio telescope, you can see its glow touch everything in space—and without one, you can see the mark it leaves in the static on an old TV set. But there's a peculiar dim in the CMB that has astronomers scratching their heads. Dubbed the Cold Spot, it was once thought to be caused by a "supervoid" where there was a dearth of galaxies. But new research says that can't be possible—and points to something much more fascinating.

The map of the CMB sky produced by the Planck satellite. The large area with anomalously low CMB temperature known as the CMB Cold Spot is highlighted in the inset.

She's A Supervoid, Supervoid

Everything is relative, including the Cold Spot. The 1.8-billion light-year region is colder than its surroundings by a mere 0.00015º C (0.00027º F), but that's plenty for astronomers to puzzle over. It was discovered back in 2004, and in 2015, astronomers thought they had an answer for its origins. See, a phenomenon known as the Sachs-Wolfe effect causes light from regions of higher density—areas with more galaxies, for example—to be slightly redshifted, and light from regions of lower density—a void of galaxies, perhaps—to be slightly blueshifted. In the CMB, those redshifts from galaxies would make the radiation look warmer than it really is, and voids would make it look cooler. That's what made astronomers suspect that the Cold Spot was due to a "supervoid" that contained 10,000 fewer galaxies than comparable areas of the universe.

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Bubbles In The Theory

But technology improves, and measurements get better. In 2017, Durham University researchers published research in Monthly Notices of the Royal Astronomical Society that says the Cold Spot wasn't caused by a supervoid of galaxies. They based this claim on high-fidelity data of 7,000 galaxies retrieved from the Anglo-Australian Telescope. Instead, they say that the Cold Spot is split into smaller voids that aren't much different than the average void you'd find elsewhere in the universe.

Simulations of the standard model of the universe suggest that the odds of the Cold Spot arising out of sheer coincidence are 1 in 50. In a press release, co-author Tom Shanks said this means that it could just be caused by a rare fluctuation in the CMB—but it could have more intriguing explanations. "Perhaps the most exciting of these is that the Cold Spot was caused by a collision between our universe and another bubble universe. If further, more detailed, analysis of CMB data proves this to be the case then the Cold Spot might be taken as the first evidence for the multiverse – and billions of other universes may exist like our own."

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