In 2004, researchers were combing through the data from a satellite called WMAP when they noticed something unusual. Part of the universe looked a lot colder than the rest. They called it, appropriately, the Cold Spot and scientists still aren’t really sure why it’s there.
But according to recent research, it has a lot to do with what’s basically a hole in the universe — a region where there’s much less matter than there should be.
The Cold Spot was originally found in the map of the Cosmic Microwave Background, or CMB. The CMB is energy left over from about 378,000 years after the Big Bang when the universe was no longer just a dense fog of hydrogen plasma and instead became made of regular old neutral atoms which radiated heat.
This background radiation forms the very edge of the observable universe. The universe has no edge and we observe it as thermal radiation — tiny variations in temperature that are mapped out as different wavelengths of visible light.
Now, the Cold Spot is weird because a huge swath of the map is colder than the rest — by about 70 microKelvin — that’s 70 millionths of a Kelvin. That might not seem like very much, but the rest of the CMB usually varies by just 18 microKelvin. And scientists have come up with plenty of possible explanations for why the Cold Spot is there.
Some have said that the spot isn’t even real, that it was just an error in the technique that was used to study the data, a technique called the Spherical Mexican Hat Wavelet Method because it’s shaped like a sombrero. You get it.
But others proposed that it was simply a much less dense region of space and now it looks like they were at least partly right.
Most models of the universe suggest that the cosmos is arranged kind of like an enormous piece of foam. When you zoom really, really far out you can see that matter, in the form of galaxies, is organized into huge filaments and sheets called superclusters. And since a lot of this matter was drawn together in these threads, it left some relatively empty spaces, like pockets in the foam.
Those regions of less dense space are usually called voids and we’ve known about them since at least the 1970s. We were able to spot them because the less dense an area of space is, the bluer and colder it looks in the CMB. But despite their name, voids aren’t totally empty.
They can contain thousands and thousands of galaxies. But that’s still way, way less than other areas, and voids are usually pretty small. So, for the Cold Spot to be caused by a void this thing would have to be much bigger than any void ever discovered before. A supervoid.
It just didn’t seem possible, but in April of this year a group of astrophysicists showed that it might be. The team compared two sets of data: a catalogue of all the known galaxies and a map of the sky seen in the infrared that would help them calculate things like position and distance.
When they counted the galaxies around where the Cold Spot had been detected they discovered that it had 10,000 fewer than they would’ve expected based on the average density of galaxies in the universe.
So, the Cold Spot had all the appearances of being an incredibly large lack of matter in a pocket of the universal foam. But the findings still couldn’t completely explain the Cold Spot.
According to researchers, the lack of mass in that region would only account for about one-tenth of the apparent temperature drop. So, we’re just stuck with the fact that there’s this big, nearly empty swath of the universe that’s colder than the rest. Since we still can’t explain 90% of the Cold Spot’s coldness, it’ll just have to stay a mystery for now.