Supervoids vs Colliding Universes!

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If you study a map of the Cosmic Microwave Background or CMB, you may notice a large deep blue splotch on the lower right. This is the cold spot. Is this feature a statistical fluke, the signature of vast super voids, or even the imprint of another universe?

Is that giant cold spot in the Cosmic Microwave Background really evidence of a collision with another universe? That's what all the media hype is saying, which means it's time for another Space Time Journal Club to sort it out. Today we're going to talk about a fascinating new publication by Mackenzie et al. 2017 titled, Evidence Against a Supervoid Causing the CMB Cold Spot. First, the name, Evidence Against a Supervoid. The leading explanation for the cold spot was that it was imprinted on the Cosmic Microwave Background radiation as that radiation passed through giant empty regions of the universe, so-called supervoids. The evidence against part tells us that the authors think that this hypothesis is wrong. This is partly what excited everyone about the alternative bubble universe collision hypothesis. Before we go into detail about either of these hypotheses, let's get the basics down. The Cosmic Microwave Background is everywhere. It's the light that was released at the moment that the first atoms formed, 380,000 years after the Big Bang.

For all of the gory details, check out our previous episode. The CMB is amazingly uniform. When it formed in the early hot universe, it was mostly infrared light with a temperature of 3,000 Kelvin. 13.5 billion years of cosmic expansion later, and it stretched to microwave wavelengths, and to a temperature very close to 2.725 Kelvin all across the sky. Although it's very smooth, the CMB does show lots of very tiny fluctuations in temperature. Those are all of those smaller blotchy spots. We think that they came from random quantum fluctuations from the very first instant after the Big Bang. These were then amplified by a period of exponential expansion in the very early universe that we call inflation.

The typical deviation from the average temperature is around 20 microkelvin. So the differences are one part in 100,000. The Cold Spot, which is in the direction of the Southern constellation Eridanus, is 150 microkelvin cooler than the average. It's also huge. 10 degrees across for the coldest patch, with a less extreme halo and hot rim that's 20 degrees across. Think 40 full moons. Now, it's possible to explain the Cold Spot as just an unusually strong random fluctuation in the CMB.

Just like those small blotches, but by chance, very large. Simulations show that you should get a spot that size in the CMB in around 1 in 50 universes. So we might be in one of those slightly rarer universes with a big CMB splotch. Really, in any given universe, there should be a few weird 1 in 50 things.

But the odds are low enough that it's worth investigating. Inu and Silk in 2006 first proposed the Cold Spot could be the imprint of a supervoid via the so-called Integrated Sax Wolf or ISW effect. The ISW effect is dark energy in action. It's a cosmological tug of war. Gravity pulls things in while dark energy pushes things out. A photon entering a matter-rich galaxy cluster gets an energy boost as it falls into the cluster's gravitational well. But by the time the photon is on its way out, the expansion of the universe has actually stretched out the cluster, weakening its gravitational pull. There's a steep slope down and a shallow slope up, just like a ski ramp. The photon exits with a net energy gain, which would register as a higher temperature on our CMB map. But the opposite happens when a photon enters a void. It loses energy going in because it's being pulled backwards by the higher density universe behind it.