Supervoids vs Colliding Universes (Script) (Patreon)

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 area, creatively named the Cold Spot. Is this feature a statistical fluke, the signature of vast supervoids, 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 imprinted on the cosmic microwave background as that radiation passed through giant empty regions – so-called supervoids. The “evidence against” part tells us that the authors think 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 3000 Kelvin. Thirteen and a half billion years of cosmic expansion has stretched it to microwave wavelengths, and to a temperature of very close to 2.725 kelvin all across the sky.

Although it’s very smooth, the CMB does show lots very tiny fluctuations in temperature. Those are all of the smaller blotchy spots. We think that they come 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 early universe that we call inflation. The typical deviation from the average temperature is around twenty microkelvin – so differences of 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: ten 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 is 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 one-in-50 things. But the odds are low enough that it’s worth investigating.

Inoue & Silk in 2006 first proposed the Cold Spot could be the imprint of a supervoid via the Integrated Sachs-Wolfe, 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. 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. The opposite happens when a photon enters a void. It loses energy going in because it’s being pulled backwards by the higher density behind it. But the galaxies have spread out a little further as it exits the void, so it doesn’t get pulled out as strongly.

The ISW effect would be tiny – negligible - in a universe without dark energy. The difference between the going-in and going-out boosts would be too small to be noticed. But around 4 billion years ago dark energy caused the expansion of our universe to begin accelerating, whereas previously it had been slowing down due to gravity. In an accelerating universe the difference in the ingoing and outgoing boosts can be large enough to detected. So if there are giant voids in the direction of the Cold Spot then these could have sapped energy from CMB photons as they passed through.

Mackenzie et al. used the Anglo-Australian Telescope in outback New South Wales to perform a spectroscopic survey of 7,000 galaxies in the direction of the cold spot out to a redshift 0.4. To use laymans terms, they split the light of those galaxies into component wavelengths and determined the shift in the wavelengths of those spectra due to the expansion of the universe – i.e., they measured redshifts. This gave distances to those galaxies, which ultimately allowed an accurate 3-D atlas of galaxies in the direction of the cold spot, all the way out to the point where dark energy started to dominate the universe. They found three, maybe a four supervoids. However the combined ISW effect they calculated for all four voids should only have produced 32 microkelvin drop in temperature, a mere fifth of the observed 150 microkelvin drop.

Mackenzie et al. also observed a control region, “G23”, in the direction of the star Fomalhaut. G23 has a similar void structure to the Cold Spot’s line of sight, plus a couple overdensities. G23’s ISW effect is calculated to yield a 14 microkelvin drop in the CMB, and that actually matches the observed deviation of 15 microkelvin. So the control sample shows that calculating the ISW effect can lead to a number that matches the true effect. Mackenzie et al. conclude that this means supervoids can’t be the sole explanation for the 150 microkelvin cold spot.

What is it, then? Well, there’s a good chance it’s actually just a statistical blip. The void hypothesis was always the “least crazy” idea. But now that that seems to be ruled out, it’s at least worthwhile talking about the “more crazy” notions.

First there’s the ever-recurring idea that “gravity is wrong” – the same notion that people have tried to use to explain dark matter. Basically, the calculation of the ISW effect – the effect of the voids – is greater than expected because we don’t understand gravity on large scales. However modified gravity is on shaky grounds because dark matter is looking more and more like real stuff, not incorrect gravity. Also, the control field gave roughly the right answer, which it shouldn’t have if our understanding of gravity was so far off.

The other weird ideas are about what happened in the inflationary era. There are ideas like the amplification of topological defects in the universe, or inhomogeneous reheating at the end of a nonstandard inflation. But the one that gets people most excited is, of course, the cold spot is the mark left due to a collision with another universe. So what’s that all about?

A popular version of inflation theory is that of eternal inflation. The idea is that the initial period of exponential expansion that we call inflation actually lasts forever.. It’s a whole big topic and we’ll do an episode on at some point. In the eternal inflation scenario, a normal universe begins when a small patch of the inflating universe stabilizes – in particular its vacuum energy takes on a stable value.  At that point it stops inflating and starts expanding normally. This can happen spontaneously anywhere in the greater inflating spacetime, resulting in “bubble” universes. And it can happen frequently or rarely depending on the completely unknown details of the string theory parameter space. But regardless, in an infinitely inflating spacetime collisions between bubbles are eventually expected.

So what happens when two bubble universes collide? Well they merge and exchange an enormous amount of energy. Chang, Kleban & Levi 2009 figured out that should result in a temperature gradient across each universe. If that merger point is distant from us then this looks like a hot or cold spots in the cosmic microwave background.

The colliding multiverse explanation is still pretty fringe. Mackenzie et al. have debunked the more standard explanation of the Cold Spot, so it’s still in the running. However more detailed observations of the CMB in that region are needed to rule out it being a statistical fluke. Which it probably is. But if not, perhaps once upon a time we really did collide with an entirely separate bubble of Space Time.