Latest debate on hot spot in universe

A new galaxy survey suggests that a supervoid isn’t responsible for the Cold Spot seen in the cosmic microwave background — the oddity may have a far more ancient origin.

The Big Bang’s fading afterglow has left most of space a remarkably uniform 2.7 degrees above absolute zero. Tiny fluctuations give the afterglow a mottled look, but most of the temperature fluctuations are unremarkable, statistically speaking. However, one cold spot on the sky is not part of this harmony.

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

Astronomers are still trying to figure out why this so-called Cold Spot exists. New survey results may help them find an explanation—but it’s unclear which explanation is the best match to what the team observes.

How Cold is the Cold Spot?

The Cold Spot’s core spans 10°, or 20 times the size of the full Moon; a slightly warmer halo spans 20°. The core is a whopping 0.00007 kelvin colder than average. That sounds like an awfully small difference, and it is, but it’s big with respect to the other variations we see — the Cold Spot’s average temperature is four times cooler than the average temperature fluctuation in the cosmic microwave background (CMB), the radiation set free 380,000 after the Big Bang.

If this blemish is just a statistical fluke, then we’re living in the only one of 50 universes where such a thing might happen by chance — a big enough fluke to make astronomers uncomfortable.

One non-fluke explanation is that a huge, mostly empty region of space called a supervoid lies along the line of sight to the Cold Spot. CMB photons lose energy when they pass through voids. As a result, photons from this direction of space would appear colder.

This explanation seemed all the more likely when, in 2014, István Szapudi (University of Hawai‘i) and colleaguesdiscovered a supervoid in the direction of the Cold Spot. They found that the void, the largest yet discovered, spanned about 1.8 billion light-years 11.1 billion years after the Big Bang. However, initial calculations showed that the supervoid didn’t fully account for the Cold Spot’s temperature drop.

New Galaxy Survey, New Questions

A new galaxy survey casts further doubt. Astronomers operating the 2dF-VST ATLAS Cold Spot galaxy redshift survey (phew, say that three times fast!) mapped the positions of almost 7,000 galaxies along the line of sight toward the 10°-wide Cold Spot core.

The survey is admittedly sparse — there are a lot more than 7,000 galaxies along that line of sight. But from the data, Ruari Mackenzie (University of Durham, UK), Szapudi, and colleagues detected three voids. The biggest is a tad closer (and therefore older, 11.9 billion years after the Big Bang) and smaller than the 2014 result suggested, and it appears to contain even fewer galaxies than Szapudi’s team had estimated.

Nevertheless, even all three voids are not enough: Mackenzie and colleagues calculate the voids' cooling effect to be just 9 microkelvin — that’s a far cry from explaining the 70 microkelvin temperature drop in the Cold Spot’s core.

So is the Cold Spot supervoid-created, or not? The team members disagree. One complication is the control field. In many scientific studies, a control serves to test what would happen if the theory weren’t true — if, say, the medication weren’t applied in a medical study. In this case, the team has selected as a control a large, average-mottled field of CMB sky. Along this line of sight, the team finds voids of similar size to the Cold Spot supervoid — but no Cold Spot to go along with them.

Based on this control, some on the team conclude the supervoid isn’t to blame, opting for exotic solutions instead (more on that below). However, Szapudi cautions, the control field also contains a big galaxy cluster, and the Cold Spot field does not. That cluster’s heating effect would offset the voids’ cooling effects. “We absolutely do not expect that [this region of space] would project a cold spot on the CMB,” he says.

This figure compares the 3D galaxy distribution in the foreground of the Cosmic Microwave Background's Cold Spot (black points) to the galaxy distribution in an area with no background Cold Spot (red points). The number and size of low galaxy density regions in both areas are similar.
Durham University, UK

The Alternatives

Still, the alternatives are fun to consider — and who knows, they might even prove to be true.

One option is that the early universe went through a phase transition, much like water freezing into ice as it cools. If that happened, it may have left a defect in the CMB, akin to an imperfect snowflake’s not-quite-crystalline pattern. Cosmic inflation ought to have smoothed out any such defects, though, making them a less viable explanation.

Other options involve going beyond the standard theory of inflation. Studies of the CMB have so far validated this theory, so if astronomers want to keep it around, then they have to resort to something pretty crazy to explain the Cold Spot — a collision with another universe.

According to most versions of the inflation paradigm, this exponential expansion, once started, ought to keep going forever. It only stops in pockets, so-called bubble universes. If we live in one such bubble universe, there’s a chance that early in this bubble’s existence, it bumped into another, leaving a cosmic bruise on the CMB.

“The paradox [is] that the craziest-sounding of the exotic models for the explanation of the Cold Spot, the bubble universe collision model, is actually the most standard in terms of the inflation model,” says study coauthor Tom Shanks (University of Durham, UK).

Only time (as well as more galaxy surveys and tests of inherent assumptions) will tell.