The Cosmic Microwave Background, the faint afterglow of the Big Bang detectable across the sky, contains a strange Cold Spot — a region measurably cooler than its surroundings — and while some physicists have wondered whether it could be the scar of a collision with another universe, others argue it may be a rare statistical fluctuation or the imprint of enormous cosmic structure.

The Cosmic Microwave Background is the oldest light there is, the faint heat left over from the Big Bang, released when the young universe first cooled enough to turn transparent. It reaches us from every direction at almost exactly the same temperature, about 2.7 degrees above absolute zero, freckled with tiny variations a hundred-thousandth of a degree across.

One patch breaks the pattern. In the direction of the constellation Eridanus sits a region colder and larger than the usual freckling, known simply as the Cold Spot. It is real, it has been confirmed by more than one telescope, and the argument is over what made it.

What the Cold Spot is

The background has been mapped in steadily finer detail by a run of missions, from COBE in the 1990s to WMAP and then the European Space Agency’s Planck satellite. Against that map, the temperature ripples are minute, a matter of tens of millionths of a degree either side of the average.

The Cold Spot is a dip of roughly 150 millionths of a degree below the mean, spread across a circle several degrees wide on the sky. It first stood out in WMAP data around 2004 and was later confirmed by Planck. What makes it notable is not that it is cold, since cold patches are expected, but that a region this cold and this broad sits awkwardly with how often the standard theory says such a thing should appear.

The simplest answer is chance

In the standard model of cosmology, the ripples in the background are essentially random, the frozen-in trace of quantum fluctuations stretched to enormous size. On that picture, a spot as cold and wide as this one is rare, but rare is not the same as impossible.

How rare is genuinely disputed, partly because of how the question is asked. We noticed the Cold Spot because it is the most extreme feature on the map, and judging the odds of the most extreme thing after you have already singled it out is a slippery business. Some analyses find the Cold Spot consistent with chance. Others put the odds of a fluke low, at a few per cent. The point is not settled.

The ordinary culprit that fell short

The most studied physical explanation is a vast hole in the distribution of galaxies. In 2015, a team led by Istvan Szapudi reported a supervoid aligned with the Cold Spot, an underdense region on the order of a billion light-years across, lying between us and the distant background.

Such a void can cool the light passing through it, by something called the integrated Sachs-Wolfe effect. A photon falls into the void and climbs out the far side, and because the universe expands while it is crossing, it does not quite recover the energy it gave up on the way in. It arrives a little cooler.

The trouble is the size of the effect. A void like the one found should chill the background by around 20 millionths of a degree, far short of the 150 observed. A 2017 galaxy survey led by Ruari Mackenzie went further, finding the voids in that direction not enough to account for the Cold Spot at all, and that similar voids sat along other lines of sight where no anomaly appears. Large-scale structure may contribute something, but on current evidence it does not do the whole job.

The exotic idea, and why to be careful

Then there is the possibility that gives the Cold Spot its headlines. In some versions of inflation, the burst of expansion thought to have begun the universe, our cosmos is one bubble among many, and bubbles can collide. A collision early on, the argument goes, might leave a mark on the background, and the Cold Spot has been put forward as a candidate for one.

It is worth being plain about the standing of this idea. It is a speculation, not a result. Searches for the particular signature a bubble collision would leave have not produced a convincing case, and the researchers whose void work prompted the multiverse headlines were careful to say their results said nothing about a multiverse in either direction. The coverage ran ahead of the science.

What to watch

The way forward is to map the matter between us and the background more thoroughly. Galaxy surveys such as the Dark Energy Survey have already taken the measure of the Eridanus region, and larger surveys to come will tighten the estimate of how much, if anything, a void there contributes.

Alongside it sits a quieter question, of whether the Cold Spot and the handful of other background anomalies are real features the standard model must explain, or the kind of coincidences that surface whenever a rich map is examined closely enough.

For now, the honest summary is short.

The Cold Spot is real, its cause is unresolved, and the ordinary explanations are still ahead of the exotic one.