The CMB Cold Spot

Summary

The cosmic microwave background is the oldest light there is, and it is astonishingly smooth. Its temperature varies from place to place by about one part in a hundred thousand.

In one patch of sky, about five degrees across, it is colder than it should be.

Various things have been proposed to explain it, including a collision with another universe.

The registry's position is that before any of that, somebody needs to establish that it needs explaining, and that this has not been done, and that the statistical argument against it is one of the most useful things in this archive.

What is documented

The map. The cosmic microwave background was mapped by WMAP and then, in far greater detail, by Planck. It is the residual glow of the hot early universe, released about three hundred and eighty thousand years after the Big Bang.

The Cold Spot. A region in the southern galactic hemisphere, in the direction of Eridanus, roughly five degrees across, which is measurably colder than the surrounding background.

It was identified in WMAP data in 2004 and it is present in the Planck data.

Its significance. Around three standard deviations, depending on how the analysis is done. This is unusual. It is not overwhelming.

The supervoid. In 2015, István Szapudi and colleagues reported the discovery of a very large underdense region, a supervoid, lying between us and the Cold Spot, in roughly the right direction and of very large extent.

This was reported as the explanation, and there is a mechanism by which it could be one. Light passing through an expanding void loses a little energy: it falls in, gains energy, and then has to climb out of a void that has grown while it was crossing, so it loses more than it gained. This is the integrated Sachs-Wolfe effect, and it is real.

But it does not do enough. Subsequent analysis, notably by Ruari Mackenzie and colleagues in 2017, examined the region in detail and concluded that the void, though it exists, is not large enough or deep enough to produce the observed cooling by this mechanism.

The void is real. It is not sufficient.

Leading explanations

The look-elsewhere effect. This is the registry's leading candidate, and it is not exotic at all.

Here is the argument, and it is important enough to set out slowly, because it applies to a very great deal of this archive.

The cosmic microwave background contains random fluctuations. It is supposed to contain random fluctuations. The whole theory predicts them.

In any random field, there will be a coldest patch. There has to be. Somewhere in that map, some region is going to be the coldest one, and by the nature of the thing, it will be quite a lot colder than average, because that is what being the coldest means.

Now: scan the whole sky, identify the coldest spot, and then ask how likely it is that a spot that cold would occur by chance.

That question is rigged. You have already searched the entire sky for the most extreme thing in it, and you are now asking how improbable that extreme thing is, without accounting for the fact that you searched.

This is the look-elsewhere effect, and it is the reason particle physicists demand five standard deviations before announcing anything, and it is why a three-sigma anomaly that was found by looking for anomalies is not worth very much.

Several analyses argue that when the search is properly accounted for, the Cold Spot's significance largely evaporates.

A supervoid. The void exists and does not appear sufficient.

Cosmic texture, or a topological defect. Proposed. Not established.

A collision with another universe. Proposed. The registry notes that it accounts for a three-sigma cold patch by invoking an entire additional universe, and that this is a poor trade.

Current status

Partially explained. A real feature exists in the map. A supervoid exists in that direction and appears insufficient to account for it. And a substantial body of statistical argument holds that the feature may be exactly as anomalous as the coldest patch in a random field is required to be, which is to say, not anomalous at all.

The registry keeps this record chiefly for the look-elsewhere effect, which is the single most useful idea in it and which applies far beyond cosmology.

If you go looking for the strangest thing in a large body of data, you will find one. It will be strange. And it will tell you nothing, unless you can say how hard you looked.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.