Space & Cosmic

The CMB Cold Spot: The Anomalously Cold Patch in the Afterglow of the Big Bang

By The Unsolved Report Editorial Team · Last reviewed 2026-06-19

A large patch of the cosmic microwave background is colder than it should be. Here is the real evidence, the supervoid explanation, and what stays unsolved.

Spread across the entire sky is the oldest light in existence: the cosmic microwave background, a faint glow released when the universe was just 380,000 years old. It is remarkably uniform, the same temperature in every direction to within a hundred-thousandth of a degree. But within those tiny variations lies an oddity that has intrigued cosmologists for two decades. In the direction of the constellation Eridanus sits a patch that is colder than it has any statistical right to be, and unusually large. It is called the Cold Spot, and it is one of the most studied anomalies in cosmology.

What the cosmic microwave background is

To understand why the Cold Spot is interesting, you have to understand the canvas it sits on. The cosmic microwave background (CMB) is the thermal afterglow of the hot, dense early universe. As the universe expanded and cooled, it eventually became transparent, and the light from that moment has been traveling ever since, stretched by cosmic expansion into the microwave band.

That light is astonishingly smooth, but not perfectly so. It carries faint temperature fluctuations, hotter and colder spots differing by mere millionths of a degree. These fluctuations are the seeds of all later structure - galaxies, clusters, the cosmic web - and their statistical pattern is one of the great confirmations of the standard cosmological model. Satellites including NASA's WMAP and ESA's Planck mapped these fluctuations across the whole sky with exquisite precision.

In the standard picture, those fluctuations should be random in a specific statistical sense: spots of various sizes and temperatures, distributed without preferred locations, like static on an old television tuned just so. The Cold Spot is interesting because it appears to be a deviation from that expected randomness.

The anomaly itself

The Cold Spot was first identified in WMAP data in the mid-2000s and later confirmed in the independent, higher-resolution Planck maps, so it is not an instrument artifact peculiar to one telescope. Its notable features:

Size: it is a large-scale feature, spanning roughly 5 to 10 degrees on the sky, far larger than the typical fluctuations.
Temperature: it is colder than the average CMB temperature by a small but, for a feature this size, surprising amount, on the order of 70 microkelvin below the mean at its center.
Surroundings: part of what makes it stand out is that the cold region is ringed by a comparatively warm halo, an unusual combination.

None of this would be remarkable for a small spot. The puzzle is the combination of size and coldness. A feature this large and this cold is rare in the simulations of a standard, random CMB sky. Estimates of how unusual it is vary depending on exactly how you define and search for it, which is itself part of the debate.

The honest statistical caveat

Before reaching for exotic explanations, scientists confront a subtle but crucial issue known as the "look-elsewhere effect," or a posteriori statistics. If you scan an entire sky full of random fluctuations and then point to the single most extreme feature you find, it will always look improbable in isolation, because you searched everywhere and selected the outlier.

This matters enormously for the Cold Spot. Some analyses argue that once you account properly for the fact that the Cold Spot was found by searching the whole sky, its significance shrinks, and it becomes a mild oddity rather than a glaring contradiction. Other analyses, using specific filtering techniques, maintain that it remains genuinely unlikely. So the very first open question is not what causes the Cold Spot but how anomalous it truly is. Reasonable cosmologists disagree.

The leading conventional explanation: a supervoid

The most discussed natural explanation involves a giant under-dense region of space lying between us and the CMB, in the direction of the Cold Spot. Surveys have indeed identified a large under-dense region in that direction, sometimes called the Eridanus supervoid, a region perhaps a billion or more light-years across where galaxies are noticeably scarcer than average.

The physical mechanism connecting a void to a cold spot is the integrated Sachs-Wolfe effect. The idea, in plain terms:

A photon of CMB light falls into a region of weaker gravity (a void), gaining a little energy as it enters.
It should give that energy back as it climbs out the far side.
But because the universe is expanding, and that expansion is accelerating due to dark energy, the void's gravitational well subtly flattens during the time the photon is crossing it.
The photon therefore climbs out of a shallower well than it fell into, ending up with slightly less energy than it started with, making that patch of sky look slightly colder.

It is an elegant idea, and the supervoid in that direction is real. But here is the key honest point: detailed studies, including analyses using the Dark Energy Survey, have concluded that the supervoid, by itself, is not large or empty enough to produce the full depth of the Cold Spot under standard physics. The most-cited estimates suggest a void of the observed size could account for only a fraction, perhaps around a fifth, of the anomaly. So the supervoid is a contributor, but probably not the whole story.

So what is left?

Given that the supervoid explanation is incomplete, cosmologists are left with a few live possibilities, and it is worth being candid that none is fully settled.

It is a rare but real statistical fluctuation. Under the standard model, extreme features do occasionally occur. The Cold Spot might simply be a rare cold patch that happens, by coincidence, to sit roughly behind a real supervoid. Combined with the look-elsewhere caveat, many cosmologists regard this as the most likely answer.
The integrated Sachs-Wolfe effect from the void is stronger than standard physics predicts. If voids cool the CMB more than expected, that would point to something interesting about dark energy or gravity on large scales, but this is speculative and not established.
More exotic ideas. Over the years, more dramatic hypotheses have been floated, including the notion that the Cold Spot is an imprint left by a collision between our universe and another "bubble" universe in a multiverse scenario.

That last idea deserves a clear warning label. The cosmic-collision or multiverse interpretation is highly speculative, not supported by direct evidence, and far outside mainstream consensus. It is the kind of idea that makes for striking headlines but rests on assumptions that cannot currently be tested. Responsible coverage should treat it as a fringe hypothesis, not a leading explanation. The serious scientific debate is between "rare statistical fluctuation" and "under-dense region plus possibly enhanced ISW," not between universes colliding.

The Cold Spot is not the only large-scale oddity

The Cold Spot is the most famous CMB anomaly, but it belongs to a small family of large-scale features that have prompted discussion. Together they are sometimes called the "CMB anomalies," and they include a hemispherical power asymmetry, in which fluctuations appear slightly stronger on one half of the sky than the other, and an apparent alignment of some of the largest-scale features that has been nicknamed the "axis of evil." Like the Cold Spot, these were noted in WMAP data and persisted in Planck data.

It is worth keeping these in perspective. Each anomaly, taken alone, is only mildly improbable, and each is subject to the same look-elsewhere caveat: with a whole sky to search and several different statistical tests to apply, finding a few unusual-looking features is partly expected even in a perfectly standard universe. Cosmologists debate whether the anomalies collectively hint at something beyond the standard model or whether they are simply the kind of chance features any random sky will throw up. The honest current position is that none of them, including the Cold Spot, rises to the level of overturning Lambda-CDM, but they are watched closely as the data improve.

What better data could settle it

Progress here depends partly on mapping the foreground structure more completely. If the Cold Spot is largely caused by intervening under-dense regions through the integrated Sachs-Wolfe effect, then ever-deeper galaxy surveys in that direction should pin down exactly how much cooling the voids can produce. The Dark Energy Survey work on the Eridanus region was a step in this direction. Future large surveys, including those from the Vera C. Rubin Observatory and the Euclid space telescope, will map the three-dimensional distribution of matter along that line of sight in far greater detail.

If those maps show that the under-dense structure can account for the Cold Spot under standard physics, the anomaly largely dissolves into ordinary cosmology. If a stubborn gap remains between what the voids can do and how cold the spot actually is, that gap becomes a more pointed question. Either way, the resolution will come from better measurement, not from speculation, which is exactly how anomalies should be handled.

Established versus open

Established:

The Cold Spot is a real feature of the CMB, seen consistently in both WMAP and Planck data.
It is unusually cold and large compared with typical fluctuations.
A genuine large under-dense region (a supervoid) exists in roughly that direction.

Open:

How statistically anomalous the Cold Spot truly is once the look-elsewhere effect is accounted for.
Whether the supervoid plus standard physics can explain it, or whether it is mostly a coincidental rare fluctuation.
Whether any non-standard physics is implicated (unresolved and not required by current evidence).

Why it matters and why it does not require aliens or other universes

The Cold Spot is a useful case study in how science handles anomalies responsibly. It is real, it is measured by independent instruments, and it is mildly puzzling. But "mildly puzzling" is not the same as "unexplained by known physics," and it is certainly not evidence of exotic phenomena. The most probable explanations are mundane: a partial contribution from a real cosmic void, plus the ordinary fact that random skies sometimes produce rare features.

That is the honest, evidence-first picture. The Cold Spot sits at the frontier where our maps of the early universe are precise enough to notice small oddities, and where good scientists argue carefully about whether an oddity is a clue to new physics or simply the universe being slightly lumpy in the way statistics allows. Both possibilities are interesting. Neither requires anything beyond careful cosmology.

Sources & further reading

Wikipedia - CMB cold spot - https://en.wikipedia.org/wiki/CMB_cold_spot
Wikipedia - Cosmic microwave background - https://en.wikipedia.org/wiki/Cosmic_microwave_background
ESA - Planck mission - https://www.esa.int/Science_Exploration/Space_Science/Planck
NASA - WMAP mission - https://wmap.gsfc.nasa.gov/
Kovacs et al. 2022, DES view of the Eridanus supervoid and the CMB Cold Spot, MNRAS - https://academic.oup.com/mnras/article/510/1/216/6468992
Wikipedia - Sachs-Wolfe effect (integrated Sachs-Wolfe) - https://en.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect