Supervoids vs Colliding Universes!

A giant cold patch in the Cosmic Microwave Background (CMB)—about 150 microkelvins below average and roughly 10 degrees across—has long fueled speculation that it might be the imprint of a “supervoid” or even the scar of a collision between bubble universes. New analysis using galaxy data argues that the supervoid explanation falls short: the observed cold spot is too deep to be produced by the integrated effect expected from the voids seen along that line of sight.

The CMB is the afterglow of the early universe, released when the first atoms formed roughly 380,000 years after the Big Bang. It is remarkably uniform at about 2.725 Kelvin across the sky, with tiny temperature fluctuations at the level of about 20 microkelvins (one part in 100,000). The cold spot stands out as an unusually large and cold region in the direction of the southern constellation Eridanus, with a colder core and a surrounding hot rim.

One leading explanation traces the cold spot to the Integrated Sachs–Wolfe (ISW) effect. In a universe where dark energy drives accelerated expansion, photons passing through large underdense regions (voids) can lose energy in a way that shows up as a temperature decrement in the CMB. The idea was first proposed by Inoue and Silk in 2006: giant supervoids, aligned with our view, could imprint a cold signature as CMB photons traverse them.

To test this, McKinsey et al. used the Anglo-Australian Telescope in Outback New South Wales to survey about 7,000 galaxies toward the cold spot out to redshift 0.4, measuring their redshifts via spectroscopy to build a three-dimensional map of the matter distribution. That map suggested three, possibly four, supervoids in the relevant region. But when the expected ISW temperature shift from those voids was calculated, the result was only about 32 microkelvins—roughly one-fifth of the observed 150 microkelvin drop.

A key part of the analysis was a control region, labeled G23, aimed toward the star Fomalhaut. G23 contains a similar void structure plus some overdensities, and the predicted ISW effect there came out to a 14 microkelvin drop, matching the observed deviation of about 15 microkelvins. That agreement indicates the method can work when the underlying structure is present, strengthening the conclusion that the cold spot’s depth cannot be explained by the supervoids alone.

With the supervoid hypothesis weakened, attention shifts to other possibilities. One route is “modified gravity,” where the ISW effect might be misestimated because gravity behaves differently on large scales. The analysis notes this is a shaky foundation given growing evidence that dark matter behaves like real matter rather than a sign of incorrect gravity.

More speculative ideas involve the early-universe physics of inflation—such as topological defects or unusual reheating—or the most sensational scenario: bubble-universe collisions in an eternal inflation framework. In that picture, collisions between bubble universes could generate temperature gradients that appear as hot or cold spots in the CMB. The collision explanation remains fringe, but the new results keep it on the table by knocking down a more conventional mechanism.

Still, the simplest possibility remains that the cold spot is a statistical fluke: simulations suggest a feature of this size could occur in about 1 in 50 universes. More detailed CMB observations in the region are needed to determine whether the cold spot is truly anomalous in a way that demands new physics—or just an unusually rare fluctuation.