The four candidate “bruises” are in the lower-right quadrant of this all-sky map of the CMB, in green, light blue, red and orange (edge of image)

Update on 16 August 2011: The researchers ran additional statistical checks on the CMB data, looking at the probability that the bubbles would appear anywhere on the sky. Lead author Stephen Feeney says: “The current data favour no bubble collisions. However, a non-zero number of bubble collisions is still allowed, and there are four patches in the WMAP data where [signals of possible bubble universes] are higher than anything we expect from systematic errors due to instrumental effects, foreground-removal artefacts etc. With data from Planck we expect our pipeline to be sensitive to much weaker collision signals, so we should be able to test whether there is something there or whether they’re just weird patches of CMB.” The updated analysis appears in Physical Review Letters (DOI: 10.1103/PhysRevLett.107.071301).

Original story published 17 December 2010:

Collisions between our cosmos and other universes may have left round “bruises” in a map of ancient cosmic radiation.


Our universe is thought to have expanded rapidly in a process called inflation in the first moments after the big bang. Some physicists suspect inflation is still happening, starting up in some regions while stopping in others, such as the part of the universe we live in. In this picture, called eternal inflation, new universes are continually popping into existence like bubbles in a vast, expanding sea of space-time.

Many of these universes should be carried away from one another as soon as they form. But universes born close together could collide if they are expanding faster than the space between them.

If our universe was hit by another bubble universe, the impact would release colossal bursts of energy. If this occurred before inflation ended in our patch of the universe, it could leave an imprint that might still be detectable today. Now Stephen Feeney of University College London and colleagues say they may have spotted such imprints in the cosmic microwave background (CMB), the all-sky glow that comes from photons emitted when the universe was less than 400,000 years old.

Hot and cold

A collision would alter how long inflation lasted in the impact zone. If the expansion continued for longer than it otherwise would, the density of matter in the impact zone would be lower than in surrounding regions. This would show up as a cold spot in the CMB. Conversely, a shorter period of inflation would create a warm spot in the CMB.

The team calculated the likely temperature profiles for such impacts and searched for them in CMB data from NASA’s Wilkinson Microwave Anisotropy Probe.

The search turned up four circular patches, each spanning an area of sky equivalent to at least eight full moons (arxiv.org/abs/1012.1995 and arxiv.org/abs/1012.3667). One is a cold spot that had already been cited as evidence of another universe interacting with our own.

“There’s no obvious, boring explanation for the features,” says team member Matthew Johnson of the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

Calling cards

If collisions with other universes did indeed create these patches, they should have left other calling cards in the CMB, such as a telltale signature in the orientation, or polarisation, of CMB photons. The European Space Agency’s Planck satellite, which launched in 2009, should be able to detect these signs. Its first full maps of the sky are expected in 2012.

Even if just one of these spots turns out to be a bubble collision, it would be “a discovery of the first magnitude”, says Thomas Levi of the University of British Columbia in Vancouver, Canada. The finding would bolster theories – such as string theory – that call for a vast number of universes with different properties.

“It is encouraging they found some candidates,” says Alexander Vilenkin of Tufts University in Medford, Massachusetts. But he adds that even if bubble universes exist, they might not form at a rate that would guarantee one would have collided with our universe.