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.
The Astronomical Research Center (A.R.C) mentioned that 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.