If you thought the cosmos was a lot to take in, think again. Theorists have long suggested that our universe is just one of many that exist in a complex "multiverse." Now researchers have found hints that this may actually be the case.
The researchers, who claim to be the first to search observational data for the presence of a multiverse, cannot yet prove that our universe is one of many. However, their analysis, published  last Friday on the arXiv preprint server , implies that more-precise data could confirm the existence of a multiverse. "It's incredibly exciting to think there is even a chance that actual observational evidence for a multiverse might be found in our lifetimes," says Alan Guth, a cosmologist at the Massachusetts Institute of Technology in Cambridge, who was not involved in the study.
The possibility of a multiverse comes from inflation theory, the idea that our universe went through a rapid expansion shortly after the big bang. Pioneered by Guth and others, inflation theory does a good job of explaining why space is fairly smooth. But researchers can't explain what started the expansion and what stopped it. These problems have led theorists to consider the possibility that inflation could occur at other places and times, generating new universes parallel to our own.
Cosmologist Hiranya Peiris of University College London and colleagues decided to test for a multiverse by examining the cosmic microwave background (CMB) radiation, a remnant of the big bang that provides a map of what the universe looked like some 380,000 years into its existence. If a nearby universe had somehow interacted with our universe before that time, it might have left its imprint on the CMB for us to discover almost 15 billion years later. According to Peiris's group, the imprint would have certain recognizable features, such as distinct edges and symmetrical shapes.
To search for such features, the researchers devised a computer algorithm that analyzed CMB data recorded by NASA's Wilkinson Microwave Anisotropy Probe. The algorithm found data that was consistent with the type of features generated by a collision between universes. Although not a discovery as such, it is a hint that suggests that a more definitive result could be found with higher-resolution observations, such as those from the Planck satellite launched last year. "We didn't expect to find anything, and we instead found these intriguing hints even in current data," says Peiris.
Alexander Vilenkin, a cosmologist at Tufts University in Medford, Massachusetts, who helped develop inflation theory, thinks it is an "encouraging" result. "If indeed we find some features in the CMB which are well fitted by a collision but are hard to explain otherwise, this would be a tremendously important development," he says.
Yet the news is not all good. The CMB, a mostly random pattern, is notoriously open to so-called a posteriori observations—that is, you can see in it what you want to see. In one famous case, scientists claimed to make out the initials of the British physicist Stephen Hawking. Although Peiris's group calibrated its algorithm to avoid such mistakes, there is always the chance that the features could have a more mundane origin. "Unless nature reveals some exceptional feature in the CMB, it will be difficult to discriminate too many details about the dynamical processes occurring in the early universe," says Arjun Berera, a cosmologist at the University of Edinburgh in the United Kingdom.
This article has been corrected to note that the researchers' algorithm analyzed all of the highest resolution CMD data provided by WMAP, not a small portion, as previously stated.