According to Kurt Vonnegut, “The universe is a big place, perhaps the biggest.”
However, it may not be the only place, nor even the biggest.
How can we tell if another universe has collided with our own? Physicists in Canada and the US believe they have the answer – it would leave “a unique and highly characteristic” imprint in the microwave background that pervades the cosmos. The physicists claim that the prediction can be tested using existing and future space telescopes, which contradicts a widespread view that the existence of a multiverse is untestable.
Chuck Bennett, an astrophysicist at Johns Hopkins University in Maryland, US, who was not involved with the study, believes the prediction helps bring multiverse theory into the realms of conventional, falsifiable science. “Science relies on being able to falsify ideas through experiment or observations of nature,” he says. “The fact that these potentialities exist enables us to call this ‘science’. That, to me, is a significant statement.”
The possibility of a multiverse comes from both string theory and inflation theory, the idea that our universe underwent a rapid expansion just after the Big Bang. Inflation theory does a good job of explaining why space is fairly smooth on large scales, but researchers can’t explain what started the expansion and what stopped it. These problems have led physicists to consider the possibility that inflation could occur at other places and times, generating new universes in addition to our own.
In order for a scientific hypothesis to be more than just theoretical speculation, there need to be testable predictions. Most people have assumed that even if other universes exist in a multiverse, there would be no way that we could detect evidence of their existence. However, that’s a speculative assumption too.
Three researchers have now suggested where and how to look for evidence.
If universes in the multiverse are like bubbles in champagne, they may occasionally collide. And if so, a collision need not be catastrophic, but might leave its marks, which the researchers call “wakes”. The evidence would be found in the cosmic microwave background (CMB), in the form of patterns of polarization of the microwave photons. It’s probably a long shot… but then, the existence of the CMB itself was a very speculative idea for over 15 years. So speculative that it had been all but forgotten, before it was discovered. (The discoverers got a Nobel Prize. The scientists who proposed the idea got… nothing.)
Current theories of the origin of the Universe, including string theory, predict the existence of a multiverse containing many bubble universes. These bubble universes will generically collide, and collisions with ours produce cosmic wakes that enter our Hubble volume, appear as unusually symmetric disks in the cosmic microwave background (CMB) and disturb large scale structure (LSS). There is preliminary observational evidence consistent with one or more of these disturbances on our sky. However, other sources can produce similar features in the CMB temperature map and so additional signals are needed to verify their extra-universal origin. Here we find, for the first time, the detailed three-dimensional shape and CMB temperature and polarization signals of the cosmic wake of a bubble collision in the early universe consistent with current observations. The predicted polarization pattern has distinctive features that when correlated with the corresponding temperature pattern are a unique and striking signal of a bubble collision. These features represent the first verifiable prediction of the multiverse paradigm and might be detected by current experiments such as Planck and future CMB polarization missions. A detection of a bubble collision would confirm the existence of the Multiverse, provide compelling evidence for the string theory landscape, and sharpen our picture of the Universe and its origins.