But Lim found that as the speed continues to increase, the pattern eventually reverses: The soliton interaction begins to decrease. By the time they are traveling at the speed of light, there is no interaction at all. “They just fly right past each other,” Lim said. “The faster you collide two solitons, the simpler they become.” The lack of interactions makes it easier to model the dynamics of colliding solitons, as well as colliding bubble universes with solitons as their “edges,” since the systems are roughly similar.

According to Johnson, Lim has uncovered a very simple rule that can be applied broadly: Multiverse interactions are weak during high-speed collisions, making it easier to simulate the dynamics of those encounters. One can simply create a new model of the multiverse, use solitons as a tool to map the new model’s expected signatures onto cosmic microwave data, and rule out any theories that don’t match what researchers see. This process would help physicists identify the most viable models for the multiverse, which — while still speculative — would be consistent both with the latest observational data and with inflationary theory.

The Multiverse’s Case for String Theory

One reason that more physicists are taking the idea of the multiverse seriously is that certain such models could help resolve a significant challenge in string theory. One of the goals of string theory has been to unify quantum mechanics and general relativity, two separate “rule books” in physics that govern very different size scales, into a single, simple solution.

But around 10 years ago, “the dream of string theory kind of exploded,” Johnson said — and not in a good way. Researchers began to realize that string theory doesn’t provide a unique solution. Instead, it “gives you the theory of a vast number of worlds,” Weinberg said. A common estimate — one that Weinberg thinks is conservative — is 10500 possibilities. This panoply of worlds implies that string theory can predict every possible outcome.

The multiverse would provide a possible means of incorporating all the different worlds predicted by string theory. Each version could be realized in its own bubble universe. “Everything depends on which part of the universe you live in,” Lim said.

Peiris acknowledges that this argument has its critics. “It can predict anything, and therefore it’s not valid,” Peiris said of the reasoning typically used to dismiss the notion of a multiverse as a tautology, rather than a true scientific theory. “But I think that’s the wrong way to think about it.” The theory of evolution, Peiris argues, also resembles a tautology in certain respects — “an organism exists because it survived” — yet it holds tremendous explanatory power. It is a simple model that requires little initial input to produce the vast diversity of species we see today.

A multiverse model tied to eternal inflation could have the same kind of explanatory power. In this case, the bubble universes function much like speciation. Those universes that happen to have the right laws of physics will eventually “succeed” — that is, they will become home to conscious observers like ourselves. If our universe is one of many in a much larger multiverse, our existence seems less unlikely.

Uncertain Signals

Ultimately, however, Peiris’ initial objection still stands: Without some means of gathering experimental evidence, the multiverse hypothesis will be untestable by definition. As such, it will lurk on the fringes of respectable physics — hence the strong interest in detecting bubble collision signatures in the CMB.

Of course, “just because these bubble collisions can leave a signature doesn’t mean they do leave a signature,” Peiris emphasized. “We need nature to be kind to us.” An observable signal could be a rare find, given how quickly space expanded during inflation. The collisions may not have been rare, but subsequent inflation “tends to dilute away the effects of the collision just like it dilutes away all other prior ‘structure’ in the early universe, leaving you with a small chance of seeing a signal in the CMB sky,” Peiris said.

“My own feeling is you need to adjust the numbers rather finely to get it to work,” Weinberg said. The rate of formation of the bubble universes is key. If they had formed slowly, collisions would not have been possible because space would have expanded and driven the bubbles apart long before any collision could take place. Alternatively, if the bubbles had formed too quickly, they would have merged before space could expand sufficiently to form disconnected pockets. Somewhere in between is the Goldilocks rate, the “just right” rate at which the bubbles would have had to form for a collision to be possible.

Researchers also worry about finding a false positive. Even if such a collision did happen and evidence was imprinted on the CMB, spotting the telltale pattern would not necessarily constitute evidence of a multiverse. “You can get an effect and say it will be consistent with the calculated predictions for these [bubble] collisions,” Weinberg said. “But it might well be consistent with lots of other things.” For instance, a distorted CMB might be evidence of theoretical entities called cosmic strings. These are like the cracks that form in the ice when a lake freezes over, except here the ice is the fabric of space-time. Magnetic monopoles are another hypothetical defect that could affect the CMB, as could knots or twists in space-time called textures.

Weinberg isn’t sure it would even be possible to tell the difference between these different possibilities, especially because many models of eternal inflation exist. Without knowing the precise details of the theory, trying to make a positive identification of the multiverse would be like trying to distinguish between the composition of two meteorites that hit the roof of a house solely by the sound of the impacts, without knowing how the house is constructed and with what materials.

Should a signature for a bubble collision be confirmed, Peiris doesn’t see a way to study another bubble universe any further because by now it would be entirely out of causal contact with ours. But it would be a stunning validation that the notion of a multiverse deserves a seat at the testable physics table.

And should that signal turn out to be evidence for cosmic strings or magnetic monopoles instead, it would still constitute exciting new physics at the frontier of cosmology. In that respect, “the cosmic microwave background radiation is the underpinning of modern cosmology,” Peiris said. “It’s the gift that keeps on giving.”

This article was reprinted on Wired.com.