Ivan Baldry, an astronomer at Liverpool John Moores University in England, who was not involved in the study, says the multiverse is like Occam’s razor, that philosophical idea that the simplest explanation is often the correct one. “We have a huge variety of galaxies and a huge variety of solar systems — it makes sense to have a huge variety of universes,” he says.
That isn’t as crazy as it sounds. “I think when you use the phrase parallel universe, people imagine some sort of portal into the underworld or some other dimension,” says Matthew Kleban, an astronomer at New York University. Instead, he likes to think of a multiverse as a series of islands.
Take Hawaii, as an example. From Maui, you might not be able to see any other islands beyond the horizon. In fact, you might not even know they exist until an exotic tree — one that clearly doesn’t grow on your island — happens to land on your beach one day. Evidence of a nearby universe would be no different than stumbling upon that tree.
Or it might be more accurate to think of the multiverse as bubbles forming in a glass of champagne. Our universe is just one of those tiny bubbles.
X Marks the Spot
The story starts to sound like science fiction when you ponder what would happen if two universes bumped into each another. After all, bubbles in a glass of bubbly collide all the time. It’s also true that if enough universes popped into existence, some might form close enough to collide with our own — leaving a temperature bruise on the cosmic microwave background that astronomers might be able to detect.
Such a collision would send a pulse of energy propagating across the universe like the wake from a ship, says Kleban, who has created detailed predictions of the collision’s effect.
“If you had something like one of these cosmic wakes, which is a kind of ripple that cuts across the universe at that time, you'll see a particular pattern in the sky,” he says. And because the CMB is a relic leftover from the early universe, that pattern imprinted on the CMB should look something like the unexplained cold spot.
"When you use the phrase parallel universe, people imagine some sort of portal into the underworld or some other dimension."
The fact that astronomers can now rule out one of the cold spot’s few plausible explanations increases the likelihood that it’s the result of a bubble collision, Kleban says. But he notes that it’s hard to draw a definitive conclusion — mostly because such an extraordinary claim would require extraordinary evidence. “This doesn’t rise to that level, but it’s perhaps a step on the way,” he says. “If this is a bubble collision then I expect more evidence to accumulate, at least I hope.”
That evidence might come soon. Astronomers are currently pulling together a map of the CMB that would also reveal light that’s polarized, or aligned, like light reflected off a lake. Kleban suspects that the microwaves will reflect off one of these cosmic wakes in a similar manner, thus revealing further evidence of a past cosmic collision.
Related: Misbehaving Particles May Answer Key Questions About Big Bang
With the map slated to come out as early as this year, Kleban is hopeful that we will find evidence of another universe — and soon. “It would be one of the most important scientific discoveries in history,” he says, comparing it to the Copernican revolution, when we realized that Earth was not the center of our solar system. “It tells us that we're even less significant, smaller and less central than we thought we were. So in that regard, I think it would be an amazing discovery. It would have a very big impact on theoretical physics. But more than that, I think it would have a big impact on people’s imaginations.”
But Shanks worries about such a discovery. “If I find the pattern that says there is a multiverse that immediately devalues our observation because it could be that this is just one of the billions of universes that accidentally show that pattern,” he says. By proving a multiverse you ultimately prove that it could just a statistical fluke — because, with so many universes out there, it might very well be. “So you’re in a catch-22 situation.”
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