“Fifteen years ago, when you talked about the multiverse, the attitude of many physicists was just ridicule,” says Alexander Vilenkin, professor of physics and director of the Institute of Cosmology at Tufts University. “But there has been a great change in attitude.” The concept of a multiverse is controversial and, at times, contentious. Today, many scientists are understandably skeptical. A few even reject the notion. But for others like Vilenkin, the arguments for the multiverse are so compelling, they demand science take it seriously.
Fewer than 100 years ago, people thought the Milky Way was the whole universe. Then it was discovered that galaxies exist beyond the confines of our home galaxy. Now, it is thought that the Big Bang might not be the singular beginning of all space and time. There may have been other events producing other universes: a multiverse.
What lies beyond the visible bounds of our universe. “When I teach elementary courses, I usually say the universe is everything that there is,” said Vahe Petrosian, a Stanford professor of physics and of applied physics. “If you go to the edge of it and there is still some more, that is part of the universe, too.”
In late 2015, Vilenkin and his colleagues proposed another way to determine if the multiverse exists: black holes. If our universe is just one of an infinite number, then once inflation stopped in ours, pockets within it that had been inflating would have then collapsed into black holes. The longer each pocket inflated, the more massive the black hole.
The paper co-authored by Vilenkin describes the possibility that bubble universes were created inside of the visible universe during the Big Bang. Many of these universes collapsed and formed black holes, Vilenkin said. If the black holes are big enough, they may have inflating universes inside of them, and these expanding universes would be connected to the visible universe by wormholes.
Inflation would thus leave behind a population of black holes with a telltale range of masses. In principle, by measuring the ripples in space and time produced in black hole collisions—like the gravitational waves discovered by LIGO last year—astronomers can take a census of black hole masses and see if they were created by inflation, which would imply the multiverse.
But there is also a universe that is limited by what people can see: the visible universe. Around 13.8 billion years ago there was a Big Bang, a really hot, dense phase when the universe started expanding. Everything in the visible universe has to be younger than that. But, theoretically, people suspect that the universe seen with telescopes – what we see within the horizon of the Big Bang – may not be all there is.
“You say this is the beginning, and then you ask, ‘What was before that?’ If you go to some edge, there is always the question, ‘Is there something beyond that edge?'” Petrosian said.
The theory of eternal inflation, proposed in part by Vilenkin, could begin to answer these questions. Eternal inflation says that space is always expanding overall, but some pockets of space will expand and create universes while others stop expanding. The universes that form are called “bubble universes” because they bubble up where energy is being concentrated.
“We used to think that beyond the visible universe there was simply more of the same,” said Vilenkin. “More planets and stars and galaxies. But other universes may have different physical laws.”
Not all bubble universes are created equally. If the mass of a proton or electron were tweaked, the universe might not have stars, planets or life. Some of the universes expand, contract and collapse in a very short time before forming everything. Some universes are like ours.
It’s possible that sometimes these bubbles interact, and that one will interact with our bubble and produce observational evidence. “Say, what are the possibilities, what happens if these bubbles interact, what sort of information will it give us,” says Petrosian.
Despite the similarities between Vilenkin’s theory and the film Interstellar, many scientists have hope for the multiverse theory.
“Once a reasonable idea comes, you can never say it’s wrong,” affirms Petrosian. “And this is not too crazy. We will probably never have answers to these questions, but it is important to ask them. So we do ask, and sometimes we are successful.”