The multiverse is one of the most divisive topics in physics, and it just became more so. The major announcement last week of evidence for primordial ripples in spacetime has bolstered a cosmological theory called inflation, and with it, some say, the idea that our universe is one of many universes floating like bubbles in a glass of champagne. Critics of the multiverse hypothesis claim that the idea is untestable—barely even science. But with evidence for inflation theory building up, the multiverse debate is coming to a head.
The big news last week came from the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) experiment at the South Pole, which saw imprints in the cosmic microwave background—the oldest light in the universe, dating from shortly after the big bang—that appear to have been caused by gravitational waves rippling through the fabric of spacetime in the early universe. The finding was heralded as a huge breakthrough, although physicists say confirmation from other experiments will be needed to corroborate the results.
If verified, these gravitational waves would be direct evidence for the theory of inflation, which suggests the universe expanded exponentially in the first fraction of a nanosecond after it was born. If inflation occurred, it would explain many features of our universe, such as the fact that it appears to be fairly smooth, with matter spread evenly in all directions (early inflation would have stretched out any irregularities in the universe).
Inflation might also mean that what we consider the universe—the expanse of everything we could see with the most perfect telescopes—is just one small corner of space, a pocket where inflation stopped and allowed matter to condense, galaxies and stars to form, and life to evolve. Elsewhere, beyond the observable universe, spacetime may still be inflating, with other “bubble” universes forming whenever inflation stops in one location.
This picture is called eternal inflation. “Most inflationary models, almost all, predict that inflation should become eternal,” says Alan Guth, a theoretical physicist at the Massachusetts Institute of Technology (MIT), who first predicted inflation in 1980.
If the BICEP2 results end up proving inflation occurred, then the multiverse may be part of the bargain. “I think the multiverse is a natural consequence of inflation ideas,” says theoretical physicist Frank Wilczek, also at MIT. “If you can start one universe form a very small seed, then other universes could also grow from small seeds. There doesn’t seem to be anything unique about the event we call the big bang. It is a reproducible event that could and would happen again, and again, and again.”
If that is true, it could help explain why our universe seems so special. The mass of the electron, for example, appears to be completely random—this value is not predicted by any known physics. And yet if the electron were any heavier or lighter than it is, atoms could not form, galaxies would be impossible, and life would not exist. The same goes for many other constants of nature, especially the cosmological constant—the theorized, but unverified, source of the so-called dark energy that is propelling the acceleration of the expansion of the universe. If the cosmological constant were different, and dark energy was more or less powerful, the universe would be drastically altered. and life as we know it wouldn’t be possible.
If our universe is the only one in existence, then we need some explanation for why it seems so fine-tuned for us to exist. If it is but one of many, however, then maybe each has different parameters, different constants, and one universe just happened to arrive at the values that enabled life.
“We live in this part of the universe because we can live there, not because the whole universe is built for our benefit,” says Stanford University physicist Andrei Linde, one of the main authors of inflation theory and the multiverse hypothesis. This idea, called the anthropic principle, is satisfying to some, and maddening to others.
“That story gives a very neat and self-consistent picture,” Guth says. But many find the anthropic principle and the multiverse distasteful. “The multiverse functions here as an all-purpose excuse for not being able to explain anything about particle physics,” mathematician Peter Woit at Columbia University wrote in a blog post responding to BICEP2 reactions. “I consider such a view to be ‘giving up’ on finding a true scientific explanation,” says Princeton University theoretical physicist Paul Steinhardt.
In addition to seeing the multiverse idea—and the anthropic principle it enables—as a cop-out, skeptics charge that it is impossible to test, because theory predicts other bubble universes would be permanently out of reach and unobservable. “Literally, anything can happen and does happen infinitely many times,” Steinhardt says. “This makes the theory totally unpredictive or, equivalently, unfalsifiable.”
An untestable idea is by definition unscientific, because science relies on verifying predictions through experimentation. Proponents of the multiverse idea, however, say it is so inextricable with some theories, including inflation, that evidence for one is evidence for the other.
“Once we have experimental proof that the cosmological constant is real, and we have experimental proof of inflationary cosmology, then suddenly we have something which I firmly believe is experimental evidence in favor of the multiverse,” Linde says. “Those people who say the theory of the multiverse does not have any experimental confirmation have not paid enough attention.”
Whether the BICEP2 results represent a piece of such confirmation is a point of contention between the pro- and anti-multiverse factions. “The BICEP2 discovery should cause dismay among multiverse skeptics—at least in this particular universe,” MIT physicist Max Tegmark wrote in a guest blog for Scientific American.
Doubters, of course, vehemently disagree. “Perhaps there is a part of the multiverse in which the #BICEP2 results provide evidence for a multiverse, but I don't think we live there,” Peter Coles, a theoretical cosmologist at the University of Sussex in England, wrote on Twitter.
And many physicists are agnostic about what, if anything, the BICEP2 results have to say about the multiverse. “The multiverse is an idea for how the inflationary period that gave rise to our universe may have come about,” says Marc Kamionkowski, professor of physics and astronomy at Johns Hopkins University. “This particular measurement doesn’t shed any direct light on that.”
Ultimately, neither side of the debate is likely to concede defeat any time soon. But one faction at least is claiming a small victory from last week’s news. “The more we move in this way, the more seriously we should take the possibility of eternal inflation and the multiverse,” Linde says, “and the idea that our universe is not just one cosmic balloon but a fractal of balloons producing new balloons producing new balloons forever.”