Why The Multiverse Isn’t Just Madness
The multiverse—the idea that infinite universes stretch beyond our own—has gained traction among physicists. But others think it’s just a multi-mess.
Alternate realities, parallel dimensions, and multiple universes. Whatever you call it, the notion of other versions of existence is one of the most popular tropes in science fiction. In some other universe, you’re not reading this sentence but skydiving. In another, you’re nothing but a cockroach. In yet another, not only is life impossible, but atoms don’t even exist.
In recent years, though, such seemingly crazy ideas have shifted from fantasy and speculation toward bona fide science. Even among physicists, the multiverse has gone mainstream.
Theoretically, infinite universes might stretch beyond our own, like endless bubbles in a sea of boiling water. Each bubble has its own laws of physics, and although we may never visit or even see another bubble, some physicists say growing evidence is making the multiverse increasingly plausible—and even probable.
“Fifteen years ago, when you talked about the multiverse, the attitude of many physicists was just ridicule,” says Alex Vilenkin, a physicist at Tufts University. “But there has been a great change in attitude.”
Still, the concept of a multiverse is controversial and, at times, contentious. 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.
Scientists have been contemplating various forms of the multiverse for decades. In 1957, for example, physicist Hugh Everett proposed a new way to interpret the bizarre paradoxes of quantum physics, such as how a particle can be in two states simultaneously, or, in a macroscopic extension, how Schrodinger’s cat can be both dead and alive. Everett suggested that when you observe the particle or check on the cat, two separate realities branch off from each other: The cat is dead in one and alive in the other. Physicists dubbed this picture the many-worlds interpretation of quantum mechanics.
These many worlds are parallel universes, coexisting in different regions of an abstract, mathematical space, forever closed off from each other. Today, although only a minority subscribes to this outlook, quantum physicists still debate the issue.
But in the early 1980s, physicists realized a different kind of multiverse may also exist—one in which other universes occur not in some abstract parallel space, but in distant physical locations within the same fabric of space and time as our universe. It was a startling proposition, but these so-called bubble universes seemed to be a consequence of a new picture of cosmology that was just taking hold.
In 1980, Alan Guth, a physicist now at the Massachusetts Institute of Technology, suggested that in the early moments after the Big Bang, the universe ballooned especially fast before settling back into its normal expansion. Driven by a kind of repulsive gravity, this temporary period of inflation, as he called it, could explain why the cosmos is smooth and flat—features that had been puzzling cosmologists.
“With the simplest assumptions, you end up with eternal inflation and the multiverse. Being conservative on that front lands you at this radical thing.”
Today, most cosmologists accept the general premise of inflation, and they’ve proposed a number of possible models to explain how this rapid ballooning happened. “As time has gone on, the observational success of the prediction of simple inflationary models has gotten better and better,” Guth says.
For instance, over the last couple decades, satellites such as WMAP and Planck have made precise measurements of the cosmic microwave background—the Big Bang’s afterglow—and have found that its subtle patterns match inflation’s predictions. Cosmologists can also use this radiation to measure the mass density of the universe, which Guth says is within half a percent of what inflation predicts.
“The evidence is almost overwhelmingly strong,” says Andrew Liddle, a cosmologist at the University of Edinburgh. “The majority of people regard inflation as sufficiently satisfactory, and there’s no expectation that it will be overturned by something else.”
During the early years of inflationary theory, Guth and other pioneers of the idea soon discovered a surprising implication of their equations: Inflation is eternal, ceasing only in certain bubble-like pockets of space. “The space between [the bubbles], which is still inflating, makes room for more bubbles to form,” Vilenkin explains. “The inflating space expands so fast that nothing can ever catch up to its boundaries, so for all practical purposes, they’re isolated, self-contained bubble universes.” According to this picture, our universe is just one out of an infinite multiverse of bubbles.
In 1983, Vilenkin found that the most common models of inflation predict the multiverse. Models that avoid it tend to be contrived and unrealistic. “With the simplest assumptions, you end up with eternal inflation and the multiverse,” says physicist Andreas Albrecht of the University of California, Davis. “Being conservative on that front lands you at this radical thing.”
Perhaps the most compelling support for the multiverse involves the mysterious force known as dark energy. In 1998, astronomers discovered that the universe was expanding faster and faster—an acceleration later attributed to dark energy. The rate of acceleration depends on a number known as the cosmological constant. What has been confounding physicists is that, based on their understanding of the forces and particles of nature, they expect the constant to be roughly 10122 times bigger than what they measure it to be. They have no idea why the measured value is so small, but an explanation might lie in the multiverse.
In the 1980s, Nobel laureate Steven Weinberg, of the University of Texas, Austin, was investigating the value of the cosmological constant and suggested that in a multiverse, that value could vary. In our universe, he determined, the constant is small, because only a small constant could allow galaxies to form and life to develop. A large number, on the other hand, results in a universe that flies apart before atoms can even coalesce. Only a tiny fraction of universes might have the small constants suitable for life. We’re just lucky enough to live in one.
You can make similar so-called anthropic arguments to explain other fundamental constants of nature, such as the mass of the neutron. If these constants deviate in value even a bit, life could not exist and no one would be around to measure them. Unless physicists find a more satisfying explanation based on first principles, the multiverse at least offers some reason for why the constants in our universe happen to be so finely tuned for life.
Weinberg’s analysis proved prescient. When astronomers discovered that the universe was accelerating—possibly due to dark energy—they measured the cosmological constant to be within a factor of 10 to what Weinberg had suggested, says Guth. Since then, physicists have refined Weinberg’s proposal and calculated a value even closer to the measured one. It’s not the most precise match, but the best yet. “To my knowledge, there’s still no better explanation of the observed magnitude of dark energy,” Liddle says.
Meanwhile, string theory—the best candidate so far for a theory of everything—may provide the theoretical framework that supports the multiverse. String theory requires extra spatial dimensions beyond our usual three (up/down, left/right, forward/backward). These dimensions—which are too small for us to perceive—are curled up in innumerable ways, each corresponding to a bubble universe with different laws of physics. The big knock on string theory, however, is that it lacks any observational evidence.
Observational evidence for the multiverse itself isn’t that much more promising. But researchers have some prospects. For example, if a neighboring bubble universe happens to bump into our own, it would leave an imprint in the cosmic microwave background. Astronomers have looked, but have yet to find anything.
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. 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.
This work is preliminary and speculative, for sure. And overall, the support for the multiverse is undeniably circumstantial. In the end, perhaps the best that proponents can hope for is indirect evidence, in the form of more refined models and unequivocal confirmation of inflation. If increasingly precise measurements of the cosmic microwave background further narrow down theories of inflation, scientists may eventually be left with one specific model that leads straight to the multiverse.
But it’s the lack of direct evidence—and the likelihood that it may be inherently impossible to test the multiverse—that leads some to scoff at the notion.
One of the strongest critics is physicist Paul Steinhardt of Princeton University. Along with physicists such as Albrecht, Guth, and Stanford’s Andrei Linde, he helped pioneer inflation in the 1980s. But when he realized inflation never stopped, hatching endless bubble universes in the process, he saw a problem. The multiverse wasn’t a feature, but a flaw.
“It’s a breakdown in the theory,” he says. It’s as if someone came to you with a theory for why the sky is blue that at first seems plausible, but after some refinements, “it produces not just a blue sky, but a purple sky, a polka-dotted sky—you name it,” he says.
A multiverse in which anything can and will happen doesn’t explain anything at all, says Steinhardt. A hallmark of the scientific process is being able to test predictions. But, he says, “what does it mean to predict something if it predicts everything?” Such a theory can’t be tested—or potentially falsified—and thus isn’t a useful scientific theory. The multiverse, he says, is a “multi-mess.”
Others say this view is too limiting. “The idea of falsifiability in a strict sense of the word is an oversimplified view of the way science works,” Guth says. “No theory in science is ever actually proven. An acceptable scientific theory is simply the best theory that scientists know of to explain a set of phenomena in nature.”
This problem of falsifiability might be a matter of philosophy. But there’s also the practical problem of just trying to do physics in a multiverse. At its core, physics relies on calculating the probability that certain phenomena will happen—such as the likelihood that a particle will decay into another. But when you’re dealing with infinite possibilities, calculating probabilities no longer makes mathematical sense.
“If you don’t have a firm idea of what you mean by probability, you can’t really have a complete picture of how physics works,” Guth says. “This problem of defining probabilities I find to be one of the most frustrating problems I’ve known about in my life.”
Attempts to solve this probability predicament—what’s known as the measure problem—have had limited success. A few years ago, building on work by theoretical physicist Don Page, Albrecht suggested that when it comes to the multiverse, the probabilistic tools that physicists normally use might not apply. “It’s conceivable,” he says, “that being more disciplined in how you use probabilities can actually resolve the measure problem.” This possibility has softened his view on the multiverse, says Albrecht, though he’s still skeptical. He now puts a 10 percent chance on the likelihood that we live in a multiverse. (Guth, on the other hand, says the odds are better than even.)
For Steinhardt, though, the attempts to address the measure problem are ad hoc modifications to a flawed idea. The problems with the multiverse are so severe, he argues, that cosmologists should abandon inflation altogether. One possible alternative is a class of “bouncing” models in which the universe didn’t start with the Big Bang. Instead, the idea goes, the universe has always existed. It had been contracting when, at some point—nominally the Big Bang—it “bounced,” then started to expand. In some models, the universe undergoes infinite cycles of expansion and collapse. These theories don’t require inflation, and thus avoid the multiverse, Steinhardt says.
Only a handful of physicists are working on this kind of bouncing cosmology. “It’s gotten more press than they actually have support in the physics community,” Guth says. Indeed, the theories are not as well developed as inflation, but Steinhardt and others continue to explore and advocate for them.
Meanwhile, some physicists are examining the possibility of a theory that maintains inflation but avoids the multiverse. That might be possible, Albrecht says, but doing so would require new physics. He’s found that such a theory can work—without being too contrived—only if you make certain extreme assumptions about the laws governing fundamental particles and forces. It would certainly be a radical approach.
Of course, so is sticking with the idea of inflation leading to a multiverse. “It’s not on very solid ground,” Albrecht says. “That’s not a terrible insult. That’s what things are like on the cutting edge of physics.”
If physicists agree on anything, it’s that a resolution won’t come easily. “If the multiverse idea is right,” Guth says, “it’ll be a long time before the human race is convinced.”