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Nothing makes scientists happier than an experimental result that completely contradicts a widely accepted theory. The scientists who first invented the theory might not be tickled, but their colleagues will be overjoyed. Science progresses when a good theory is superseded by an even better theory, and the most direct route to building a better theory is to be confronted by data that simply don’t fit the old one.

Nature is not always so kind, however. Fields like particle physics and cosmology sometimes include good theories that fit all the data but nevertheless seem unsatisfying to us. The Hot Big Bang model, for example, which posits that the early universe was hot, dense, and rapidly expanding, is an excellent fit to cosmological data. But it starts by assuming that the distribution of matter began in an incredibly smooth configuration, distributed nearly homogeneously through space. That state of affairs appears to be extremely unnatural. Of all the ways matter could have been distributed, the overwhelming majority are wildly lumpy, with dramatically different densities from place to place. The initial conditions of the universe seem uncanny, or “finely tuned,” not at all as if they were set at random.

Good scientific theories can fit all the data but still seem unsatisfying to us.

Faced with theories that fit all the data but seem unnatural, one can certainly shrug and say, “Maybe that’s just the way it is.” But most physicists take the attitude that almost none of our current models are exactly correct; our best ideas are still approximations to the underlying reality. In that case, apparent fine-tunings can be taken as potential clues that might prod us into building better theories.



Last week’s announcement of the observation of gravitational waves from the earliest moments of the history of the universe will — if the observation holds up — represent a resounding victory for this kind of approach. The observations seem to verify a prediction of the theory of cosmic inflation, proposed by the physicist Alan Guth in 1980 (with both important predecessors and subsequent elaborations). Guth was primarily motivated by a desire to provide a more natural explanation of why our universe looks the way it does.

Guth’s proposal was that the extremely early universe was dominated for a time by a mysterious form of energy that made it expand at a super-accelerated rate, before that energy later converted into ordinary particles of matter and radiation. We don’t know exactly what the source of that energy was, but physicists have a number of plausible candidates; in the meantime we simply call it “the inflaton.” Unlike matter, which tends to clump together under the force of gravity, the inflaton works to stretch out space and make the distribution of energy increasingly smooth. By the time the energy in the inflaton converts into regular particles, we are left with a hot, dense, smooth early universe: exactly what is needed to get the Big Bang model off the ground. If inflation occurred, the smoothness of the early universe is the most natural thing in the world.

Inflation has become a starting point for much contemporary theorizing about the beginning of the universe. Cosmologists either work to elaborate the details of the model, or struggle to find a viable alternative. Which is why excitement was so high last week when cosmologists announced that they had found the imprint of primordial gravitational waves in the cosmic microwave background, the leftover radiation from the Big Bang. These gravitational waves are a direct prediction of inflation. Before last week, our reliable knowledge of the universe stretched back to about one second after the Big Bang; this observation pushes our reach back to one trillionth of a trillionth of a trillionth of a second.

The theory of cosmic inflation was motivated by the simple desire to have a more natural explanation of the early universe.

Cosmic inflation is an extraordinary extrapolation. And it was motivated not by any direct contradiction between theory and experiment, but by the simple desire to have a more natural explanation for the conditions of the early universe. If these observations favoring inflation hold up — a big “if,” of course — it will represent an enormous triumph for reasoning based on the search for naturalness in physical explanations.

The triumph, unfortunately, is not a completely clean one. If inflation occurs, the conditions we observe in the early universe are completely natural. But is the occurrence of inflation itself completely natural?

That depends. The original hope was that inflation would naturally arise as the early universe expanded and cooled, or perhaps that it would simply start somewhere (even if not everywhere) as a result of chaotically fluctuating initial conditions. But closer examination reveals that inflation itself requires a very specific starting point — conditions that, one must say, appear to be quite delicately tuned and unnatural. From this perspective, inflation by itself doesn’t fully explain the early universe; it simply changes the kind of explanation we are seeking.

Fortunately — maybe — there is a complication. Soon after Guth proposed inflation, the physicists Alexander Vilenkin and Andrei Linde pointed out that the process of inflation can go on forever. Instead of the inflaton energy converting into ordinary particles all throughout the universe, it can convert in some places but not others, creating localized “Big Bangs.” Elsewhere inflation continues, eventually producing other separate “universes,” eventually an infinite number. From an attempt to explain conditions in the single universe that we see, cosmologists end up predicting a “multiverse.”

This may sound like a very peculiar result. But in the news conference after last week’s announcement, both Guth and Linde suggested that evidence for inflation boosts the case for the multiverse. And perhaps the multiverse repays the favor. The fundamental laws of physics obey the principles of quantum mechanics: Rather than predicting definite outcomes, we attach probabilities to members of an ensemble of many different experimental outcomes. If inflation begins in any part of this quantum ensemble, and that inflation goes on forever, it creates an infinite number of individual universes. So even if inflation itself seems unlikely, multiplying by the infinite number of universes it creates makes it quite plausible that we find ourselves in a post-inflationary situation.

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If you find the logic of the previous paragraph less than perfectly convincing, you are not alone. Not that it is obviously wrong; but it’s not obviously right, either. The multiverse idea represents a significant shift in the philosophy underlying inflation: Rather than explaining why we live precisely in this kind of universe, eternal inflation admits there are many kinds of local universes, and expresses the hope that ones like ours are more likely than other kinds.

Perhaps they are. At this point, however, we simply don’t know how to do the math. The multiverse is a provocative scenario, but the specific models that predict it are very tentative, far from the pristine rigor one expects of a mature physical theory. How many universes are there? How do we decide which sets of conditions are most “likely” within the giant ensemble of possibilities? How do we balance the intrinsic probabilities of quantum mechanics with the possible infinite proliferation of local regions? Does absolutely everything happen within the multiverse, or are only some possibilities actually realized?

The questions are daunting, but they’re not necessarily hopeless. Physical theories are often vague when first proposed, and only come into focus after a great deal of effort. Inflation was originally motivated by a quest for naturalness, and its audacious extrapolations away from established physics have apparently been vindicated by new observations. The multiverse, in its modern cosmological guise, has a similar origin. It’s not that a group of theoretical physicists were unwinding with some adult beverages and starting wondering out loud, “What if there were billions and billions of universes?” It’s that the equations we invented to explain observational data (the smoothness of our initial conditions) ended up pointing in the direction of this provocative possibility, and it’s the responsibility of scientists to take the predictions of their models as seriously as possible.

Naturalness is a subtle criterion. In the case of inflationary cosmology, the drive to find a natural theory seems to have paid off handsomely, but perhaps other seemingly unnatural features of our world must simply be accepted. Ultimately it’s nature, not us, that decides what’s natural.

Sean Carroll, a theoretical physicist at the California Institute of Technology, is the author of “The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World.”