What’s certain is that current and future missions will provide ever more refined data that will sharpen the inflationary approach. Bear in mind that inflation is a paradigm, not a unique theory. Theorists have now implemented the core idea of the bang-as-repulsive-gravity in hundreds of ways (different numbers of inflaton fields, different interactions between those fields and so on), with each generally yielding slightly different predictions. The Bicep2 data has already winnowed the viable models significantly, and forthcoming data will continue the process.
This all adds up to an extraordinary time for the inflationary theory. But there’s an even larger lesson. Barring the unlikely possibility that with better measurements the swirls disappear, we now have a new observational window onto quantum processes in the early universe. The Bicep2 data shows that these processes happen on distance scales more than a trillion times smaller than those probed by our most powerful particle accelerator, the Large Hadron Collider. Some years ago, together with a group of researchers, I took one of the first forays into calculating how our cutting-edge theories of the ultra-small, like string theory, might be tested with observations of the microwave background radiation. Now, with this unprecedented leap into the microrealm, I can imagine that more refined studies of this sort may well herald the next phase in our understanding of gravity, quantum mechanics and our cosmic origins.
Inflation and the Multiverse
Finally, let me address an issue I’ve so far carefully avoided, one that’s as wondrous as it is speculative. A possible byproduct of the inflationary theory is that our universe may not be the only universe.
In many inflationary models, the inflaton field is so efficient that even after fueling the repulsive push of our Big Bang, the field stands ready to fuel another big bang and another still. Each bang yields its own expanding realm, with our universe being relegated to one among many. In fact, in these models, the inflationary process typically proves never-ending, it’s eternal, and so yields an unlimited number of universes populating a grand cosmic multiverse.
With evidence for the inflationary paradigm accumulating, it’s tempting to conclude that confidence in the multiverse should grow too. While I’m sympathetic to that perspective, the situation is far from clear-cut. Quantum fluctuations not only yield variations within a given universe—a prime example being the microwave background variations we’ve discussed—they also entail variations between the universes themselves. And these variations can be significant. In some incarnations of the theory, the other universes might differ even in the kinds of particles they contain and the forces that are at work.
In this enormously broadened perspective on reality, the challenge is to articulate what the inflationary theory actually predicts. How do we explain what we see here, in this universe? Do we have to reason that our form of life couldn’t exist in the different environments of most other universes, and that’s why we find ourselves here—a controversial approach that strikes some scientists as a cop-out? The concern, then, is that with the eternal version of inflation spawning so many universes, each with distinct features, the theory has the capacity to undermine our very reason for having confidence in inflation itself.
Physicists continue to struggle with these lacunae. Many have confidence that these are mere technical challenges to inflation that in time will be solved. I tend to agree. Inflation’s explanatory package is so remarkable, and its most natural predictions so spectacularly aligned with observation, that it all seems almost too beautiful to be wrong. But until the subtleties raised by the multiverse are resolved, it is wise to reserve final judgment.
If inflation is right, the visionaries who developed the theory and the pioneers who confirmed its predictions are well-deserving of the Nobel Prize. Yet, the story would be bigger still. Achievements of this magnitude transcend the individual. It would be a moment for all of us to stand proud and marvel that our collective creativity and insight had revealed some of the universe’s most deeply held secrets.