Cosmic Inflation

Theoretical physicist Sean Carroll at The Opinionator, a New York Times blog, gives us a very readable, very helpful explanation of the cosmological theories called cosmic inflation and the multiverse. These have been in the news recently because of the possible discovery of gravitational waves predicted by inflation theory.

Carroll is excited by this because, like many scientists, he's disturbed by the extraordinary fine-tuning that the early universe must have exhibited if inflation never happened. Since fine-tuning points to a cosmic designer, and since Carroll is an atheist who desires that the universe be completely natural, he's emotionally and intellectually invested in inflation theory.

Here are a few excerpts from the essay:

The Hot Big Bang model, 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.

If the Standard Big Bang model fits all the data and is simpler than the inflation model, why posit the inflation model?

The problem is that the universe displays a smooth distribution of matter and energy which is extraordinarily improbable if the origin of the universe is a random event. The inflation hypothesis is a way to escape the conclusion that this smoothness was somehow not random. In the first trillionth, trillionth, trillionth of a second of the universe's existence some unknown force, perhaps negative gravity, inflated the universe to a vast size almost instantly smoothing it out like an inflating balloon smooths out the wrinkles in the latex:

Alan Guth’s proposal (in the 1980s) 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.

The inflaton, in other words, is a source of energy that hasn't been observed or measured but which is hypothesized to have existed in order to avoid the metaphysical implications of an extraordinarily improbable distribution of mass-energy at the origin of the universe.

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.

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.

It seems that the only reason for suggesting that this immense expansion occurred and that it was caused by a force that science has no other reason to think exists, is the desire to avoid any non-naturalistic entities involved in the creation. Carroll acknowledges, however, that even if inflation should turn out to be the case it only pushes the problem of fine-tuning back a step:

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.

Carroll goes on to explain how inflation may lead to a multiverse, another hypothesis advanced primarily to avoid the fact that much of the rest of the cosmos also seems to be fine-tuned for life. Such precision is literally incredible if our universe is the only universe and if it arose by chance. But if ours is just one of an infinite number of different universes then ours becomes not only more likely but actually inevitable.

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.

This sounds a bit circular to the layman. A highly improbable inflation produces the multiverse, and the multiverse makes inflation a near certainty. Carroll himself seems a bit abashed by the reasoning:

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.

I think that last sentence gives the game away. Inflation is based on little more than a hope that somehow the universe can be explained without recourse to an intelligent designer.

In any case, Carroll has more to say on these matters at the link. It really is a good article, not only because it explains some very abstruse concepts in easy-to-understand fashion, but also because it's a nice illustration of how scientists, far from being the objective pursuers of the truth wherever it may lie, are really driven by metaphysical and religious commitments that dictate what theories they'll consider and which ones they won't.