Is Inflationary Cosmology Science?


by Sean Carroll

[tl;dr: Check out this article in Scientific American by Ijjas, Steinhardt, and Loeb suggesting that inflation isn’t science; this response by Guth, Kaiser, Linde, and Nomura that was co-signed by a bunch of people including me; and this counter-response by the original authors.]

Inflationary cosmology is the clever idea that the early universe underwent a brief period of accelerated expansion at an enormously high energy density, before that energy converted in a flash into ordinary hot matter and radiation. Inflation helps explain the observed large-scale smoothness of the universe, as well as the absence of unwanted relics such as magnetic monopoles. Most excitingly, quantum fluctuations during the inflationary period can be amplified to density perturbations that seed the formation of galaxies and large-scale structure in the universe.

That’s the good news. The bad news — or anyway, an additional piece of news, which you may choose to interpret as good or bad, depending on how you feel about these things — is that inflation doesn’t stop there. In a wide variety of models (not necessarily all), the inflationary energy converts into matter and radiation in some places, but in other places inflation just keeps going, and quantum fluctuations ensure that this process will keep happening forever — “eternal inflation.” (At some point I was a bit skeptical of the conventional story of eternal inflation via quantum fluctuations, but recently Kim Boddy and Jason Pollack and I verified to our satisfaction that you can do the decoherence calculations carefully and it all works out okay.) That’s the kind of thing, as we all know, that can lead to a multiverse.

Here’s where things become very tense and emotional. To some folks, the multiverse is great. It implies that there are very different physical conditions in different parts of the cosmos, which means that the anthropic principle kicks in, which might in turn imply a simple explanation for otherwise puzzling features of our observed universe, such as the value of the cosmological constant. To others, it’s a disaster. The existence of infinitely many regions of spacetime, each with potentially different local conditions, suggests that anything is possible, and therefore that inflation doesn’t make any predictions, and hence that it isn’t really science.

This latter perspective was defended in a recent article in Scientific American by three top-notch scientists, Anna Ijjas, Paul Steinhardt, and Avi Loeb. They argue that (1) the existence of a wide variety of individual inflationary models, and (2) the prediction of a multiverse in many of them, together imply that inflation “cannot be evaluated using the scientific method” and that its proponents are “promoting the idea of some kind of nonempirical science.”

Now, as early-universe cosmologists go, I am probably less inclined to think that inflation is part of the final answer than most are. Many of my colleagues are more or less convinced that it’s correct, and it’s just a matter of nailing down parameters. I am much more concerned about the fine-tuning difficulties that make inflation hard to get started in the first place — in particular, the hilariously low entropy that is required. Nevertheless, inflation has so many attractive features that I still give it a fairly high Bayesian credence for being correct, above 50% at least.

And inflation is indubitably science. It is investigated by scientists, used to make scientific predictions, and plays a potentially important explanatory role in our understanding of the early universe. The multiverse is potentially testable in its own right, but even if it weren’t that wouldn’t affect the status of inflation as a scientific theory. We judge theories by what predictions they make that we can test, not the ones they make that can’t be tested. It’s absolutely true that there are important unanswered questions facing the inflationary paradigm. But the right response in that situation is to either work on trying to answer them, or switch to working on something else (which is a perfectly respectable option). It’s not to claim that the questions are in principle unanswerable, and therefore the field has dropped out of the realm of science.

So I was willing to go along when Alan Guth asked if I would be a co-signer on this response letter to Scientific American. It was originally written by by Guth, David Kaiser, Andrei Linde, and Yasunori Nomura, and was co-signed by an impressive group of physicists who are experts in the field. (A quick glance at the various titles will verify that I’m arguably the least distinguished member of the group, but I was happy to sneak in.) Ijjas, Steinhardt, and Loeb have also replied to the letter.

I won’t repeat here everything that’s in the letter; Alan and company have done a good job of reminding everyone just how scientific inflationary cosmology really is. Personally I don’t object to ISL writing their original article, even if I disagree with some of its substantive claims. Unlike some more delicate souls, I’m quite willing to see real scientific controversies play out in the public eye. (The public pays a goodly amount of the salaries and research budgets of the interested parties, after all.) When people say things you disagree with, the best response is to explain why you disagree. The multiverse is a tricky thing, but there’s no reason to expect that the usual course of scientific discussion and investigation won’t help us sort it all out before too long.

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