In the Dark

I thought I’d just draw the collective attention of my vast readership (Sid and Doris Bonkers) to a bit of a row that has broken out between two groups of cosmologists concerning the theory of cosmic inflation.

This kerfuffle started with an article entitled Pop Goes The Universe in Scientific American by Anna Ijjas, Paul Steinhardt, and Avi Loeb that (among other things) asserts that inflation “cannot be evaluated using the scientific method” and is consequently not a scientific theory. Another group of cosmologists (including Alan Guth, the author of the paper that launched the inflationay universe model) penned a response that was signed by a long list of leading scientists, thirty-three of them to be precise. The original authors then issued a response to the response. Sean Carroll (who was one of those who signed the response the original paper has written a nice blog post summarizing the points of disagreement.

I’m not going to attempt to post a detailed response to every issue raised in this correspondence, but I will make a few points.

First, I think it’s important to realize that there isn’t a single simple definition of `the scientific method’: there are lots of scientific methods, each of which may employed to a greater or lesser degree in different disciplines. Most scientists would probably agree that some notion of `testability’ has to be included if a theory is said to be scientific, but it seems to me that testability is not an absolute, in the sense that not all predictions of a theory need to be observable for the theory as a whole to be testable to a degree. A theory might predict the existence of a phenomenon A that is impossible for all practical purposes to observe, but if that theory also has another necessary consequence B that is observed then the theory does not deserve to be dismissed as unscientific.

One aspect of modern inflationary theory that is singled out for criticism has been the incorporation of the idea of a multiverse. I have to make the confession here that I don’t like the concept of the multiverse, nor do I like the way it has become fashionably mainstream in the field. I’ve never seen it as a necessary (or even useful) addition to inflation theory. However, suppose you have a model of inflation that leads to something like Linde’s version of the multiverse. Causally disconnected domains of this multiverse may indeed not be observable, but if the theory has other necessary implications for things we can observe in our local universe then it is testable to a degree.

My position (such as it is) is that I like the idea of inflation, largely because: (a) it’s very neat; and (b) it provides a simple mechanism for generating fluctuations of the right form to account for so many of the observable properties of our universe, especially the fluctuations we measure in the cosmic microwave background seen by Planck:

These observations don’t prove that inflation is right, nor do they narrow down the field of possible inflationary models very much, but they do seem to be in accord with the predictions of the simplest versions of the theory. Whether that remains true for planned and future observations remains to be seen. Should someone come up with a different theory that matches existing data and can account for something in future data that inflation can’t then I’m sure cosmologists would shift allegiance. The thing is we don’t have such an alternative at the moment. Inflation is the preferred theory, partly for want of compelling alternatives and partly because we need more data to test its predictions.

That said, there are one or two points on which I agree with Ijjas, Steinhardt and Loeb. In particular there has developed what I consider to be a pathological industry dreaming up countless variations of the basic inflation model. There is now a bewildering variety of such models, few of which have any physical motivation whatsoever. I think this is a particularly a grotesque manifestation of the absurd way we measure scientific `success’ in terms of counting publications and how that has driven unhealthy research practice.

No doubt many of you disagree or wish to comment for other reasons either on the original communications or on my comments. Please feel free to offer your thoughts through the box below!