Still no Light on the Origin of the Universe

Back in March a team of astronomers announced that by studying the polarization of the Cosmic Microwave Background Radiation they had detected evidence of primal gravitational waves and thus of the cosmological model known as inflation. This seemed like an exciting finding, finally bringing some experimental science to bear on our speculations about the origin of the universe. Sadly, it didn't take long for other physicists to point out that the tiny amount of twisting they found in ripples of polarization could have been produced by all sorts of other things, from light scattered by galactic dust to electrons moving around galactic magnetic fields. Oops.

Physicist Paul Steinhardt of Princeton has now written an editorial for Nature slamming the BICEP2 experiment and the whole business of speculative cosmology. Steinhardt seems to be part of the hard-nosed brigade that has never liked untested theories about things like the inflaton field or P-branes, and his editorial pulls no punches:

The BICEP2 incident has also revealed a truth about inflationary theory. The common view is that it is a highly predictive theory. If that was the case and the detection of gravitational waves was the ‘smoking gun’ proof of inflation, one would think that non-detection means that the theory fails. Such is the nature of normal science. Yet some proponents of inflation who celebrated the BICEP2 announcement already insist that the theory is equally valid whether or not gravitational waves are detected. How is this possible?

The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests. First, inflation is driven by a hypothetical scalar field, the inflaton, which has properties that can be adjusted to produce effectively any outcome. Second, inflation does not end with a universe with uniform properties, but almost inevitably leads to a multiverse with an infinite number of bubbles, in which the cosmic and physical properties vary from bubble to bubble. The part of the multiverse that we observe corresponds to a piece of just one such bubble. Scanning over all possible bubbles in the multi­verse, every­thing that can physically happen does happen an infinite number of times. No experiment can rule out a theory that allows for all possible outcomes. Hence, the paradigm of inflation is unfalsifiable. . . . Taking this into account, it is clear that the inflationary paradigm is fundamentally untestable, and hence scientifically meaningless.

Fundamental physics of both the very small and the very large has run up against a wall, and scientists in some fields have been banging their heads against it for decades. I sense that string theory is fading, not because it has been proved wrong, but because a huge effort stretching across decades has not made any progress in proving it right. Fascination with multiverse theories seems to many people (including me) as the counsel of despair; we can't figure out why our universe is the way it is, so let's assume infinitely many universes, at least one of which was bound to end up like this one.

Maybe that's the best we can do. But if so, how disappointing.