This week’s Nature has an article by Paul Steinhardt, with the title Big Bang blunder bursts the multiverse bubble. The subtitle of the piece describes the BICEP2 frenzy of last March as “premature hype”, and the description in the body of the article is:
The results were hailed as proof of the Big Bang inflationary theory and its progeny, the multiverse. Nobel prizes were predicted and scores of theoretical models spawned. The announcement also influenced decisions about academic appointments and the rejections of papers and grants. It even had a role in governmental planning of large-scale projects.
Given recent arguments that BICEP2 may be seeing dust, not primordial gravitational waves, the March media frenzy quite possibly was highly premature, if not completely misguided. Steinhardt goes on to argue that in the future
announcements should be made after submission to journals and vetting by expert referees. If there must be a press conference, hopefully the scientific community and the media will demand that it is accompanied by a complete set of documents, including details of the systematic analysis and sufficient data to enable objective verification.
He also takes the occasion to note the odd fact that while BICEP2 results have been claimed to be proof of inflation and the multiverse, if they turn out to be wrong, that’s fine too:
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.
Steinhardt was on a panel last Friday night here in New York at the World Science Festival, which can be watched here. The panel included Guth and Linde (who earlier in the week got $1 million for their work on inflation), as well as John Kovac of BICEP, and Amber Miller, Dean of Science here at Columbia. The last part of the video includes an unsuccessful attempt by Steinhardt to pin down Kovac on the significance of the BICEP2 evidence for primordial gravitational waves claim, as well as an exchange with Guth and Linde. They both defend inflation as the best model of the alternatives.
Multiverse promotion continues apace, with Steinhardt one of a rather small number of physicists publicly objecting. On Monday Alexander Vilenkin will explain to the public at the American Museum of Natural History that “the Big Bang was not a unique event in cosmic history and that other Big Bangs constantly erupt in remote parts of the universe, producing new worlds with a great variety of physical properties” (see here). A recent story on livescience has Brian Greene on the multiverse. Over at Massimo Pigliucci’s Scientia Salon Coel Hellier is starting a multipart series arguing against multiverse skeptics with The multiverse as a scientific concept — part I. Nothing in Part I about the problematic issues (untestable claims that fundamental physics is “environmental”), maybe in Part II…