Ethan Siegel at Starts With a Bang has a post up arguing that the multiverse must exist. His reasoning has to do with cosmic inflation. Inflation is the theory that the universe expanded at an exponential rate in the first billionth of a trillionth of a trillionth of a second of the big bang timeline. After that, space started expanding at a slower rate along the lines we see today.
Inflation was originally developed as a theory to explain why the universe seemed flat (in terms of spacetime), why temperature fluctuations throughout the early universe were so consistent, and a few other things such as the absence of magnetic monopoles. These motivations for the theory seem important. I think they should be remembered when evidence for the theory is being discussed. In other words, can we cite the motivations for the theory as evidence for it?
Anyway, Siegel notes that, due to quantum uncertainty, the particle physics that led to inflation ending would not have been a strictly deterministic event. Therefore inflation may not have ended everywhere in space at the same time. Our universe may be a bubble of non-inflation in a sea of inflating space. And if there is one bubble, there are likely others, other universes, the multiverse.
But all of this seems to hinge on the idea of cosmic inflation, specifically a variant of it called eternal inflation. And here’s my issue. The majority of physicists do seem to accept that the theory of inflation is true, but not all of them. And the evidence for it seems to be a combination of the original problems the theory was developed to solve and circumstantial evidence. Some notable physicists, including Paul Steinhardt, one of the theory’s early supporters, seem to think it actually creates more questions than it answers.
So, should we consider inflation settled science? This seems like an example of an unsettling trend in physics in recent years of accepting theories that can’t be empirically tested. Given that a huge swath of similar theories in particle physics were reportedly invalidated by the LHC results (or rather, non-results), it seems like a very questionable strategy, an abandoning of a key aspect of scientific investigation that been successful for centuries.
Science has credibility for a reason. That reason is the value it puts on testing every proposition. Talking about untestable theories as though they’ve been validated seems to put that credibility in jeopardy. There’s a danger that the public will see start to see theoretical physics as metaphysical navel gazing.
There is also a danger, identified by Jim Baggott some years ago, that many scientists may simply not look at alternative theories because they think inflation has solved the issue, or that they may eschew some speculative theories just because they’re not compatible with inflation. But if inflation is still really just a speculative theory, then they’re giving up on one speculative theory because it’s not compatible with another speculative theory, perhaps cutting off a fruitful line of inquiry.
It may turn out that inflation does eventually pass some test we simply haven’t thought of yet. Or we may eventually figure out a way to test the idea of bubble universes. But until we do, talking as though these are settled issues makes my skeptic meter jump through the roof.
Physics has a reputation for being a very hard science. Sometimes I wonder how warranted that reputation really is, at least in the theoretical branches.
Unless of course I’m missing something?