Prompted by reading about the recent Munich conference on the philosophy of science, I am reminded that many people regard the idea of a multiverse as so wild and wacky that talking about it brings science into disrepute. My argument here is the reverse: that the idea of multiple Big Bangs, and thus of a multiverse, is actually more mundane and prosaic than the suggestion that there has only ever been one Big Bang. I’m calling this a “philosophical” argument since I’m going to argue on very general grounds rather than get into the details of particular cosmological models.
First, let me clarify that several different ideas can be called a “multiverse”, and here I am concerned with only one. That “cosmological multiverse” is the idea that our Big Bang was not unique, but rather is one of many, and that the different “universes” created by each Big Bang are simply separated by vast amounts of space.
Should we regard our Big Bang as a normal, physical event, being the result of physical processes, or was it a one-off event unlike anything else, perhaps the origin of all things? It is tempting to regard it as the latter, but there is no evidence for that idea. The Big Bang might be the furthest back thing we have evidence of, but there will always be a furthest-back thing we have evidence of. That doesn’t mean its occurrence was anything other than a normal physical process.
If you want to regard it as a one-off special event, unlike any other physical event, then ok. But that seems to me a rather outlandish idea. When physics encounters a phenomenon, the normal reaction is to try to understand it in terms of physical processes. Continue reading
This article was first posted in two parts on Scientia Salon.
The multiverse concept is often derided as “unscientific” and an example of physicists indulging in metaphysical speculation of the sort they would usually deplore. For example commenters at Scientia Salon have said that the multiverse is “by definition not verifiable and thus outside the bounds of empirical science”, and that “advocates of multiverses seem to be in need of serious philosophical help”. 
Critics thus claim that the multiverse amounts to a leap of faith akin to a religious belief. Indeed, the religious often accuse atheistic scientists of inventing the multiverse purely to rebut the “fine-tuning” argument that they say points to a creator god (though the fine-tuning argument is readily refuted in several other ways, and anyhow physicists really don’t care enough about theology these days to let that worry them; further, the concepts leading to a multiverse were developed well before theologians started taking note of the issue).
The purpose of this article is to argue that the multiverse is an entirely scientific hypothesis, arrived at for good scientific reasons and arising out of testable and tested cosmological models. To be clear, I am not asserting that the multiverse has been proven true, even on the balance of probability, but I am asserting that it is a serious scientific concept that will eventually be accepted or rejected on scientific grounds.
Several different concepts could be labelled a “multiverse”, but I am advocating one particular multiverse concept, that arising from what cosmologists call the “eternal inflation” version of Big Bang cosmology.  I’ll outline why cosmologists have arrived at this model, which is now a mainstream account of the origin of our universe, and which leads naturally to a multiverse. Continue reading