The multiverse as a scientific concept

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”. [1]

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. [2] 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.

What does “existence” mean?

During a recent online discussion I discovered, somewhat to my surprise, that there is no general agreement on what the word “exist” means. Everyone has an intuitive understanding of it but when it comes to an explicit definition of the word there is no consensus, and indeed philosophers have written a vast literature on the topic of ontology, or what exists.

Dictionaries don’t really help; for example Oxford Dictionaries gives a nicely circular set of definitions:

Exist: 1. have objective reality or being
Reality: 1. the state of things as they actually exist
Being: 1. existence.

Of course physicists have a perfectly good operational definition: something exists if it is capable of making a detector go ping. Try arguing that, however, and you’re immediately accused of materialism, physicalism, scientism, being blind to possibilities beyond a very narrow world-view, and a host of similar sins (I plead guilty to at least the first three).

A fine-tuned universe argues for atheism

A favourite and fashionable argument for God is the argument from a fine-tuned universe. The argument is that, were it not for many aspects of our universe being “just right” for us to exist, then we wouldn’t be here, therefore [and that “therefore” is the big leap] the universe must have been fine-tuned to produce us.

Such an argument is advanced by theologians such as Alvin Plantinga and William Lane Craig, while Francis Collins has even gone so far as to claim that Richard Dawkins has admitted that he is troubled by the argument (this led to a witty response by Dawkins).

However, this argument is not just flawed, it actually contains six major flaws. The argument is one of those that is so unconvincing that it never leads anyone to believe in God, it only bolsters the faith of those who already believe, in which state their credulity renders them unable to examine the argument objectively.