Multiverse and Parsimony
The following is built up completely by excerpts from the article "Does the Universe Need God?" by Sean Carroll. However, the text has been cut and pasted in an arrangement that is not found in the original article, with the intention of producing a continuous prose relevant to the topic. In this post, I am restricting myself only to the issue of whether the postulation of multiple 'universes' (Multiverse) is un-parsimonious or not. [Parsimony is preference for the least complex explanation for an observation, and it is used as rule of thumb in science for judging hypothesis.]
Multiverse in physics means that in different regions of the universe the physical parameters take on different values. These different regions are traditionally called "universes" even if they spatially connected. It simply posits regions of spacetime outside our observable horizon, in which conditions are very different – including, in principle and often in practice, the parameters specifying the laws of physics, such as the mass of the neutron or the vacuum energy.
This has garnered a great deal of attention in recent years, in part because it seems to be a natural outcome of two powerful ideas that were originally pursued for other reasons: inflationary cosmology, and superstring theory. The multiverse comes to life by combining inflation with string theory. Once inflation starts, it produces a limitless supply of different "pocket universes," each in one of the possible phases in the landscape of vacuum states of string theory. Given the number of potential universes, it wouldn't be surprising that one (or an infinite number) were compatible with the existence of intelligent life. Once this background is in place, the "anthropic principle" is simply the statement that our observable universe has no reason to be representative of the larger whole: we will inevitably find ourselves in a region that allows for us to exist. The multiverse is not a theory; it is a prediction of a theory, namely the combination of inflationary cosmology and a landscape of vacuum states. Both of these ideas came about for other reasons, having nothing to do with the multiverse. If they are right, they predict the existence of a multiverse in a wide variety of circumstances. It's our job to take the predictions of our theories seriously, not to discount them because we end up with an uncomfortably large number of universes.
One popular objection to the multiverse is that it is highly non-parsimonious; is it really worth invoking an enormous number of universes just to account for a few physical parameters? As Swinburne says:
'To postulate a trillion trillion other universes, rather than one God in order to explain the orderliness of our universe, seems the height of irrationality.'
That might be true, even with the hyperbole, if what one was postulating were simply "a trillion trillion other universes." But that is a mischaracterization of what is involved. What one postulates are not universes, but laws of physics. Given inflation and the string theory landscape (or other equivalent dynamical mechanisms), a multiverse happens, whether you like it or not.
This is an important point that bears emphasizing. All else being equal, a simpler scientific theory is preferred over a more complicated one. But how do we judge simplicity? It certainly doesn't mean "the sets involved in the mathematical description of the theory contain the smallest possible number of elements." A scientific theory consists of some formal (typically mathematical) structure, as well as an "interpretation" that matches that structure onto the world we observe. The structure is a statement about patterns that are exhibited among the various objects in the theory. The simplicity of a theory is a statement about how compactly we can describe the formal structure (the Kolmogorov complexity), not how many elements it contains.
A multiverse that arises due to the natural dynamical consequences of a relatively simple set of physical laws should not be discounted because there are a lot of universes out there. Multiverse theories certainly pose formidable problems, especially when it comes to making predictions and comparing them with data; for that reason, most scientists would doubtless prefer a theory that directly predicted the parameters we observe in nature over a multiverse ensemble in which our local environment was explained anthropically. But most scientists (for similar reasons) would prefer a theory that was completely free of appeals to supernatural agents.