With the recent BICEP2 results, the question of the Multiverse has raised it's less than pretty head. The Multiverse is a topic which polarizes people, some who embrace the idea that our Universe is just one of countless trillions and trillions and trillions of other universes out there. Others get a little cross with the popularity of the topic. And then there are those (looking at you slashdot commenters) who generally talk nonsense.
Over at Starts with a Bang Ethan Siegal tells us that The Multiverse is not the answer. The essential crux of Ethan's argument is that, in calling on the Multiverse, cosmologists are giving up on science. What does he mean?
The Multiverse is not a single concept, there is more than one kind, but the basic idea is that, as well as our Universe, which we find quite cosy for life, there are a myriad of other universes. These universes are not identical to our own, but possess different physics, different masses of the fundamental particles, different strengths of the fundamental forces, different forces even.
I won't go into detail here (as I am going to go into detail elsewhere - watch this space!) but most of these universes would be sterile, completely devoid of life. But aren't we lucky to find ourselves in a universe in which we can live? Of course, we have to find ourselves in such a universe, otherwise we would not be here to ask the question. This is the Anthropic Principle (a concept more wildly and crazily discussed than the Multiverse).
So, what's Ethan's problem? Well, if are going to think that we are just one of this myriad of universes, then there is no point asking any questions about why the electron has the mass it does, or why gravity is weaker than the weak force. It's just the roll of the cosmological dice that gave these values, and these values are just right for us to live comfortably. Calling on the Multiverse is akin to saying "God did it" and so it not science. We can pack-up our cosmological bag, and go home.
But I don't think Ethan is correct. As he points out in his article, we still have mysteries to solve. One of the most pressing is to get gravity and the other forces to play nicely together in extreme conditions of the early Universe. This could be the long sought after theory of quantum gravity which will allow us to see back into the creation of the Universe. Some think that the constants of nature will fall out of this ultimate theory, as we will know why an electron has the mass it does.
But there are other things we need to understand, such as the details of inflation, minor things like how it started, how it stopped, and, well, most other things about it. We could end up with several competing theories that describe our inflating universe, and producing multiple universes as it goes. The differing models may imprint themselves on our observations, and we can tell them apart, but we may never see the other universes out there as they are beyond our horizon.
Your multiverse generating model does, however, have a big hurdle to overcome, and that is that it must produce at least one universe, one out of the potential bazzion universes it can produce, that is the Universe, the one we find ourselves living in. If your model cannot account for our cosmic home, then off to the rather large and overflowing dustbin of science.
So, I don't think anyone is giving up. I don't think cosmology is moving from science into pseudo or non-science. And while we may never be able to see the sibling universes all around us, our mathematical picture of the evolution of the multiverse is still testable, and is still science. Freaky science, but science all the same.
There's a lot more to come on this topic, so watch this space.