Consider this summary of an article in Nature written by George Ellis, a famous cosmologist based in Cape Town, South Africa.

Excerpt:

Multiverse thinking or the belief in the existence of parallel universes is more philosophy or science fiction than science. ”Cosmology must seem odd to scientists in other fields”.

George Ellis, a well-known mathematician and cosmologist, who for instance has written a book with Stephen Hawking, is sceptical of the idea that our universe is just another universe among many others.

A few weeks ago, Ellis, professor emeritus of applied mathematics at the University of Cape Town, reviewed Brian Greene’s book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Knopf/Allen Lane, 2011) in the journal Nature. He is not at all convinced that the multiverse hypothesis is credible: ”Greene is not presenting aspects of a known reality; he is telling of unproven theoretical possibilities.”

According to professor Ellis, there is no evidence of multiverses, they cannot be tested and they are not science.

Ellis is not the only multiverse sceptic in this universe. A few months ago, science writer John Horgan wrote a column in Scientific American, expressing his doubt in multiverses.

Multiverse thinking is basically an atheistic attempt to solve some huge problems with the big bang and explain why the universe could have been formed without a design or a Designer.

Now how do the theorteical physicists respond to this?

Here’s an article from Scientific American responding to Ellis. (H/T Uncommon Descent)

Excerpt:

This picture of the universe, or multiverse, as it is called, explains the long-standing mystery of why the constants of nature appear to be fine-tuned for the emergence of life. The reason is that intelligent observers exist only in those rare bubbles in which, by pure chance, the constants happen to be just right for life to evolve. The rest of the multiverse remains barren, but no one is there to complain about that.

Some of my physicist colleagues find the multiverse theory alarming. Any theory in physics stands or falls depending on whether its predictions agree with the data. But how can we verify the existence of other bubble universes? Paul Steinhardt and George Ellis have argued, for example, that the multiverse theory is unscientific, because it cannot be tested, even in principle.

[…]Our universe appears surprisingly fine-tuned for life in the sense that if you tweaked many of our constants of nature by just a tiny amount, life as we know it would be impossible. Why? If there’s a Level II multiverse where these “constants” take all possible values, it’s not surprising that we find ourselves in one of the rare universes that are inhabitable, just like it’s not surprising that we find ourselves living on Earth rather than Mercury or Neptune.

[…]George argues that if we take seriously that anything that could happen does happen, we’re led down a slippery slope toward even larger multiverses, like the Level IV one. Since this is my favorite multiverse level, and I’m one of the very few proponents of it, this is a slope that I’m happy to slide down!

Is the multiverse theory a good response to the fine-tuning argument? It seems to me that the fine-tuning rests on solid experimental evidence, while the multiverse rests on what-if speculations about unobservable entities.