When I became interested in The Big Bang Theory, at some point it occurred to me that if there was one Big Bang, why couldn't there have been more, in the past, at this instant for all we know, and indefinitely into the future? Then I came across the multiverse hypothesis. I have two issues with this idea, first officially coined by William James in 1895. My first question is whether alternate hypothetical universes would really qualify as other universes. Whatever the origin of The Big Bang nearest and dearest to us earthlings, wouldn't it make sense that other Big Bangs likely came about from the same mysterious source? Break down the word universe and it literally means, "one-all." We don't generally consider the moon or Mars or a distant galaxy or the nearest convenience store to be in another universe, but to be aspects of the one-all. So mightn't these other results of, presumably, the same source for a Big Bang as ours be seen as branches of the same cosmic tree, and therefore part of one much larger multidimensional universe?
My other issue with the multiverse is when it is employed for the express purpose of justifying a purely random explanation for elements of fine tuning that allow the existence of life here on earth. My feeling is that complex order exists, not as a bizarre mathematical anomaly, but because that is a tendency of nature. Consider some unique life forms recently discovered such as the algae dubbed picobiliphytes. http://news.nationalgeographic.com/news/2007/01/070111-new-lifeform... In fact I think that if one frames life as a weird fluke to the point where it might seem a near impossibility that happened anyway, this could as much lend to a perception of life as a miracle as a purely random event.
I have no issue with the essence of the idea of "other" universes, but I wouldn't be at all surprised if there is life elsewhere in "this" self organizing universe and in any or all of these "other" universes.
If the multiverse idea is employed to support atheism, I would go so far as to say that inventing or supporting alternate universes for that purpose could be likened to fundamentalists who claim that fossil evidence was created by Satan to undermine faith in scripture. I would also say that such a hypothesis bares possibly even less credence than the idea that there is a loose element of intention in all of nature, complexity, and evolution.