It isn't just "creationists" who make these estimates. Physicists and cosmologists think that tiny changes in a number of fundamental physical constants (like the charge of the electron, etc.) would have resulted a universe in which life of any kind would be impossible.
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Without entering into the question of Gods, what makes a physicist or cosmologist believe that he is capable of determining whether an alternate universe could support life?
"Without entering into the question of Gods, what makes a physicist or cosmologist believe that he is capable of determining whether an alternate universe could support life?"
And you think they are not qualified? I think you should read what they say. The ones I have read are very well informed about other fields of science. Nor do I know of any biologists or other scientists who can contradict them, or even try. Remember, we are not talking about conditions which are merely different in the way submarine hot springs are, or the surface of Mars is. We are talking about conceptual universes in which no matter is possible at all, or in which the Big Bang would already have reversed and collapsed, without leaving enough time for life to form.