Are Multiverses possible?

Not only possible, but inevitable. However, you are using the term in a much stronger sense than we use it in theoretical physics. The Observable Universe is only one infinitesimal corner of the potentially (and at the very least, practically) infinite Universe. Each of the infinitude of other infinitesimal corners makes a separate non-observable universe, and thus the Universe can well be regarded as a multiverse in its own right. These other corners may well (and in some cases will necessarily) have very different low energy physics to that which we are accustomed. This alone gives the perfect multiverse to explain anthropically just about every "fine-tuning" example that may be raised.There are many other examples of multiverses, as Max explains on his site (that is referenced in the post above), and we can explore these in greater detail if required.

Well, other than the entirety of quantum mechanics Go chat to Alain Connes and he'll explain how p-adics explain the whole of physics. I couldn't really follow him 18 years ago, so I'm not even going to try now!