Q&A: Cosmology

Q:
Can there be a parallel existence?

A:
Many scientists have speculated that there may be universes other than ours. Proving it is another matter. See the book by Martin Rees, Before the Beginning for a discussion of how the universe may consist of many different interacting universes. Another interesting idea is that a "shadow" or "mirror" universe exists that could explain the dark matter.
See
http://www.eurekalert.org/pub_releases/1999-01/OSU-SING-070199.php

Another is the many-worlds interpretation of quantum mechanics. Quoting from an interview with David Deutsch, one of the foremost expositors of this theory today: "It is rather like the discovery of other planets or other galaxies. Having previously known only the Milky Way, we did not just find that there are vast numbers of stars out there, far more than in the Milky Way. There are more galaxies out there than there are stars in the Milky Way. We also found that most of the stars outside the Milky Way are actually arranged in other little Milky Ways themselves. And that is exactly what happens with parallel universes. It is of course only an analogy but quite a good one; just like the stars and galaxies, the unseen parts of reality are arranged in groups that resemble the seen part. Within one of these groups, which we call a parallel universe, the particles all can interact with each other, even though they barely interact with particles in other universes. They interact in much the same way as the ones in our seen universe interact with each other. That is the justification for calling them universes. The justification for calling them parallel is that they hardly interact with each other, like parallel lines that do not cross. That is an approximation, because interference phenomena do make them interact slightly. "
http://www.qubit.org/people/david/Articles/PhilosophyNow.html
Also see the article by M. Tegmark and J. Wheeler in the Feb. 2001 issue of Scientific American: 100 Years of Quantum Mysteries