Parallel Universes : are thought to be universes that exist alongside that of our own. They could be very similar in nature to our universe or they could in fact be very different. In his book, The Universe Next Door: Twelve Mind Blowing Ideas From The Cutting Edge of Science, Marcus Chown illustrates that the fiction of a parallel universe may indeed be closer to fact.
There is some speculation that our universe may have been created by superior beings in another universe. The reason that this theory arises, is because there is a belief among cosmologists that the universe began initially with a tiny speck of matter - weighing in at possibly only a thousandth of a gram. This particle then underwent a strange process known as inflation. As the mechanics of the process are already known, this has led to the notion that perhaps the whole process could indeed be recreated in a laboratory. In other words the manufacture of a whole new universe done by scientists in their labs. Currently this is beyond our present technical capabilities, because the pressures involved to squeeze the matter to an extraordinarily high density are too great.
However this does not exclude the possibility that an alien civilization some few hundred or thousand of years ahead of us could be engaging in universe manufacture as we speak. This notion has led to the British physicist Edward Harrison to speculate that, perhaps our own universe was made by intelligent beings living in another universe.
On taking this idea further, try to image that the reality that is currently being experienced by everyone is only one of numerous realities. For example somewhere out there in a different time, place or space, Germany won the second world war or the North Pole was hot! Although it may appear to be a curious notion, there is some scientific evidence to suggest that the idea may be closer to fact. Quantum Theory, which successfully describes the world of the atom, shows us that everything may not be as it first seems. A nice example of this, is when one single atom has been shown to exist in many places at once! This phenomenon has been shown in experiments, but it doesn't explain why the stuff we are all made of can be in more than one place at any given time, but we apparently cannot. More and more scientists are becoming convinced that the world of atoms is the same as our own. Should this prove to be true then indeed there must be alternative realities in alternative universes that exist alongside our own.
The consequences and implications for us are quite amazing; just because you may die in this universe does not necessarily mean that you have died in another. The only versions of you that continue to perceive the world are those that remain alive. Could this be the closest thing thing that we will get to immortality? See also: bilocation, quantum theory and Versailles Time-Slip.
Source: Information for this topic taken from an article in The Daily Express - Could we really be living in a parallel universe? - Saturday 19 January 2002