The idea that our universe may be one of an infinite number of universes has gained popularity. American Alan Guth (gooth) and Russian Andrei Linde are believers. According to the multiverse theory, there are infinite Big Bangs before and after ours. New universes are forever coming into existence. Expansion begins with inflation, a period during which universes expand faster than light. If we could turn off the galaxies, we would see a sea of radiation left over from the Big Bang. In fact, the COBE and WMAP spacecraft have mapped this Cosmic Background Radiation.
Alan Guth shelved the idea for a while, but Linde stayed with it. Believers contend that dark energy points to a multiverse. They say string theory, too, lends credence. String theory suggests that there may be more dimensions than height, width & depth. There may be duplicates of ourselves in parallel universes. Naturally, there are scoffers, cosmologists who think the multiverse is fantasy.
Swedish cosmologist Max Tegmark claims there are 4 kinds of parallel universes and has written about them.