In the theoretical multiverse, there are countless universes, with new ones bubbling constantly out from existing universes.

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Branes, Crunches and Other Big Ideas

Here's a thought: What if our universe is but the offspring of another, older universe? Some astrophysicists speculate that this story is written in the relic radiation left over from the big bang: the cosmic microwave background (CMB).

Astronomers first observed the CMB in 1965, and it quickly created problems for the big bang theory -- problems that were subsequently addressed (for a while) in 1981 with the inflation theory. This theory entails an extremely rapid expansion of the universe in the first few moments of its existence. It also accounts for temperature and density fluctuations in the CMB, but dictates that those fluctuations should be uniform.

That's not the case. Recent mapping efforts actually suggest that the universe is lopsided, with more fluctuations in some areas than in others. Some cosmologists see this observation as supporting evidence that our universe formed out of a parent universe.

In chaotic inflation theory, this concept goes even deeper: an endless progression of inflationary bubbles, each becoming a universe, and each of these birthing even more inflationary bubbles in an immeasurable multiverse [source: Science News].

Still other models revolve around the formation of the pre-big bang singularity itself. If you think of black holes as cosmic trash compactors, they stand as prime candidates for all that primordial compression, so our expanding universe could theoretically be the white hole output from a black hole in another universe. A white hole is a hypothetical body that acts in the opposite manner of a black hole, giving off serious energy and matter rather than sucking it in. Think of it as a cosmic exhaust valve. Some scientists propose that our universe may have been born inside a black hole, and every black hole in our own universe could each contain separate universes as well.

Other scientists place the formation of the singularity inside a cycle called the big bounce in which our expanding universe will eventually collapse back in on itself in an event called the big crunch. A singularity once more, the universe will then expand in another big bang. This process would be eternal and, as such, every big bang and big crunch the universe ever experiences would be nothing but a rebirth into another phase of existence.

The last explanation we'll discuss also supports the idea of a cyclical universe, courtesy of string theory. It surmises that new matter and energy spring into existence every trillion years when two extra-dimensional membranes, or branes, collide in a zone outside our universe.

What existed before the big bang? It's still an open question. Perhaps nothing. Perhaps another universe or a different version of our own. Perhaps a sea of universes, each with a different set of laws dictating its physical reality.

Explore the links on the next page to tackle other lofty, cosmological questions.