Notes from an article by Max Tegmark in Scientific America, May 2003

This is great discussion on the hypothesized four different types of multiverses (summary--some quotations--from the picture pages of the article):

Level 1 Multiverse: The simplest type of parallel universe is simply a region of space that is too far away for us to have seen yet. THe farthest that we can observe is currently about 4 x 10^26 meters, or 42 billion light-years--the distance that light has been able to travel since the big bang began. (The distance is greater than 14 billion light-years because cosmic expansion has lengthened distances.) Each of the Level 1 parallel universes is basically the same as ours. All the differences stem from variations in the initial arrangement of matter.

Support for this universe comes from the argument that if the universe is infinitely large (which it seems to be) and has enough mass, then, just by chance alone, every variation of our local universe (which is made up of 10^118 subatomic particles) will exist somewhere. That means that an identical parallel universe would be 10^10^118 meters away from ours.

Level II Multiverse: A somewhat more elaborate type of parallel universe emerges from the theory of cosmological inflation. The idea is that our Level 1 multiverse--namely, our universe and contiguous regions of space, including the copies and variations--is a bubble embedded in an even vaster but mostly empty volume. Other bubbles exist out there, disconnected from ours. They nucleate like raindrops in a cloud (or bubbles in a rising bread). During nucleation, variations in quantom fields endow each bubble with properties that distinguish it from other bubbles.

Cosmologists infer the presence of Level II parallel universes by scrutinizing the properties of our universe. These properties, including the strength of the forces of nature and the number of observable space and time dimensions, were established by random processes during the birth of our universe. Yet they have exactly the values that sustain life. That suggests the existence of other universes with other values.

Level III Multiverse: Quantom mechanics predicts a vast number of parallel universes by broadening the concept of "elsewhere." These universes are located elsewhere, not in ordinary space but in an abstract realm of al possible states. Every conceivable way that the world could be (within the scope of quantom mechanics) corresponds to a different universe the parallel universes make their presence felt in laboratory experiments, such as wave interference and quantum computation.

According ot the principle of Ergodicity, quantum parallel universes are equivalent to more prosaic types of parallel universes (like those found in Level I. The key idea is that parallel universes, of whatever type, embody different ways that events could have unfolded.

Most people think of time as a way to describe dchange. At one moment, matter has a certain arrangement (think of rolling a die); a moment later, it has another. The concept of multiverses suggest an alternative view. If parallel universes contain all possible arrangements of matter, then time is simply a way to put those universes into sequence. The universes themselves are static; change is an illusion, albeit an interesting one. (Think movie theater--or, better yet, twist-a-plot books, since free-will should exist thanks to quantum mechanics.)

Level IV Multiverse: The ultimate type of parallel universe opens up the full realm of possibility. Universes can differ not just in location, cosmological properties or quantum state but also in the laws of physics. Existing outside of space and time, they are almost impossible to visualize; the best one can do is to think of them abstractly, as static sculptures taht represent the mathematical structure of the physical laws that govern them. For example, consider a simple universe: Earth, moon, and sun, obeying Newton's laws. To an objective observer, this universe looks like a circular ring wrapped in a braid (moon). Other shapes embody other laws of physics. This paradigm solves various problems concerning the foundations of physics.

In other words, universes are just mathematical structures. Think of people as an animal crawling on the ground, and a bird as the one seeing the entire universe at once (this analogy is much better in the article).