You hear about parallel universes all the time in science fiction (see Fringe, probably a billion episodes of Star Trek, Stephen King’s Dark Tower series, etc.). But did you know that scientists take parallel universes seriously and consider them possible. There are several types of possible parallel universes. This is the third post in a series of five posts. The first can be found here and the second here.
Up till now, we’ve dealt with universes that followed the same laws of physics and are much like our own universe. Both the parallel observable universes and the many worlds interpretation of quantum mechanics started with the same initial conditions (what the universe was like in the instants after the big bang) and have the same laws of physics. Now we start getting into universes that have different initial conditions and different laws of physics.
Everyone knows that the universe began with a big bang and has been expanding ever since. Fewer people know about the theory of inflation, which refined the big bang to explain some of its quirks. Under the theory of inflation, the universe in its very, very, very, very early stages (specifically between 10^-36 to 10^-32 [a fraction that has 32 zeroes between the period and the number] seconds after the big bang) experienced a repulsive force so strong that it expanded 10^78 (a one with 78 zeros after it) times its initial size. That is a ridiculously fast expansion given how huge the universe got in that ludicrously small amount of time.
Overall, the inflationary model is highly successful. But, as with everything in science, there is still debate about some of the finer aspects of inflation. So there are several inflationary models. One of the most popular models right now is called chaotic inflation. The idea is pretty simple: the universe is still undergoing inflation; but within that inflation, there are regions where the inflation stops and creates universes, including our own. Each of these bubble universes would be another type of parallel universe. However, the space between our universe and these bubble universes is growing exponentially and could never be crossed. The chaotic inflation continues infinitely and generates an infinity of universes, and the “big bang” is only the start of each universe and does not correspond to the beginning of everything. Max Tegmark calls these types of parallel universes Level II parallel universes in his taxonomy.
These bubble universes, unlike the other universes we’ve looked at, can be wildly different places from our own. Some may have more dimensions, which is impossible for us to even wrap our head around (try watching this video about 4-dimensional hypercubes as an example; it’s boring and poorly shot, but it explains the concepts beautifully). There could also be different types of subatomic particles, leading to strange, different states of matter. Further still, these bubble universes could have different physical constants from our own, which would lead to matter interacting differently. For example, one of the main physical constants is the cosmological constant, which determines the rate of expansion of the universe. Too high and the universe will fly apart too quickly for galaxies and us to form; too low and the universe just collapses back in on itself before it even has a chance to get started.