Extra Universes

String theory suggests that we live in a multidimensional universe or a multiverse. Do we really need extra universes, or is one enough?

That question is hard to answer because no one has discovered other universes. It's all suggested in the math used to describe Einstein's relativity and string theory.

When we use the term 'parallel universes' this usually suggests that universes like ours are all packed together in a multiverse. This is a product of the many worlds interpretation of quantum physics. This is a result of the Schrödinger equation that predicts that a quantum event can have a number of different outcomes at the same time. This suggests that you could exist in many different worlds and experience different lives. The question arises of how we could travel to these other universes.

One way would be to travel through hyperspace, which lies in extra dimensions. The other way is find a way to travel across the barrier between these extra universes. These imaginary barriers could be right next to us. Again, this might involve going into other dimensions, or it could be by using quantum tunneling. This is where a particle can jump from one location to another across a barrier. There is always a probability, although small, that a particle can jump across a barrier. However, particles can only jump from a higher energy state to a lower one. I think we're going to have to be more advanced to discover a way to use this method.

There is also the multiverse idea in which these parallel universes were created by inflation after the Big Bang, and these extra universes have different physical constants. Both of these ideas come from 'Brane' theory. When two branes collide they create Big Bangs, producing new universes. The reason why branes collide is that they are like sheets flapping in the wind.
A D-brane is where strings are attached, and they represent a Brane world. Our world is a 3-brane. However, we could be living in a D-brane with infinite dimensions where gravity and time are different in each of these dimensions.

Another idea is the Randall-Sundrum model that predicts that our universe is a five-dimensional anti-de Sitter space (a maximally symmetric vacuum solution to Einstein's field equation with a negative cosmological constant) where all elementary particles are localized on a (3 + 1) dimensional brane. The exception is the graviton (gravity carrying) particle. The reason for this is that this theory predicts that gravity is different in these worlds or universes, depending upon the dimensional configuration of the brane that the universe is located. Basically, in the RS1 model the strength of gravity depends upon its position in the fifth dimension but it is consistent throughout the brane. In the RS2 model predicts localized gravity where graviton particles can't wander that far through other dimensions.

The truth is that these stringy theories are not consistent and do not predict dark matter in a workable manner. There could be not only infinite dimensions but also infinite universes. The problem is to find a way to detect them. That will happen eventually. Stay tuned.

Thanks for reading.