Everyone likes the idea of a Multiverse. It may be the appeal of an ideal chance where you have second chances and things could turn out differently. So is there really a place in science where all this happens? Physicists, when they say ‘Universe’, may mean the observable Universe which is only a part of the whole Universe. Just imagine that an alien is on the edge of our observable Universe. It will see the parts of the Universe that we can’t see because light from that part have not reached us yet. This is a widely understood observation, but it is not the related to the idea of a parallel Universe or of a Multiverse at all.
There are three different Multiverse models in physics. Type one: The Universe is known as ‘bubble’ Universes. This is a straight forward kind of Multiverse. The basic idea of this model is, perhaps there are other bubbles just like ours and are so distant that we will never be able to see them or they lie in the center of a black hole. This kind of model was created as an attempt to explain why our Universe is so good at making stars, galaxies, black hole and life. As the argument goes, if each of these separate mutually unseen able bubbles have different laws of physics, then we can only exist in a bubble which has right physical laws that allow us to exist. We have to live in a Universe where the Earth can form because if Earth couldn’t form then we couldn’t be here. If you are not convinced by this logic, then don’t worry because there is no experimental evidence for this type of a Multiverse.
Multiverse Type two is formed on the basis of a Membrane and extra dimensions. String theorists have proposed the idea that what we think is our Universe is nothing but a three dimensional layer of many layers like a single page among many in a book (Read more about string theory and multi dimensions here), Just like every page in a book has its own two dimensional area. These layers are called membranes. There is no evidence for this kind of Multiverse too.
Multiverse type three is a ‘many world’ picture of quantum mechanics. As we know the quantum world is based on probabilities and every action of ours can have many outcomes, but when we perform that action, only one will exist. Suppose you are standing on a junction and there are two ways that you can go. So you have to make a choice, go left or go right. The current Universe in which this situation is happening gives rise to two other ‘daughter’ Universes depending on the options you have: one in which you can go right, and one in which you can go left. There is some experimental evidence for this model as experimentalists are becoming extremely able to manipulate and control ever larger quantum mechanical systems in their labs. These systems approach the line between the quantum realm and our everyday life. We must remember that physics is a science not a philosophy and in our attempts to explain the Universe that we observe, we have to make claims that can in principle be tested.