Universe, is a rather overwhelming concept. We have always studied the Universe as everything there is. There is nothing outside the Universe or rather, there is no outside to the Universe. Universe is all there is and everything that exists is in this one particular Universe. And then, we study the origin of this universe, its history, its expanse and even its future, but what if there are other Universes? And we, being minuscule parts of one of those Universes cannot even think of existence of others? But, the Scientists have thought and thus, brainstormed a rather fantasy-like Multiverse theory.
The Name gives away everything in this case. The multiple Universes that co-exist with our own universe. Since 1930s there have been plenty of attempts in explaining why this is a possibility. One of the theories suggests that the Universe made up of particles, can only have a finite number of arrangements of those particles. Even though the number is very very huge, it is not infinite. And as the Mathematics gives, there can be 10^10^122 possible arrangements. So, after these many arrangements, there must be repetitions thus giving rise to similar other universes. There can certainly be other universes which differ an arrangement of just one particle or just two particles.This can explain the possibility of Multiverse existence.
Another theory says that space-time is flat, and thus it is spread infinitely unlike the Universe. So, there must be other Universes along that infinitely spread space time. We humans, are on a tiny planet inside a gigantic Universe. We cannot see beyond our universe, it does not mean there is nothing beyond. Although, these theories are a little far-fetched and do not have any supporting evidence, they make a cool subject for numerous Sci-fis.
So, let’s consider there are parallel universes besides ours. So, what does that exactly mean? It means there can be infinite parallel Universes. Each decision you make is reflected as an alternative decision in one of the universes. So, if you take a decision to take a job and move to China, in some other universe, you might not accept the job, in some other you might accept and not go to china and so on. There are almost infinite possibilities theoretically. Another alternatives maybe that the parallel universes maybe ahead in time or back in time. So, there can exist an exact same parallel universe just that it is 10 years ahead in time than ours. There are really infinite possibilities and thus makes Multiverse theory still a fiction.
Some astrophysicists have proposed the theories that could explain Multiverses but, there being no evidence or even a concrete theory, Scientists are always skeptical when it comes to this subject. Astrophysicist, Ethan Siegal once written that as per The Big Bang theory, our universe has just started 13.9 billion years ago, at the start of universe, due to the enormous energy, the expansion was exponential but as the time passed, things started to cool down and slow down. He then suggested that for particles to achieve all possible combinations it is highly far-fetched and the universe will eventually come to a halt with a certain most stable combination of particles.
There have been imaginative theories regarding the Multiverse but, it has no evidence or a concrete proof as of now. But, ironically, there have not been any solid proofs of denying the theory either, which leaves us with yet another dangling unsolved mystery. May be there are parallel universes or maybe there aren’t, this has become a topic more sort of philosophical importance than of physical significance. As of now, this topic will still remain only as a fictional favorite.