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For many years, the theories that there are multiple universes other than our own have appeared as a sign that, perhaps, we are not the only ones existing on the existential plane. There is no truly scientific evidence to confirm the existence of multiple universes, but that does not mean that there are no events that have hinted otherwise. In fact, the only reason that there is no physical theory to back it up is that we cannot see it, nor study it, nor observe it, so we do not know if it really exists.
Multiple Universes different from the visible Universe.
The theory that could support the existence of parallel universes is string theory.
It was born in 1968 due to the enormous failure between quantum theory and relativity theory. While quantum theory explains the forces that govern the universe in a microscopic way, relativity theory does so on a cosmic scale. The problem is that gravity as we know it is possible to be explained on a cosmic scale, but it is impossible to do so on a microscopic scale, which generates a contradiction in the most scientifically accepted theories about our universe.
String theory explains that the particles that make up the universe are long, vibratory objects that resemble strings (hence their name) and not dots as they are believed to be today.
In this way, the vibration of this string depends on the energetic flow it has, which could also affect the behavior of the rest of the particles that form it, which would also have the shape of strings.
The particles of the universe according to this theory would be open strings that when joined together would form a circle or a closed string. In our case, gravity would be formed by a closed string, which means that when one or two more strings join together, gravitons would be created, the particles that form gravity on a cosmic and quantum scale.
Real possibility of a Multi Universe.
There is another point to take into account in the theory of strings that could affirm the existence of other universes, which are called Branas. Branes would become the “walls” to which the end of each string is attached. Branes would also have different dimensions. They could stretch and join with others. Thus, the properties of each particle depend on them, from its colour to its mass.
In this way, the strings would have millions of possibilities to change and modify themselves, creating other strings from a specific alteration.
String theory would not only be a form of Theory of Everything that would unify all the laws of physics, but it would be, for the first time, the possibility of finally talking about the existence of different universes.