Traditionally, cosmology imagined the Universe as a linear model. That is, a unique Universe with a beginning and probably an end.
For the linear model, the Big Bang is the beginning of everything: space, time, physical laws and all matter and energy.
If this is true, there is only one Universe and it encompasses everything that exists. But the linear model has contradictions and does not solve all issues. Therefore, other models were raised.
Einstein proposed the cyclic Universe model. For him cyclic model, the Universe is born and dies infinitely. There is no single Universe, but infinities. Each Universe is the cycle between a Big Bang and the next. The Universe expands and contracts, so it starts with a Big Bang and ends with a new Big Bang. Every time a Universe dies, a new one begins.
Universe of Branas or membranes
String theory poses a cyclic model called "Universe of Branas". According to this model, each Universe is a brane or membrane. There are endless branes. The branes vibrate and sometimes collide. When two branes collide, a new Big Bang is produced and another Universe is born.
For the Parallel Universes model, there are many universes at the same time. But, although they exist at the same time, they are independent and it is impossible to move from one to another. Their physical laws are different and only some could have life. The best known models of parallel universes are bubble universes and the multiverse.
Bubble Universes: New Universes are born from others that already exist. A new Universe can be born at any time and place in the Cosmos. There is an inflation of space, like the one created by our Universe, and a new Universe begins within another.
Multiverse: It is a consequence of the laws of probability of quantum mechanics. It states that, if time is infinite, all probabilities end up being fulfilled. Each probability is fulfilled in a parallel Universe. So there would be parallel Universes with another exactly identical "I", and parallel Universes with a totally different "I". It is a difficult idea to understand.
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