Parallel Universes or "Multiverses" that most people used to see in Sci-fi are one of the serious research topics of many physicists, mathematicians, and cosmologists. According to a group of scientists which includes David Gross, the idea of the multiverses are far from reality and not an elegant one. However, for some scientists like Alan Guth and Andrei Linde who have established themselves in science, this concept is very elemental one and it is the necessity of the mathematical equations' results. There are also String Theoreticians like Brian Greene who has already written books on it.
If Multiverses really exist, they could be very complex for our reality perception than we guess. If so, unlike we think it is unique and sole, our 13,7 billion years old universe could be a little part of a huge and procreant structure!
The foresight of the multiverses had started on the probability calculations of having a twin brother somewhere in the universe if it is infinite. Some quantum mechanics physicists proposed that if one of the values of all quantum states for a quantum system happens in this universe, then other probabilities would have occurred in other universes as a physical reality. Since this approach is an interpretation issue, science environment didn't take it that much seriously at first. But the subject which is telling that the existence of other universes beyond ours came to the fore upon the proposal of inflation and string theories which the astronomers and the physicists mooted in the 1980s. The topic had earned its reputation back under the name of "multiverses" or "multiverses model". And this time, not as a speculation and an interpretation of the physics equations, but as the foresight of the equations itself. Up to that time, the Standard Big Bang Model was the most successful universe model.
The inflation theory is one of the recognised and popular model at the moment because it confirms uniformity data of the universe. However, according to Andrei Linde, who is a Russian scientist from Stanford University, there is no need to accept the explanations of the universe's expansion which are only based on thermodynamic principles.
As Andrei Linde moots, new universes could be born in each of the space-time regions where the energy density is high.
Linde took into account each energy value in the beginning of the universe, which the scalar field probably could have. and he calculated the states which ones could cause the inflation. The scalar field was not smooth and flat like a lake but wavy and fluctuant like an ocean. In a way of more scientific explanation, in the scalar field what the vacuum is made of, has continuous fluctuations of energy and quantum waves. Those fluctuations happen in a very small period of time and impossible to observe. They sometimes cause macroscopic-sized inflations and thus they initiate newly born universes. As the universe expands, the fluctuations are also propagated. Therefore energy density increase where the waves superpose each other. And also the energy density decreases where those fluctuation waves interfere each other destructively. Thus some parts of the universe became cooler whereas the other parts were hotter.
The fields which have high energy density ceases expanding and after that it explodes and inflates. The fields which have low energy density goes on expanding. The explosion and respective inflation of high-energy fields refer to the creation of the new universes. And so the islands of the universe are being created in the Mega-Universe ocean. Inflation theory tells us the big bang didn't happen only one time. In the history of Mega Universe, this happened many times and will be going on forever.
Let's turn back to the question of "what was there before Big Bang? and how everything is created from nothing?". As Linde's theory says, there was a scalar field before the Big Bang and everything, our universe, and other universes existed because of this field's quantum fluctuations. And as a consequence, one could ask that "what was there before the scalar fields?" Another scientist, Alexander Vilenkin has some answers to this question. He theorizes that there were only Mathematical equations at first. He modeled the approaches of quantum tunneling and four-dimensional sphere for his solutions in order to explain for the coming out of nothing.
Before the Inflation and String theories, multiverses are never been interested in the academic literature. It has always been a subject of Sci-fi industry. But the 1980s were a milestone for this theory. Those theories opened a door to the scientists for a comprehensive explanation of our existence by its compatibility with the Quantum Mechanics. But the discussions on this topic never seem to end in the near future. Before anything else, acceptance of the theories which will never be proven by any experiments should be well defined.
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