The Parallel Universe Theory
The multiverse(or metaverse) is a hypothetical group of multiple separate universes including the universe in which humans live.
Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, the physical laws and the constants that describe them. The different universes within the multiverse are called the "parallel universes", "other universes" or "alternative universes".
The structure of the multiverse, the nature of each universe within it, and the relationships among these universes vary from one multiverse hypothesis to another.
Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, and literature, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternate universes", "quantum universes", "interpenetrating dimensions","parallel dimensions","parallel worlds", "parallel realities", "quantum realities", "alternate realities", "alternate timelines", "alternate dimensions", and "dimensional planes".
The physics community continues to debate the multiverse hypotheses. Prominent physicists are divided in opinion about whether any other universes exist.
Some physicists say the multiverse is not a legitimate topic of scientific inquiry. Concerns have been raised about whether
attempts to exempt the multiverse from experimental verification could erode public confidence in science and ultimately damage the study of fundamental physics. Some have argued that the multiverse is a philosophical rather than a scientific hypothesis because
it cannot be falsified. The ability to disprove a theory by means of scientific experiment has always been part of the accepted scientific method. Paul Steinhardt has famously argued that no experiment can rule out a theory if the theory provides for all possible outcomes.
In 2007, Nobel laureate Steven Weinberg suggested that if the multiverse existed, "the hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our Big Bang is doomed, for their values
 would be an accident of the particular part of the multiverse in which we live."
Search for evidence:
Around 2010, scientists such as Stephen M. Feeney analyzed Wilkinson Microwave Anisotropy Probe (WMAP) data and claimed to find evidence suggesting that our universe collided with other (parallel) universes in the distant past. However, a more
thorough analysis of data from the WMAP and from the Planck satellite, which has a resolution 3 times higher than WMAP, did not reveal any statistically significant evidence of such a bubble universe collision. In addition, there was no evidence of any gravitational pull of other universes on ours.
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More Information :
From science fiction to science fact, there is a concept that suggests that there could be other universes besides our own, where all the choices you made in this life played out in alternate realities. The concept is known as a "parallel universe," and is a facet of the astronomical theory of the multiverse.
The idea is pervasive in comic books, video games, television and movies. Franchises ranging from "Buffy the Vampire Slayer" to "Star Trek" to "Doctor Who" to "Digemon" use the idea to extend plotlines. (A fuller list of parallel universes in fiction is at the bottom of the article.)
There actually is quite a bit of evidence out there for a multiverse. First, it is useful to understand how our universe is believed to have come to be.
Arguing for a multiverse
Around 13.7 billion years ago, simply speaking, everything we know of in the cosmos was an infinitesimal singularity. Then, according to the Big Bang theory, some unknown trigger caused it to expand and inflate in three-dimensional space. As the immense energy of this initial expansion cooled, light began to shine through. Eventually, the small particles began to form into the larger pieces of matter we know today, such as galaxies, stars and planets.
One big question with this theory is: are we the only universe out there? With our current technology, we are limited to observations within this universe because the universe is curved and we are inside the fishbowl, unable to see the outside of it (if there is an outside.)
There are at least five theories why a multiverse is possible, as a 2012 Space.com article explained:
1. Infinite universes. We don't know what the shape of space-time is exactly. One prominent theory is that it is flat and goes on forever. This would present the possibility of many universes being out there. But with that topic in mind, it's possible that universes can start repeating themselves. That's because particles can only be put together in so many ways. More about that in a moment.
2. Bubble universes. Another theory for multiple universes comes from "eternal inflation." Based on research from Tufts University cosmologist Alexander Vilenkin, when looking at space-time as a whole, some areas of space stop inflating like the Big Bang inflated our own universe. Others, however, will keep getting larger. So if we picture our own universe as a bubble, it is sitting in a network of bubble universes of space. What's interesting about this theory is the other universes could have very different laws of physics than our own, since they are not linked.
3. Daughter universes. Or perhaps multiple universes can follow the theory of quantum mechanics (how subatomic particles behave), as part of the "daughter universe" theory. If you follow the laws of probability, it suggests that for every outcome that could come from one of your decisions, there would be a range of universes — each of which saw one outcome come to be. So in one universe, you took that job to China. In another, perhaps you were on your way and your plane landed somewhere different, and you decided to stay. And so on.
4. Mathematical universes. Another possible avenue is exploring mathematical universes, which, simply put, explain that the structure of mathematics may change depending in which universe you reside. "A mathematical structure is something that you can describe in a way that's completely independent of human baggage," said theory-proposer Max Tegmark of the Massachusetts Institute of Technology, as quoted in the 2012 article. "I really believe that there is this universe out there that can exist independently of me that would continue to exist even if there were no humans."
5. Parallel universes. And last but not least as the idea of parallel universes. Going back to the idea that space-time is flat, the number of possible particle configurations in multiple universes would be limited to 10^10^122 distinct possibilities, to be exact. So, with an infinite number of cosmic patches, the particle arrangements within them must repeat — infinitely many times over. This means there are infinitely many "parallel universes": cosmic patches exactly the same as ours (containing someone exactly like you), as well as patches that differ by just one particle's position, patches that differ by two particles' positions, and so on down to patches that are totally different from ours.
Famously, physicist's Stephen Hawking's last paper before his death also dealt with the multiverse. The paper was published in May 2018, just a few months after Hawking's demise. About the theory, he told Cambridge University in an interview published in The Washington Post, "We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse to a much smaller range of possible universes."