Die-hard science fiction fans and gamers are not the only people who think the multiverse might be real. For the past decade, cosmologists, astronomers, and theoretical and quantum physicists have pondered and posited the same idea. Right now, there are roughly four views of the multiverse: Level I, Level II, Level III and Level IV, as explained by MIT professor and cosmologist Max Tegmark.
In the Level I view, the other universes at the edge of the known expanding universe – 42 billion light years away – exist much like the one humans live in today, operating under the same laws of physics. Those who support the Level II idea of the multiverse suggest that different universes, some teaming with life, some sterile, with different histories and physics, exist beyond this one. In the Level III view, parallel universes exist randomly elsewhere, outside the boundaries of space in abstract states. The Level IV view states that physical reality, typically described by mathematics, insists what humans know as "real life" exists as mathematics, and that this universe lives as a mathematical object among many.
TL;DR (Too Long; Didn't Read)
Until scientists and researchers can find a way to mathematically prove the existence of the multiverse, its reality remains in the prediction realm of theories, not yet proven as fact. Multiple theories explain how the universe functions and predict the existence of these parallel worlds and universes. The most plausible theory at this point suggests that at the edge of the known but expanding universe, other universes exist that operate under the same laws of physics as the known universe.
The Birth of Quantum Physics
German physicist Max Planck received the Nobel Prize in 1918 for authoring the quantum theory of energy, in which his work led to a deeper understanding of the atomic and subatomic processes. His work with thermodynamics includes his definition of energy in discrete, quantized amounts that he called packets – energy quanta – and a formula now known as Planck's constant that defines the behavior of both particles and waves at the atomic level.
In 1900, Planck announced his findings, and Albert Einstein used Planck's quantum theory to describe the properties of light in 1905 and demonstrated that light had both the characteristics of a wave and a particle. Niels Bohr, another well-known physicist, used Planck's theories to develop a brand new and more precise model of the atom. In later years, Planck's work earned him the title of the father of quantum physics.
The Role of String Theory in Parallel Universes
The theory of inflation, developed in the 1980s, describes the nature of the big bang that formed and changed the global view of the universe. Essentially it explained this universe as a bubble universe among many others, and included testable predictions confirmed by observations that led to it being the current and main cosmological paradigm. String theory contributes to the theory of inflation by being the main contender for describing the fundamental theory of nature. String theory replaces the basic dot-like particles in particle physics with one-dimensional strings to provide a foundation for the joining of quantum physics and gravity. String theory essentially predicts and joins parallel universes or the multiverse along this one-dimensional string.
Doppelgangers and Parallel Universes
As described in the movie, "Sliding Doors," a parallel universe often includes another copy of you who took roads you didn't take in this lifetime. Some theorists posit that at key choice points along your path in life, when you made a decision at a metaphorical crossroads, in which the you in this life took the left fork, other "yous" in other parallel universes, which make up the multiverse, may have taken other paths. Instead of going to college, for example, another you skipped school and became an artist without a degree. Because of the infinite nature of the multiverse, there possibly exists an infinite number of yous in other worlds and universes.
Effects of the Multiverse on the Known Universe
The Level III view of the multiverse insists that monumental events in one universe often bleed over into another. As in the metaphor of parallel universes arising from making different choices at a crossroads, some theorists postulate that the actions a person takes often bleed over into other universes, resulting in different consequences. In literature, writers approach this with alternate history themes, where in another universe, for example, Hitler and the Nazis won World War II or John F. Kennedy didn't die. The television series, "The Man in the High Castle" provides a glimpse into this kind of storyline on TV.
Most of these theories remain as conjecture at this point, until future discoveries can unlock the secrets of the multiverse and give validity to the theories that predict its existence.