Do Parallel Universes exist ?

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Is our universe unique? 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.

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.

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.

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

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

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.

Not everyone agrees with the parallel universe theory, however. A 2015 article on Medium by astrophysicist Ethan Siegal agreed that space-time could go on forever in theory, but said that there are some limitations with that idea.

The key problem is the universe is just under 14 billion years old. So our universe's age itself is obviously not infinite, but a finite amount. This would (simply put) limit the number of possibilities for particles to rearrange themselves, and sadly make it less possible that your alternate self did get on that plane after all to see China.

Marvel Comics and DC Comics feature stories set in parallel universes that are part of the multiverse.

Many anime series, such as "Digimon," "Dragon Ball" and "Sonic the Hedgehog" feature alternate versions of their characters from other universes.

Parallel universes appear in games such as "Dungeons & Dragons," "BioShock Infinite," the "Final Fantasy" franchise, "Half-Life," "League of Legends," "Mortal Kombat" and "The Legend of Zelda."

"Flatland: A Romance of Many Dimensions" (1884), by Edwin A. Abbott, is a story about a two-dimensional world that includes living geometric figures such as circles, triangles and squares. The novel also includes other universes such as Lineland, Spaceland and Pointland. This book was adapted into a feature film in 2007.

"Men Like Gods" (1923), an H.G. Wells novel, included a "paratime" machine and explored the multiverse.

"The Chronicles of Narnia" (1950-56), a C.S. Lewis book series, features several children who move between our world and the world of Narnia, where there are talking animals. Some of these books were released as feature films earlier in the 2000s.

An episode of "Star Trek" featured a "mirror universe" in which the characters were more ruthless and warlike. The concept was repeated in nearly every subsequent "Star Trek" series. In 2009, the "Star Trek" universe got a reboot in a movie that put the characters from the 1960s original series in an alternate universe.

In "The Dark Tower," a Stephen King series that began in 1982, travellers go through portals to different levels of the titular tower (in other words, parallel Earths). Part of the series was adapted into a feature film in 2017.

The "Back to the Future" movie series (which began in 1985) follows the adventures of the McFly family, including visits to 1885, 1955 and 2015. The second film in particular shows the drawbacks of an alternate reality, when one character uses it to get rich by nefarious means. The series starred Michael J. Fox.

In the "His Dark Materials" series by Philip Pullman, children move between multiple worlds. The first book, "The Golden Compass," was adapted into a film in in 2007.

"Sliding Doors" (1998) is a film that showed two parallel universes depending on whether the main character caught a train or not. It starred Gwyneth Paltrow and John Hannah.

"Run Lola Run" (1998) is a film starring Franka Potente. The film shows multiple alternatives as a woman tries to get 100,000 Deutsche marks in only 20 minutes to save her boyfriend's life.

"Timeline" (1999) by Michael Crichton follows historians who go back in time to the Middle Ages. (While the book is mostly a time travel book, the multiverse is used in it as well.) A film based on the book was released in 2003.

In 1954, a young Princeton University doctoral candidate named Hugh Everett III came up with a radical idea: That there exist parallel universes, exactly like our ­universe.