What is a parallel universe? Does it really exist?

well, the multiverse is a theory in which our universe is not the only one, but states that many universes exist parallel to each other. These distinct universes within the multiverse theory are called parallel universes. A variety of different theories lend themselves to a multiverse viewpoint.

Not all physicists really believe that these universes exist. Even fewer believe that it would ever be possible to contact these parallel universes.

The idea of Level 1 parallel universes basically says that space is so big that the rules of probability imply that surely, somewhere else out there, are other planets exactly like Earth. In fact, an infinite universe would have infinitely many planets, and on some of them, the events that play out would be virtually identical to those on our own Earth.

We don’t see these other universes because our cosmic vision is limited by the speed of light — the ultimate speed limit. The light started traveling at the moment of the big bang, about 14 billion years ago, and so we can’t see any further than about 14 billion light-years (a bit farther, since space is expanding). This volume of space is called the Hubble volume and represents our observable universe.

The existence of Level 1 parallel universes depends on two assumptions:

• The universe is infinite (or virtually so).
• In an infinite universe, every single possible configuration of particles in a different volume takes place multiple times.

Level 1 universe

If Level 1 parallel universes do exist, reaching one is virtually (but not entirely) impossible. For one thing, we wouldn’t know where to look for one because, by definition, a Level 1 parallel universe is so far away that no message can ever get from us to them, or them to us. (Remember, we can only get messages from within our own different volume.)

Level 2 universe

In a Level 2 parallel universe, regions of space are continuing to undergo an inflation phase. Because of the continuing inflationary phase in these universes, space between us and the other universes is literally expanding faster than the speed of light — and they are, therefore, completely unreachable.

Two possible theories present reasons to believe that Level 2 parallel universes may exist: eternal inflation and ekpyrotic theory.

In eternal inflation, recall that the quantum fluctuations in the early universe’s vacuum energy caused bubble universes to be created all over the place, expanding through their inflation stages at different rates. The initial condition of these universes is assumed to be at a maximum energy level, although at least one variantchaotic inflation,predicts that the initial condition can be chaotically chosen as any energy level, which may have no maximum, and the results will be the same.

The findings of eternal inflation mean that when inflation(Big bang) starts, it produces not just one universe, but an infinite number of universes.

Level 3 universe

A Level 3 parallel universe is a consequence of the many-worlds interpretation (MWI) from quantum physics in which every single quantum possibility inherent in the quantum wavefunction becomes a real possibility in some reality. When the average person (especially a science fiction fan) thinks of a “parallel universe,” he’s probably thinking of Level 3 parallel universes.

Level 3 parallel universes are different from the others posed because they take place in the same space and time as our own universe, but you still have no way to access them. You have never had and will never have contact with any Level 1 or Level 2 universe (we assume), but you’re continually in contact with Level 3 universes — every moment of your life, every decision you make, is causing a split of your “now” self into an infinite number of future selves, all of which are unaware of each other.

Though we talk of the universe “splitting,” this isn’t precisely true. From a mathematical standpoint, there’s only one wavefunction, and it evolves over time. Thesuperpositionsof different universes all coexist simultaneously in the same infinite-dimensional Hilbert space. These separate, coexisting universes interfere with each other, yielding the bizarre quantum behaviors.

Level 4 universe

A Level 4 parallel universe is the strangest place (and most controversial prediction) of all because it would follow fundamentally different mathematical laws of nature than our universe. In short, any universe that physicists can get to work out on paper would exist, based on themathematical democracy principle:Any universe that is mathematically possible has an equal possibility of actually existing.

According to quantum mechanics, it is possible that there can be multiple earths are present in this infinite universe. we can understand this through an example: 

suppose we have 3 digit number say for "123" so if ask you to count how many different combinations you can make with this number? you arranged and say only 6 combinations can be made from the given number, Now if I say what combinations come just after the 6th combination? here mind boggles any previous combination can occur after the 6th one.

likewise, numbers any matter or object can be back to its initial form after going through multiple stages similarly, the universe is infinitely long and it can have almost every type of probability of matter so maybe the galaxies reconfigure themselves in the universe which must be distant from us but it can be a possibility which gives meaning to the multiverse.

Cases that blow your mind to think about the parallel universe.

1. The man who arrived at Tokyo international airport with a passport from a country that doesn't exist.

Allegedly, this man told customs he was in Japan for business and had legitimate visa stamps, currency from multiple European countries, checks from an unknown bank, and a driver’s license issued in Taured. After, seeing this everyone was shocked and for further verification, He was sent to a hotel while things were sorted out, while two immigration officers stood outside of his hotel door. But when they went to check on him in the morning, he’d vanished. This was from 15 stories up, above a busy street, so escaping through a window was unlikely, if not impossible. The Tokyo Metropolitan Police Department searched but never located the man from Taured. 

2. The strange man who wound up shipwrecked Germany from an unknown world.

In 1851 a man called himself Jophar Vorin So this story goes, a peculiar guy popped up in a village, where he was picked up and questioned by authorities. The only comprehensible language he spoke was a broken variation of German, The traveler spoke and wrote in unknown languages he called " Laxarian " and "Abramian " but he said his name was Jophar Vorin and claimed to come from a part of the world known as Sakria. He conveyed that he was searching for his long-lost brother, whom he'd been separated from during a shipwreck. While he had geographical information regarding his home planet, such as five compartments known as Sakria, Aflar, Aslar, Auslar, and Euplar, he couldn't trace his route on shore, and he would wind up living in Berlin, where he was a topic of discussion amongst the scientific, and simply curious, communities.

3. The Green-skinned children who simply just appeared here.

Back in the 12th century, two children, a brother, and sister, allegedly popped up in the village of Woolpit, in Suffolk, England. Both had greenish-colored skin, spoke an unknown language, wore bizarre clothing, and initially refused to eat any food other than raw beans. They were taken in, cared for, and eventually, they adapted, developing a taste for food and losing the green hue in their skin. Shortly after, the boy became sick and died, but after learning to speak English, the girl explained that she and her brother were from a place where the sun didn’t shine bright, and it was perpetually twilight. She and her brother were there, herding their dad’s cattle, when they heard some loud noise, and were suddenly in a new place where they were eventually discovered, in Woolpit. The girl grew up and integrated herself into her new surroundings. Though it remained unknown how she and her brother got there, the girl went on to get married and live her life here, in a dimension that might not actually be her place of origin.