The universe we live in might not be the only one out there in space. In fact, our universe could be just one of an endless number of universes making up a "multiverse." Though the concept may draw out credulity, there is good physics behind this idea. And there is not just one approach to get to a multiverse — several theories of physics alone point to such a result. Here are the four most conclusive theories of physics pointing towards the fact that, we live in a multiverse:
1. Infinite Universes
Scientists are not sure about the shape of space-time, but most likely, it’s flat and spreads out infinitely. But if space-time keeps on going till the end, then it must start repeating at some stage, because there are many ways, to arrange particles in space and time. So if you look far enough, you would most likely see another version of you — in fact, uncountable versions of you. Some of these versions of you might be doing exactly the same thing that you're doing right now, while others may have some different clothes on, and still others might have made many different professions and decisions. Because the visible universe extends only as far as light has had a chance to get in the 13.7 billion years since the Big Bang (that would be 13.7 billion light-years), the space-time after that point can be considered to be its own distinct universe. In this way, a huge number of universes coexist next to each other in a giant mess of universes.
2. Bubble Universes
In addition to the multiple universes created by immeasurable spreading space-time, other universes might have arisen from a theory called "eternal inflation." Inflation is the idea that the universe extended quickly after the Big Bang, inflating like a balloon. Eternal inflation, first coined by Tufts University cosmologist Alexander Vilenkin, proposed that some pockets of space stop inflating, while other areas kept on to inflate, thus producing many inaccessible "bubble universes." Thus, our universe itself, where inflation has stopped, allowed infinite stars and galaxies to form, is but a small bubble in a gigantic ocean of space, some of which is still inflating, that contains many other bubbles just like our own. And in few of these bubble universes, the laws of physics and fundamental constants might not be the same as ours, converting some of these universes to be odd indeed.
3. Parallel Universes
Another idea that arises from string theory is the idea of "braneworlds" — parallel universes that float just out of our own reach, coined by Princeton University's Paul Steinhardt and Neil Turok of the Perimeter Institute for Theoretical Physics in Ontario, Canada. The idea comes from the possibility of the existence of many dimensions to our own world than the three dimension of space and one dimension of time known to us. In addition to our own idea of three-dimensional "brane" of space, other three-dimensional branes may happen in a higher-dimensional space.
A further fature of this theory proposes that these brane universes may not always be parallel and out of our reach. Sometimes, they might bang into each other, causing frequent Big Bangs that reset the universes repeatedly.
4. Daughter Universes
The theory of quantum mechanics, which reigns over the tiny world of subatomic particles, proposes another way multiple universes might arise. Quantum mechanics states the world in terms of opportunities, rather than specific results. And the mathematics of this theory might suggest that all possible results of a situation do occur — in their own distinct universes. For instance, if you get to a junction where you can go right or left, the present universe give rise to two daughter universes: one in which you go right, and one in which you go left.
"And in each universe, there's a copy of you witnessing one or the other outcome, thinking — incorrectly — that your reality is the only reality," Greene wrote in "The Hidden Reality."
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