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For decades, skeptics, physicists and mathematicians have debated over the probability of other dimensions and alternate facets of reality. One that's gotten a lot of attention recently is that of a Multiverse. In short, it's the idea that our Universe, and all that's contained within it, is just one small region of a larger existence that includes many similar, and possibly many different, universes like our own.

On the one hand, if our current theories of physics are true, the multiverse absolutely must exist. Why must the multiverse exist? Quite simply: there must be more universe than the part that is observable to us. If you look just at the portion of the universe we can see, you can measure its spatial curvature, and find that it's incredibly close to flat. No regions repeat; no locations connect or loop back on one another; no large-curvature regions show themselves on a scale approaching that of the universe we can observe. If the universe were a hypersphere, the four-dimensional analogue of a sphere, it must have a radius of curvature hundreds of times the size of what we can observe. There must be more universe out there than what we can access.

But this isn't just a conclusion from observations; it's the same conclusion that we'd draw from our leading theory of the universe's origin: cosmological inflation. Prior to the hot Big Bang, the fabric of the universe was expanding at an exponential rate, where every 10-35 seconds or so, it would double in scale in all dimensions. Inflation went on for at least as long as 10-33 seconds or so, but could have lasted far longer: seconds, years, millennia, trillions of years or an arbitrarily long length of time. When inflation ends, the universe we're left with is stretched flat, the same temperature everywhere, and far, far vaster than anything we can ever hope to observe. Considering the finite nature of all we can see, inflation is the natural way to create a multiverse of possibilities. There are countless regions where inflation did not end, and still continues today.

The idea that the Big Bang happened everywhere at once may apply to our universe, but certainly ought not to apply to the vast majority of universes existing in the multiverse. Assuming that inflation is a quantum field, like all fields we know of, it must spread out over time, meaning that in any region of space, it has a probability of ending at a certain time, but also a probability of continuing on forever. Accepting all of this leads to an inescapable conclusion: we live in a multiverse, and our universe is just one of countlessly many that exist within it.

Sources: Quora, Wikipedia, Durham University, Brittanica, Space.

Image: Pinterest.