Do Parallel Universe really exist?

Parallel Universe

A parallel universe, also known as a parallel dimension, alternate universe, or alternate reality, is a hypothetical self-contained plane of existence, co-existing with one's own. The term "parallel universe" is more general, without implying a relationship, or lack of relationship, with our own universe. A universe where the very laws of nature are different for example, one in which there are no Laws of Motion – would in general count as a parallel universe but not an alternative reality and a concept between both fantasy world and earth.

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. These universes are all related to ours; indeed, they branch off from ours, and our universe is branched off of others. Within these parallel universes, our wars have had different outcomes than the ones we know. Species that are extinct in our universe have evolved and adapted in others. In other universes, we humans may have become extinct.

Do Parallel Universe really exist?
                        The Universe, as far as the most powerful telescopes can see (even in theory), is vast, huge and massive. Including photons and neutrinos, it contains some 10^90 particles, clumped and clustered together into hundreds-of-billions-to-trillions of galaxies. Each one of those galaxies comes with around a trillion stars inside (on average), and they’re strewn across the cosmos in a sphere some 92 billion light years in diameter, from our perspective. But, despite what our intuition might tell us, that doesn’t mean we’re at the center of a finite Universe. In fact, the evidence indicates something quite to the contrary.
      The reason the Universe appears finite in size to us — the reason we can’t see anything that’s more than a specific distance away — isn’t because the Universe is actually finite in size, but is rather because the Universe has only existed in its present state for a finite amount of time. If you learn nothing else about the Big Bang, it should be this: the Universe was not constant in space or in time, but rather has evolved from a more uniform, hotter, denser state to a clumpier, cooler and more diffuse state today.
        This has given us a rich Universe, replete with many generations of stars, an ultra-cold background of leftover radiation, galaxies expanding away from us ever-more-rapidly the more distant they are, with a limit to how far back we can see. That limit is set by the distance that light has had the ability to travel since the instant of the Big Bang.

But this in no way means that there isn’t more Universe out there beyond the portion that’s accessible to us. In fact, from both observational and theoretical points-of-view, we have every reason to believe there’s plenty more, and perhaps even infinitely more. Observationally, we can measure a few different interesting quantities, including the spatial curvature of the Universe, how smooth and uniform it is in both temperature and density, and how it’s evolved over time.

Now, that’s what we do expect, based on the known laws of physics and the observables that exist in our Universe to tell us about the inflationary state. That said, we don’t know quite a few things about this inflationary state, and what this does is bring up a huge number of both uncertainties and also possibilities:

1. We don’t know for how long the inflationary state lasted before it ended and gave rise to the Big Bang. The Universe could barely be larger than the part observable to us, it could be many ridiculous orders of magnitude larger than what we see, or it could be truly infinite in scale.
2. We don’t know if the regions where inflation ended are all the same, or whether they’re vastly different than our own. It’s conceivable that there are (unknown) physical dynamics that cause things like the fundamental constants — particle masses, strengths of forces, the amount of dark energy — to be exactly what they are for us in all regions where inflation ends. But it’s also possible that different regions where inflation ends, what we might consider different Universes, have arbitrarily different physics.
3. And if the Universes are all the same as one another as far as physical laws go, and if the number of these Universes is truly infinite, and if the many-world interpretation of quantum mechanics is completely valid, does that mean that there are parallel Universes out there, where everything in it evolved exactly the same as our own Universe did, except one tiny quantum outcome was different?

In other worlds, would it be possible that there’s a Universe out there where everything happened exactly as it did in this one, except you did one tiny thing different, and hence had your life turn out incredibly different as a result?

Where you chose the job overseas instead of the one that kept you in your country?

Where you stood up to the bully instead of letting yourself be taken advantage of?

Where you kissed the one-who-got-away at the end of the night, instead of letting them go?

And where the life-or-death event that you or your loved one faced at some point in the past had a different outcome?

It’s an incredible notion: that there’s a Universe out there for every outcome that’s conceivable. There’s one where everything with a non-zero probability of having happened is actually the reality in that Universe. But there are an awful lot of ifs that are mandatory to get there. For one, the inflationary state must have happened for not just a long amount of time — not just for the 13.8 billion years that our Universe has been around — but for an infinite amount of time.

Why is that, you ask? Surely, if the Universe has been expanding exponentially — not just for a tiny fraction of a second but for 13.8 billion years, or around 4 × 10^17 seconds — we’re talking about a tremendous volume of space! After all, even though there are regions of space where inflation ends, most of the volume of the Universe is dominated by regions where it hasn’t ended. So realistically, we’re talking about at least 10^10^50 Universes that started off with initial conditions that might be very similar to our own. 

Universes, which might be one of the biggest numbers you’ve ever imagined. And yet, there are numbers that are bigger that describe how many possible outcomes there are for particle interactions.

There are 10^90 particles in each Universe, and we need for all of them to have the exact same history of interactions for 13.8 billion years to give us a Universe identical to our own, so that when we choose one path over another, both Universes still wind up existing. For a Universe with 10^90 quantum particles in it, that’s asking an awful lot — for fewer than 10^10^50 possibilities to exist for how those particles will interact with one another over 13.8 billion years. The number you see above, for instance, is just 1000! (or (10^3)!), or 1000 factorial, which describes the number of possible permutations there are for 1000 different particles to be ordered at any instant in time. Consider, mind you, how much bigger this number is — (10^3)! — than (10^1000) is.

(10^3)!, for those of you wondering, is more like 10^2477.

But there are not 1000 particles in the Universe, but 10^90 of them. Every time two particles interact, there’s not just one possible outcome, but an entire quantum spectrum of outcomes. As sad as the case is, there are way more than (10^90)! possible outcomes for the particles in the Universe, and that number is many googolplexes times larger than a paltry number like 10^10^50.

In other words, the number of possible outcomes from particles in any Universe interacting with one another tends towards infinity faster than the number of possible Universes increases due to inflation. Even setting aside issues that there may be an infinite number of possible values for fundamental constants, particles and interactions, and even setting aside interpretation issues such as whether the many-worlds-interpretation actually describes our physical reality, the fact of the matter is that the number of possible outcomes rises so quickly — so much faster than merely exponentially — that unless inflation has been occurring for a truly infinite amount of time, there are no parallel Universes identical to this one.

The singularity theorem tells us that an inflationary state is past-timelike-incomplete, and hence, most probably did not last a truly infinite amount of time, but rather arose some distant-but-finite point in the past. There are a huge number of Universes out there — possibly with different laws than our own and possibly not — but there are not enough of them to give us alternate versions of ourselves; the number of possible outcomes grows too rapidly compared to the rate that the number of possible Universes grows.

So what does this mean for you?

It means it’s up to you to make this Universe count. Make the choices that leave you with no regrets: take the dream job, stand up for yourself, navigate through the pitfalls as best you can, and go all-out every day of your life. There is no other Universe that has this version of you in it, and there is no future for you other than the one you live yourself into.

Make it the best one possible.