### Post by Joanna on May 14, 2018 22:47:21 GMT -5

**Are There Parallel Universes and Alternate Realities 'Out There'?**

Is our universe unique? From science fiction to science fact, there is a concept that suggests 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

*Dark Shadows*to

*Buffy the Vampire Slayer*to

*Star Trek*to

*Doctor Who*to

*Digemon*have utilized the idea to extend plotlines.

There actually is quite a bit of evidence out there for a multiverse. First, one must understand how our universe is believed to have come into existence.

**Arguing for a multiverse.**Around 13.7 billion years ago, simply speaking, everything we know of in the cosmos was an infinitesimal singularity. Then, according to the Big Bang theory, some unknown trigger caused it to expand and inflate in three-dimensional space. As the immense energy of this initial expansion cooled, light started to shine through. Eventually, the small particles began to form into the larger pieces of matter we know today, such as galaxies, stars and planets. One big question with this theory is: are we the only universe out there? With our current technology, we are limited to observations within this universe because the universe is curved and we are inside the fish bowl, unable to see the outside of it (if there is an outside.) There are at least five theories why a multiverse is possible, as a 2012 Space.com article explained:

*We don’t know the shape of space-time. One prominent theory is that it is flat and continues forever. This would present the possibility of many universes out there. But with this topic in mind, it is possible that universes can repeat themselves because particles can be put together only in so many ways. More about this in a moment.*

1. Infinite universes.1. Infinite 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 cease inflating like the Big Bang inflated our own universe. Others, however, will continue growing larger. So if we picture our own universe as a bubble, it is sitting in a network of bubble universes of space. What’s interesting about this theory is the other universes could have very different laws of physics than our own because they are not linked.*

2. Bubble universes.2. Bubble 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 from one of your decisions, there would be a range of universes and each of which there would be a different outcome. So in one universe, you took that job in China. In another, perhaps you were on your way and your plane landed somewhere else and you decided to stay. And so on.*

**3. Daughter universes.***Another possible avenue is exploring mathematical universes, which, simply put, explains the structure of mathematics may change depending on the universe in which you reside. “A mathematical structure is something 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, “I really believe that there is this universe out there that can exist independently of me that would continue to exist even if there were no humans.”*

**4. Mathematical universes.***And last but not least is the idea of parallel universes. Returning 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. So, with an infinite number of cosmic patches, the particle arrangements within them must repeat – infinitely many times over. This means there are infinitely many “parallel universes”: cosmic patches exactly the same as ours (containing someone exactly like you), as well as patches that differ by just one particle’s position, patches that differ by two particles’ positions and so on down to patches that are completely different from ours.*

**5. Parallel universes.**Famously, physicist Stephen Hawking’s last paper before his death also dealt with the multiverse. The paper was published in May 2018, just a few months after his demise. In an interview in

*The Washington Post*, he told Cambridge University: “We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse to a much smaller range of possible universes.”

**Arguing against a parallel universe.**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 indicated there are limitations. 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. Simply put, this would 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 for China.

Also, the expansion at the beginning of the universe occurred exponentially because there was so much “energy inherent to space itself,” Siegal said. But over time, this inflation obviously slowed – those particles of matter created at the Big Bang are not continuing to expand, he asserted. Among his conclusions: this means multiverses would have different rates of inflation and different times – longer or shorter – for inflation. This decreases the possibilities of universes similar to our own. “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,” Siegal continued, “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.”

But rather than seeing this lack of other universes as a limitation, Siegal instead philosophizes that it shows how important it is to celebrate being unique. He advises people to make the choices that work for them, which “leave you with no regrets.” This is because there are no other realities where the choices of your dream-self take place; you, therefore, are the only one who can make those choices happen.

**Source:**Elizabeth Howell, Space.com, May 9, 2018.