We’re often told that our Universe is everything, and all that exists, has existed, and will exist in the future is a part of it. The universe as we know it is infinitely big to us, but could there possibly be more universes out there that are different – or perhaps almost identical – to our own?
What we know of our universe currently is actually the observable universe – the areas that we can observe due to light from them having the time to reach us here on Earth since the Big Bang. This area, with us placed at the centre, is currently a huge sphere approximately 91 billion light-years in diameter. Though light from further regions will eventually reach Earth, the constant expansion that is taking place means that much of the universe will remain unobservable to us. Not being able to visualise what is beyond our limits has led many people to speculate on what could exist beyond our reach. The suggestion of possible parallel universes is a controversial topic which divides a lot of physicists. The fact that the existence of multiple parallel universes (referred to as the ‘multiverse’) is impossible to prove at this point only makes the topic more controversial, but the idea of alternate worlds and realities has captured the imagination of physicists, sci-fi fans, and laymen alike. What do the pro-multiverse scientists think is out there, and what makes their arguments so compelling?
There are many different theories on ways that a multiverse could exist, and they are often split into separate categories. Max Tegmark, a Swedish-American cosmologist, has proposed four different ‘levels’ of multiverse, which can be summed up as universes beyond our horizon, “bubble” universes which have different physical constants (such as the speed of light), the many-worlds interpretation of quantum mechanics, and lastly a combination of all of the above, all of which are given labels of Level 1 to 4 respectively.
A Level 1 multiverse would contain infinite numbers of Hubble volumes (defined as the spherical region surrounding an observer, outside of which objects are moving away at a faster rate than the speed of light due to universal expansion). These Hubble volumes, due to existing as part of the same system, would all obey the same rules of physics. As the universe is infinitely big there would be an infinite number of different Hubble volumes throughout, with most having different cosmological configurations to ours. However, the concept of an infinitely big universe would also suggest that Hubble volumes with extremely similar or identical configurations to our own would also exist.
Level 2 bubble universes are suggested to have arisen due to cosmic inflation of the early universe, which caused already existing quantum uncertainties to expand into pockets different to the surrounding areas. These bubble universes could have their own completely different laws of physics, due to the initial quantum uncertainty that they developed from growing through the whole of the universe as their pocket was inflated. As physicists are sure that nothing happens only once in an infinite universe it’s likely that many more bubble universes were created, all with their own rules and constants of physics that could be completely unique or even identical to ours.
The Multiverse featured in 'Rick and Morty' follows the many w-uUURGH-orlds interpretation
The most popular kind of multiverse often seen in Science Fiction is the Level 3 many-worlds interpretation, a quantum mechanics interpretation which states that every possible outcome in the universe occurs within the multiverse simultaneously. This means that for every decision you’ve made in your life there is a universe where you did the alternative, and from then on made a series of decisions which also diverged in yet more alternative universes. This endless branching of universes would be massive when based around one individual alone, but when taking into account the entire universe and the deviations that can happen within it the possibilities for the multiverse are infinite. Following this theory there could be parallel universes that exist in which you’ve stopped reading this article, or universes where you never clicked the article in the first place (unlikely as that may seem)!
As it would be impossible for us to see the other universes and prove their existence - due to either the constraints of quantum mechanics or the distance between them being too vast - some people say that the multiverse theory is one of philosophy and not science. However, that doesn’t deter theoretical physicists from looking for the answers. Evidence of the bubble universe theory is being investigated, as changes in the cosmic microwave background of our universe could be suggestive of a collision evens with other bubble universes.
Though we may never know for sure what exists outside of our own visible universe, the multiverse theory will always capture the interest and imagination of our scientists. One thing is certain - if the multiverse does prove true, accessing it will be a lot harder than using a portal gun!