The origin of Multiverse lies in both fiction and science. It is an enticing idea proposing the existence of multiple universes at the same time with each one having its own definite set of rules. But the million dollar question is, does the multiverse exist? Do we live in one?
As far as we currently know, there is a single expanding blob of spacetime speckled with trillions of galaxies - that's our Universe. If there are others, we have no compelling evidence for their existence. That said, theories of cosmology, quantum physics, and the very philosophy of science have a few problems that could be solved if our blob of 'everything' wasn't, well, everything. That doesn't mean other universes must exist. But what if they do?
What is a universe?
It should be a simple question to answer. But different areas of science will have subtly different takes on what a universe even is. Cosmologists might say it describes the total mass of stuff (and the space in between) that has been slowly expanding from a highly concentrated volume over the past 13.77 billion years, becoming increasingly disordered with age. It now stretches 93 billion light years from edge to edge, at least based on all of the visible (and invisible) stuff we can detect in some way. Beyond that limit, there are either things we can't see, an infinite expanse of nothingness, or – in the unlikely scenario that all of space bends back around on itself – a round-trip back to the start across a hyper spherical universe.
If we're talking quantum physics, though, a universe might refer to all fields and their particles, and their combined influences over one another. As a general rule, a universe (like ours, at least) is a closed system, meaning it can't suddenly lose or gain a significant sum of energy.
Philosophically speaking, a universe might be a discrete set of fundamental laws that governs the behavior of everything we observe. A universe would be defined by its own rules that set its unique speed for light, tell particles how to push or pull, or space how it should expand.
What is a multiverse in cosmology?
A century of astronomical observations has told us a lot about the age, size, and evolution of galaxies, stars, matter and the four dimensions we sum up as spacetime. One thing we know with great confidence is that everything we see now is expanding at an accelerating rate. This logically implies the Universe, at least the one we live in, used to be a lot smaller. We can theoretically squeeze all of the matter of the Universe down to a point where the concentration of energy reduces atoms to a soup of simpler particles and forces combine until we can't tell them apart. Any smaller than that? Big shrugs.
If we go with what's known as a cyclic model of cosmology, the parent universe preceded ours in some way. It might even be a lot like this one, only running in reverse compared with ours, shrinking over time into a concentrated point only to bounce back out for some reason. Played out for eternity, we might imagine the respective universes bounce back and forth in an endless yo-yo effect of growing and collapsing. Or, if we go with what's known as a conformal cyclic model, universes expand over trillions upon trillions of years until their cold, point-like particles are so spread out, for all mathematical purposes everything looks and acts like a brand new universe. If you don't like those, there's a chance our Universe is a white hole – the hypothetical back end of a black hole from another universe. Which, logically, just might mean the black holes in our Universe could all be parents, pinching off new universes like cosmic amoebae.
Will we ever discover other universes?
Given the very definition of a universe relies on some kind of physical fence keeping influencing factors apart, it's hard to imagine ways we might ever observe the existence of a sibling for our universe. If we did, we might as well see it as an extension of our own Universe anyway. That said, there could be some cheats that could give us a glimpse. Any experiment to find one would have to rely on that 'fence' having some holes in it that allow particles or energy to leak across, either into ours, or away from it. Or, in the case of universes existing in our past, monumental events that left enough of a scar that not even a rebirth could erase.
For now, we still have no good reason to think our blob of everything is anything but unique. Given we're still learning how our own Universe works, the current gaps in physics could yet be plugged without any need to imagine a reality other than ours. In countless other versions of this article scattered throughout the multiverse, however, the question of whether we are alone just might have a different answer.