Many Worlds ...

The theory of cosmic inflation predicts a multiverse: an enormous number of Universes that experience hot Big Bangs, but each of those regions where a Big Bang occurs are completely separated from one another, with nothing but continuously inflating space between them. We cannot detect these other Universes, but their existence may not be avoidable in the context of inflation. (Credit: Geraint Lewis and Luke Barnes)

We are but a dust mote in god's eye, not only pertaining to Earth but also to the Milky Way as there are billions upon billions of galaxies out there but researchers posit the radical notion our universe is just one of an infinite of many, a concept first expressed by Hugh Everett III back in 1956, a theory substantiated by the probabilistic nature of quantum mechanics. Until recently, his work was rejected, but not now due to new data pointing out the fact his take on reality may be the right one after all.

Hugh Everett in 1964

One of the most successful theories of 20th century science is cosmic inflation, which preceded and set up the hot Big Bang. We also know how quantum fields generally work, and if inflation is a quantum field (which we strongly suspect it is), then there will always be more "still-inflating" space out there. Whenever and wherever inflation ends, you get a hot Big Bang. If inflation and quantum field theory are both correct, a Multiverse is a must.

Exponential expansion, which takes place during inflation, is so powerful because it is relentless. With every ~10^-35 seconds (or so) that passes, the volume of any particular region of space doubles in each direction, causing any particles or radiation to dilute and causing any curvature to quickly become indistinguishable from flat. (Credit: E. Siegel (L); Ned Wright’s Cosmology Tutorial (R))

Initial conditions rule

Whenever we come up against a question of initial conditions — basically, why did our system start off this way? — we only have two options. We can appeal to the unknowable, saying that it is this way because it’s the only way it could’ve been and we can’t know anything further, or we can try to find a mechanism for setting up and creating the conditions that we know we needed to have. That second pathway is what physicists call “appealing to dynamics,” where we attempt to devise a mechanism that does three important things.

1. It has to reproduce every success that the model it’s trying to supersede, the hot Big Bang in this instance, produces. Those earlier cornerstones must all come out of any mechanism we propose.
2. It has to explain what the Big Bang cannot: the initial conditions the Universe started off with. These problems that remain unexplained within the Big Bang alone must be explained by whatever novel idea comes along.
3. And it has to make new predictions that differ from the original theory’s predictions, and those predictions must lead to a consequence that is in some way observable, testable, and/or measurable.

The only idea we’ve had that met these three criteria was the theory of cosmic inflation, which has achieved unprecedented successes on all three fronts.

Probilitis, probabilities

The simplest model of inflation is that we started off at the top of a proverbial hill, where inflation persisted, and rolled into a valley, where inflation came to an end and resulted in the hot Big Bang. If that valley isn’t at a value of zero, but instead at some positive, non-zero value, it may be possible to quantum-tunnel into a lower-energy state, which would have severe consequences for the Universe we know today. (Credit: E. Siegel/Beyond the Galaxy)

But now, here’s where things get interesting. Inflation, like all the fields we know of, has to be a quantum field by its very nature. That means that many of its properties aren’t exactly determined, but rather have a probability distribution to them. The more time you allow to pass, the greater the amount that distribution spreads out. Instead of rolling a point-like ball down a hill, we’re actually rolling a quantum probability wavefunction down a hill.

The Many Worlds factor

Wherever inflation occurs (blue cubes), it gives rise to exponentially more regions of space with each step forward in time. Even if there are many cubes where inflation ends (red Xs), there are far more regions where inflation will continue on into the future. The fact that this never comes to an end is what makes inflation ‘eternal’ once it begins, and where our modern notion of a multiverse comes from. (Credit: E. Siegel/Beyond the Galaxy)

Put simply, if each hot Big Bang occurs in a “bubble” Universe, then the bubbles simply don’t collide. What we wind up with is a larger and larger number of disconnected bubbles as time goes on, all separated by an eternally inflating space.

An illustration of multiple, independent Universes, causally disconnected from one another in an ever-expanding cosmic ocean, is one depiction of the Multiverse idea. The different Universes that arise may have different properties from one another or they may not, but we do not know how to test the multiverse hypothesis in any way. (Credit: Ozytive/Public Domain)

End result, we are but one of many.

One of them is the existence of a myriad of Universes, of disconnected regions each with their own hot Big Bang, that comprise what we know as a multiverse when you take them all together. This doesn’t mean that different Universes have different rules or laws or fundamental constants, or that all the possible quantum outcomes you can imagine occur in some other pocket of the multiverse. It doesn’t even mean that the multiverse is real, as this is a prediction we cannot verify, validate, or falsify. But if the theory of inflation is a good one, and the data says it is, a multiverse is all but inevitable.

Read the entire Big Think piece, you will learn a lot. I know I have. :)