system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the multiverse theory have an infinite number of universes?

Every time I see the multiverse theory mentioned on TV, movies, books, comics, etc. they have the same basic concept of infinite universes, some very like ours, some very different.

While the idea that there are different universes with different laws of physics sits fine with me, the concept of there being one universe exactly like mine but I'm made of corn, in another the only difference is I have a different colour shirt on, etc. seems ridiculous. 

Why does the theory have 'infinite' rather than many universes?
gpt
A cosmos which produced *all possible* universes would be a simpler thing than a cosmos which produced *only some* possible universes. The first one requires only a rule or pattern that produces universes, the second also requires additional rules to identify which ones actually get produced.

Let's look at a 'Level I' multiverse to illustrate this :

> Level I: A generic prediction of cosmological inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions - including an identical copy of you about 10^10^29 meters away

Translated, that means: the universe is infinite, but the observable universe isn't. Let's abuse tterminology and call the observable universe the "universe", and the universe the "multiverse" - we can never ever get to the other "universes", there are infinitely many of them, and each one contains a collection of interesting galaxies and stars. Exactly what they contain is determined by their initial conditions (in practice, what they were like shortly after the big bang). 

A universe that started off sufficiently like ours would still be very like ours, although a tiny difference in the starting state will *eventually* make the two diverge. In some (only a mindbogglingly tiny fraction), that divergence starts to become obvious today, with you picking a different shirt (although the divergence was always there, in fact, we just wouldn't have noticed until today). 

Once you've picked a diffferent shirt, of course, human histories diverge radically. Different people spend different amounts of time talking to you, which slightly affects traffic flows in your city, which means different people die in car accidents or fall in love or argue with their boss or get pregnant. A week later there are different news headlines, different presidential tweets, different entrepreneurs starting businesses. The weather in a month is noticeably different, in a few decades the climate is. Our technology goes at a different pace, and a different direction. In one universe, our robots are off to colonise a nearby star, in another, we're off, and in a third, the robots are their own and we are no more. 

To say this is not so, you have to explain why an infinitely large space-time would contain a universe that made you, but *not* others that made an almost-you. You need to specify extra (strange) laws of physics to prevent such a thing from happening.