(Some of these may be equivalent or the same thing!)
Simulation multiverse: Advanced future civilizations create ancestor simulations, so there’s a multiverse of millions of potential realities being simulated by a single or various future civilizations in the main universe.
Cyclic time Multiverse: There is only one universe but after the big crunch we get another run, ad infinitum so all possibilities eventually play out, multiple times. This also works if after the end of an expanding cooling universe plays out a quantum fluctuation can cause another singularity. Basically, if time is eternal and things keep happening, either by crunch or rip, eventually the same things happen again and again, with or without variations.
Unbounded universe: Beyond our light cone there’s more and more and more space ad eternum, where everything can happen to arrange itself like here, but different. If you keep going in one direction eventually you’ll find a galaxy identical to ours, but where you’re half a centimetre taller. And all other possible iterations. It’s the same as the previous eternal multiverse but in Space instead of Time. A closely related concept includes bubbles with alternate physical constants created by inflation after the big bang.
Quantum many worlds multiverse: All quantum events play out in simultaneous parallel dimensions that don’t interact with each other as the wave functions decohere.
Tegmark’s mathematical multiverse: All reality is maths and we’re just one possible configuration of all mathematical structures that may exist. We are self aware mathematical structures and reality is what it looks like from within. Maths is infinite so all possible universes are contained.
Boltzmann brain: – not exactly a universe, unless you count “every man is a world”, in a universe that has reached thermal equilibrium at the end of time quantum fluctuations can achieve configurations that match the current state of your brain, so that you, the universe you have lived in and remember and everything that surrounds you is just a dream of a temporary evanescent collection of virtual particles.
Black hole multiverse: Each black hole contains an expanding universe inside it’s event horizon. Universes with physical constants that allow for black holes spawn more black hole universes within themselves recursively. Universes with life may be more common if life results in technological civilizations that eventually make artificial black holes to power their starships.
And, amusingly none of these are mutually incompatible. We could be living in a simulation that is running on a physical substrate that is itself splitting along quantum branching points in an eternally cyclic universe that is unbounded in space that exists within the internally expanding event horizon of a black hole that is actually a mathematical construct.
It’s turtles all the way.
It’s turtles all the way.
(Did I miss any? I’d love to hear if there are more!)