How many multiverses?
"The multiverse" isn't one idea but several. Max Tegmark famously sorted them into four levels.
1 min read
People reach for the multiverse to explain fine-tuning: with enough universes trying different constants, some are bound to be habitable, and of course we live in one of those. But the word covers at least four very different ideas, which the physicist Max Tegmark sorted into levels.
Level I — beyond the horizon
Space may simply go on far past what we can see. Given enough of it, distant regions repeat every arrangement of matter — including, somewhere, another you. Same physics, different scenery.
Level II — other bubbles
In eternal inflation, space keeps sprouting new bubble universes, and each can freeze in with different constants. This is the level that actually speaks to fine-tuning: a vast lottery of physics, and we occupy a winning ticket.
Levels III and IV
Level III is the many-worlds reading of quantum mechanics — every possibility realized in a branching wavefunction. Level IV is the boldest: every mathematically consistent structure exists as its own universe. The further you go, the harder they are to test — which is the standing objection to all of them.
A multiverse can explain why our universe is habitable. Whether that counts as an explanation or a dodge is still fiercely argued.