Multiverse
The proposal that our universe is one region among many, with the constants and laws varying from region to region — the pool that observer selection filters.
Lukasz Szramuk ·
The multiverse is the proposal that what we call 'the universe' is one patch of something larger — and that the numbers we call constants of nature take different values in different patches. If that is true, the fine-tuning puzzle flips: of course we find ourselves in a patch where the constants permit observers, because observers can arise nowhere else. The multiverse supplies the pool; the anthropic principle is the filter.
Max Tegmark usefully sorted multiverses into four levels. Level I: space is infinite, so far regions repeat our Hubble volume's arrangements — same laws, different dice rolls. Level II: eternal inflation spawns bubble universes whose low-energy constants differ — the level that does the anthropic work. Level III: quantum mechanics' many worlds. Level IV: every consistent mathematical structure exists physically. Most fine-tuning talk means Level II, often via string theory's landscape of perhaps 10^500 vacuum states.
The case for it: inflation (independently motivated) naturally runs eternally in most models; string theory (independently pursued) naturally has a landscape; and Weinberg's 1987 anthropic prediction of a small positive cosmological constant — the multiverse's one genuine forecast — came true a decade later. The case against: the other bubbles are unobservable by construction, the 'measure problem' makes probabilities in an infinite multiverse ill-defined, and a theory that explains any constant by selection may explain nothing at all. Whether 'multiverse' is physics or metaphysics is a live fight among serious people — Smolin and Ellis against, Linde and Guth for.
On this site, every universe you tune is a small multiverse of one: the Graveyard of Universes is exactly the idea — an ensemble where most patches are stillborn and the living ones are the ones anyone writes home about.