Science › Creation theories
The multiverse
1986· Idea
SpeculativeA serious idea with no experimental proof yet.
The speculative idea that our universe is one of many, possibly with different conditions or physical constants.
See it on the Science map →How it works
Several separate theories point this way. Eternal inflation suggests inflation never stops everywhere, endlessly budding off bubble universes. String theory allows perhaps 10^500 possible sets of laws, which such bubbles could realize. The many-worlds view of quantum mechanics is a different kind: branching versions of one reality. These are consequences of theories, not observations. Other universes, if they exist, are probably unreachable by any signal.
Why it matters
If real, it would reframe questions like why our constants suit life: we would simply live in one of the rare hospitable bubbles.
How we know
None directly. Searches for marks of bubble collisions in the microwave background found nothing. Critics argue an idea that cannot be tested is philosophy, not physics; supporters say it follows from tested theories.
Still a mystery
Whether it can ever be tested, and how to compute probabilities when infinitely many universes exist, are deep unresolved problems.
Connections (4)
▸Fine-tuning: a universe built for life? is one explanation via The multiverseSpeculative
The relationship
If there are vast numbers of universes with different constants, most would be lifeless. Observers can only arise in rare ones where conditions permit. So finding ourselves in a life-friendly universe would be no surprise, just as fish should not be surprised to find water. The multiverse turns apparent tuning into a selection effect.
How the link was found
Brandon Carter framed anthropic reasoning in 1973. Steven Weinberg applied it to dark energy in 1987. String theory's huge 'landscape' of options, described around 2000-2003 by Bousso, Polchinski and Susskind, gave it a possible physical basis.
Is it proven?
Not proven. Weinberg's 1987 estimate of dark energy was roughly confirmed in 1998, which supporters cite. Other universes remain unobserved.
Open questions and mysteries
Critics say this explains anything and so predicts nothing. Whether it counts as science is actively debated.
▸String theory has a "landscape" of universes, a kind of The multiverseSpeculative
The relationship
String theory's six hidden dimensions can be curled up in an enormous number of ways, perhaps 10^500. Each arrangement gives a different set of particles, forces and vacuum energy, effectively different laws of physics. Combined with eternal inflation, which keeps spawning new bubble universes, each bubble could land on a different arrangement. This collection is called the string "landscape."
How the link was found
Bousso and Polchinski (2000) and Kachru, Kallosh, Linde and Trivedi (2003) argued that many stable arrangements exist; Leonard Susskind named it the "landscape" in 2003.
Is it proven?
Not proven. The count is an estimate, the stability of these solutions is contested, and other universes cannot, as far as we know, be observed.
Open questions and mysteries
Do these solutions really exist? The "swampland" debate since 2018 questions it. Can a theory that predicts everything be tested?
▸Many-worlds interpretation is a different kind of The multiverseSpeculative
The relationship
Many-worlds says every quantum outcome happens, each in its own branch of reality. That gives a multiverse too, but a different kind. The branches share the same laws and constants and grow from one history, rather than being separate regions of space with different physics, as in inflation's bubbles. They overlap the same space but cannot interact.
How the link was found
Hugh Everett proposed many-worlds in 1957. Max Tegmark in 2003 classified multiverse types, listing many-worlds as a separate level from inflationary bubbles.
Is it proven?
Not testable directly so far. Many-worlds makes the same predictions as other readings of quantum mechanics, so choosing between them is largely philosophy.
Open questions and mysteries
Some physicists suspect inflationary and quantum multiverses may be the same thing described two ways. That is unproven.
▸Eternal inflation and bubble universes creates bubble universes, a kind of The multiverseSpeculative
The relationship
Eternal inflation supplies the many pockets, but different laws in different pockets need more: an energy landscape with many valleys. With only one valley, every pocket has the same constants, just copies of one kind of universe. String theory's landscape lets each pocket land in a different valley, with different particles, forces and dark energy. Eternal inflation then makes every option real somewhere.
How the link was found
Linde (1986) described pockets that could differ in laws and constants. KKLT (Shamit Kachru, Renata Kallosh, Linde and Sandip Trivedi, 2003) offered string-theory vacuums that eternal inflation could fill.
Is it proven?
Not proven, and other pockets are probably unobservable forever. Each ingredient, eternal inflation and the landscape, is a consequence of untested theory, not an observation.
Open questions and mysteries
Counting pockets goes wrong: some counting rules predict that brains assembled by random fluctuations would vastly outnumber evolved observers like us. No agreed counting rule exists.