Idea on the edgeSpeculative · Universe

Multiverse

Our universe may be one of a vast ensemble. Each member has its own physics, a consequence that falls out of some inflation and string theory models.

Speculative · not yet proven

Idea: Many universes with different properties
Origin: Eternal inflation, the string landscape
Status: Speculative, and possibly not directly observable

One bubble among many

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One universe among many

In eternal inflation, expansion never fully stops. Different regions stop inflating at different times and become separate bubbles, each its own universe with its own local physics. Our observable universe would be one such bubble.

The idea follows from inflation rather than being added by hand, which is part of why it is taken seriously.

The string landscape

String theory allows an enormous number of possible low-energy states, each with different particle physics and constants. Together with eternal inflation, this forms a landscape where every bubble can realize a different set of laws.

Can we test it

Evidence is mostly indirect. Bubbles might leave a mark if two of them collided near our own, but no such signature has been found. For now the multiverse sits beyond direct observation.

Key publications

Year	Paper	Venue	Link
2000	Guth, Inflation and eternal inflation	Physics Reports 333-334, 555	DOI
2003	Tegmark, Parallel Universes	Scientific American 288, 40	DOI
2003	Susskind, The Anthropic Landscape of String Theory	arXiv preprint	arXiv

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

Some regions would have physics unlike ours.
The idea is a consequence, not an assumption, in some models.
Direct evidence may be impossible to obtain.