Alternative theoryCosmology · Universe

Cyclic universe

The Big Bang is one turn in an endless cycle. Each universe expands, empties, and eventually contracts or resets, so there is no single beginning.

Alternative theory · unproven

Core idea: Repeating cycles of expansion and contraction
Proponents: Ekpyrotic and cyclic models
Key feature: No single first moment
Status: Unproven, with entropy challenges

Expansion, contraction, repeat

Wikimedia Commons, CC BY 3.0

Animation of a cyclic universe expanding and contracting — A cyclic universe: expansion followed by contraction, then another Big Bang. Image: Wikimedia Commons, CC BY 3.0.

No first moment

Cyclic models remove the need for a unique beginning. Our Big Bang follows an earlier contracting phase, which itself followed an earlier expansion. The timeline has no first tick.

In some versions the cycle runs forever in both directions.

How a cycle works

A cycle starts with expansion. Over vast timescales the universe thins out, then contraction or a bounce turns it around, and a new Big Bang begins. Some versions use colliding branes from string theory as the reset, rather than a collapsing classical universe.

Difficulties

Entropy grows from cycle to cycle. Without a way to shed it, each new cycle should start hotter and more disordered than the last, which conflicts with what we observe. Models have to explain why our cycle looks so clean.

Key publications

Year	Paper	Venue	Link
2001	Khoury et al., Ekpyrotic universe: Colliding branes and the origin of the hot big bang	PRD 64, 123522	DOI
2002	Steinhardt & Turok, Cosmic evolution in a cyclic universe	PRD 65, 126003	DOI
2019	Ijjas & Steinhardt, A new kind of cyclic universe	Phys. Lett. B 795, 666	DOI

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

Worth knowing

The universe may have been around long before our Big Bang.
Some versions use colliding branes from string theory.
Each cycle would still contain its own Big Bang.