Alternative theoryCosmology · Quantum gravity

Loop quantum gravity

Space-time is not continuous but made of tiny loops. In this picture the Big Bang singularity is replaced by a bounce from a previous contraction.

Alternative theory · unproven

Core idea: Space-time is granular, made of spin networks
Key result: A minimum area and volume
Big Bang: Replaced by a quantum bounce
Status: Active, but not yet observationally confirmed

Space as a network

Markus Poessel / Wikimedia Commons, CC BY-SA 3.0

Granular space-time

Loop quantum gravity quantizes geometry directly. There is no background space; the fabric itself is built from loops joined into networks. Area and volume come in discrete units, so space cannot be divided forever.

This differs from string theory, which keeps a smooth background and adds strings on top of it.

No singularity

In the early universe the discrete structure resists infinite compression. Instead of a singularity, the universe reaches a maximum density and bounces, turning a contraction into our current expansion.

The bounce would leave subtle marks on the cosmic microwave background, but none has been detected.

Tests

The main predictions are small. Light of different energies might travel at slightly different speeds, and the CMB could carry signatures of the bounce. Both effects sit below current sensitivity, so the theory remains unverified.

Key publications

Year	Paper	Venue	Link
1995	Rovelli & Smolin, Discreteness of area and volume in quantum gravity	Nucl. Phys. B 442, 593	DOI
2004	Ashtekar & Lewandowski, Background independent quantum gravity: a status report	Class. Quantum Grav. 21, R53	DOI
2006	Ashtekar, Pawlowski & Singh, Quantum Nature of the Big Bang	PRL 96, 141301	DOI

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

Worth knowing

It builds gravity from loops of geometry.
The Big Bang may have been a bounce, not a beginning.
Space itself would have a smallest unit.