Alternative theoryCosmology · Universe

Inflation

A brief burst of exponential expansion is the leading explanation for why the universe is smooth and flat. The open question is which version of inflation happened, if any.

Leading model · many variants · mechanism unproven

Core idea
A brief period of exponential early expansion
Explains
Flatness, smoothness, and the seeds of structure
Proposed
The 1980s, by Guth and others
Variants
Single field, multi-field, string-motivated
Test
Primordial gravitational waves in CMB polarization

The universe, stretched flat

Wikimedia Commons, CC BY-SA 3.0
Diagram comparing the standard Big Bang with an inflationary phase
A comparison of the standard Big Bang with an inflationary phase early in the universe's history. Image: Wikimedia Commons, CC BY-SA 3.0.

What inflation does

In a tiny fraction of a second, space expanded by an enormous factor. That stretching explains why distant parts of the sky look alike and why the universe measures flat. Quantum fluctuations during the burst were stretched to cosmic size and became the seeds of galaxies.

The model is popular because one mechanism answers several puzzles at once.

Many versions

Inflation is not a single model. Slow-roll versions use one field rolling down a shallow potential, multi-field versions add several, and others are motivated by string theory. Each predicts slightly different ripples in the early universe.

Testing it

The clearest signature would be primordial gravitational waves, visible as a particular swirl in the cosmic microwave background polarization. Searches have not found them, which rules out some versions and keeps the rest alive. The exact mechanism is still unknown.

Key publications

YearPaperVenueLink
1980Starobinsky, A new type of isotropic cosmological models without singularityPhys. Lett. B 91, 99DOI
1981Guth, Inflationary universe: A possible solution to the horizon and flatness problemsPRD 23, 347DOI
1982Linde, A new inflationary universe scenarioPhys. Lett. B 108, 389DOI
2020Planck Collaboration, Planck 2018 results. X. Constraints on inflationA&A 641, A10DOI

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

Worth knowing

  • Inflation lasted a tiny fraction of a second.

  • It stretched quantum fluctuations to cosmic size.

  • The exact mechanism is still unknown.

Recommended videos

  • Cosmic Inflation Explained Perimeter Institute for Theoretical Physics
  • How Cosmic Inflation Flattened the Universe PBS Space Time

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