Cosmic epoch · Cosmology

The Big Bang

Cosmology & UniverseEstablished Science

Definition

The hot, dense early state from which the observable Universe has been expanding for about 13.8 billion years.

Scientific overview

The Big Bang is not an explosion in space but an expansion of space itself from an extremely hot, dense beginning. The framework explains the expansion of the Universe, the abundances of the light elements, and the cosmic microwave background — three independent lines of evidence that together make it the foundation of modern cosmology.

Historical development

Georges Lemaître proposed an expanding Universe from a 'primeval atom' in 1927–1931; the discovery of the cosmic microwave background in 1965 confirmed the hot early state and settled the debate with the rival Steady State model.

Evidence

The observed expansion of the Universe
The cosmic microwave background
The primordial abundances of hydrogen, helium, and lithium

Open questions

What set the initial conditions?
What preceded the hot Big Bang, if anything?

Rests on

The Big Bang

General Relativity

Predicts

The Cosmic Microwave Background

Big Bang Nucleosynthesis

Related discoveries & objects

The Cosmic Microwave Background

The Expansion of the Universe

Related scientists

Georges Lemaître

Knowledge connections

PredictsThe Cosmic Microwave Background
PredictsBig Bang Nucleosynthesis
Part of modelThe Lambda-CDM Model
Depends onThe Big Bang
Depends onGeneral Relativity
Related toThe Cosmic Microwave Background
Related toThe Expansion of the Universe
Studied byGeorges Lemaître
Required byBig Bang Nucleosynthesis
Required byRecombination
Related toEntropy & the Arrow of Time
Related toThe Lambda-CDM Model
Related toCyclic & Bouncing Cosmologies
Required byPrimordial Black Hole
Followed byCosmic Inflation

Sources & references

Facts on this topic will be cited from these primary and reference sources.

NASANational Aeronautics and Space Administration
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.
Planck CollaborationESA Planck mission / Planck Collaboration
Cosmic microwave background maps and the cosmological parameters of the ΛCDM model.

Quick facts

Type: Cosmic epoch
Field: Cosmology
Consensus: Established Science
When: 13.8 billion years ago (t = 0)
Part of model: The Lambda-CDM Model

Scientific consensus

Established Science

Observationally confirmed and consistent across all leading models.

Quality & authority

75%

Reviewed

Completeness: Complete
Source coverage: Complete
Citation coverage: Complete
Relationship coverage: Complete
Review coverage: Complete
Image coverage: None

Curated from authoritative sources. See source quality.

Provenance & sources

review 2026.1

Asteria Scientific Review Process · verification: sourced · accurate

High evidenceconfirmed
The Big Bang is the prevailing cosmological model: the universe expanded from an extremely hot, dense early state about 13.8 billion years ago, an age constrained by the cosmic microwave background.
Source: ESA — Planck Collaboration
- Planck 2018 results. VI. Cosmological parameters, Astronomy & Astrophysics 641, A6 (2020)doi:10.1051/0004-6361/201833910

Reviewed by the internal Asteria Scientific Review Process — not an external institutional review. See the evidence framework and authority dashboard.

Scientific citations

Real, source-backed references — primary papers first, then datasets and institutional sources. Formatted through the citation engine; nothing is fabricated.

Peer-reviewed paperPlanck 2018 results. VI. Cosmological parameters

Planck Collaboration · Astronomy & Astrophysics 641, A6 · 2020

ESA — Planck Collaboration · doi:10.1051/0004-6361/201833910

Cite (APA / BibTeX)

Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics 641, A6. ESA — Planck Collaboration. https://doi.org/10.1051/0004-6361/201833910

@misc{cite:planck-2018-cosmological-parameters,
  title        = {Planck 2018 results. VI. Cosmological parameters},
  author       = {Planck Collaboration},
  organization = {ESA — Planck Collaboration},
  howpublished = {Astronomy & Astrophysics 641, A6},
  year         = {2020},
  doi          = {10.1051/0004-6361/201833910},
  url          = {https://doi.org/10.1051/0004-6361/201833910},
  note         = {Cosmological parameters from the final Planck data release.}
}

Institutional pageThe Big Bang — NASA Universe

NASA

Cite (APA / BibTeX)

NASA (n.d.). The Big Bang — NASA Universe. NASA. https://science.nasa.gov/

@misc{cite:nasa-cosmo-the-big-bang,
  title        = {The Big Bang — NASA Universe},
  organization = {NASA},
  year         = {n.d.},
  url          = {https://science.nasa.gov/},
  note         = {NASA science overview: The Big Bang.}
}

Historical referenceThe Nobel Prize in Physics 2019

2019

The Nobel Prize

Cite (APA / BibTeX)

The Nobel Prize (2019). The Nobel Prize in Physics 2019. The Nobel Prize. https://www.nobelprize.org/prizes/physics/2019/

@misc{cite:nobel-physics-2019,
  title        = {The Nobel Prize in Physics 2019},
  organization = {The Nobel Prize},
  year         = {2019},
  url          = {https://www.nobelprize.org/prizes/physics/2019/},
  note         = {Peebles (physical cosmology); Mayor & Queloz (first exoplanet around a Sun-like star).}
}

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Scientific overview

Sources & references

Facts on this topic will be cited from these primary and reference sources.

NASANational Aeronautics and Space Administration

Mission data, planetary science, space telescopes, and public-domain imagery.

Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.

Planck CollaborationESA Planck mission / Planck Collaboration

Cosmic microwave background maps and the cosmological parameters of the ΛCDM model.

Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics 641, A6. ESA — Planck Collaboration. https://doi.org/10.1051/0004-6361/201833910 @misc{cite:planck-2018-cosmological-parameters, title = {Planck 2018 results. VI. Cosmological parameters}, author = {Planck Collaboration}, organization = {ESA — Planck Collaboration}, howpublished = {Astronomy & Astrophysics 641, A6}, year = {2020}, doi = {10.1051/0004-6361/201833910}, url = {https://doi.org/10.1051/0004-6361/201833910}, note = {Cosmological parameters from the final Planck data release.} }

NASA (n.d.). The Big Bang — NASA Universe. NASA. https://science.nasa.gov/ @misc{cite:nasa-cosmo-the-big-bang, title = {The Big Bang — NASA Universe}, organization = {NASA}, year = {n.d.}, url = {https://science.nasa.gov/}, note = {NASA science overview: The Big Bang.} }

The Nobel Prize (2019). The Nobel Prize in Physics 2019. The Nobel Prize. https://www.nobelprize.org/prizes/physics/2019/ @misc{cite:nobel-physics-2019, title = {The Nobel Prize in Physics 2019}, organization = {The Nobel Prize}, year = {2019}, url = {https://www.nobelprize.org/prizes/physics/2019/}, note = {Peebles (physical cosmology); Mayor & Queloz (first exoplanet around a Sun-like star).} }