Age of the Universe
The age of the universe is the time elapsed since the Big Bang, calculated by integrating the expansion history of the standard ΛCDM model; the Planck 2018 analysis gives 13.787 ± 0.020 billion years. The value is model-dependent and must exceed the ages of the oldest stars it contains.
Curious
The universe is expanding, so in the past it was smaller and denser. Tracing the expansion back to its beginning gives a start date. The answer depends on how fast the universe is expanding now (the Hubble constant) and on what the universe is made of, because matter slows the expansion while dark energy speeds it up.
The number is also checked against the oldest known objects. Stars in ancient globular clusters are estimated to be around 12–13 billion years old, and the universe cannot be younger than the things in it. The two lines of evidence are consistent, which is a strong test of the whole picture of the Big Bang.
Exploring
| Estimate | Age (billion years) |
|---|---|
| Planck 2018 (ΛCDM, CMB + BAO) | 13.787 ± 0.020 |
| Hubble time, H0 = 73.0 | 13.4 (scale only) |
| Oldest globular-cluster stars | ~12–13 (lower limit) |
Deep Dive
Calculation. In a flat universe with matter fraction Ωm and cosmological constant fraction ΩΛ, the age is t0 = (2/(3H0√ΩΛ)) · arcsinh(√(ΩΛ/Ωm)), neglecting radiation, which contributes only a very small correction. The result scales as 1/H0, so the Hubble tension (see Hubble Constant) feeds directly into the inferred age: a higher H0 gives a younger universe unless other parameters change.
Independent checks. The age must exceed the age of the oldest stars. Ages of globular clusters come from stellar evolution models (see Stellar Evolution) fitted to the Hertzsprung–Russell diagram turnoff point, and white dwarf cooling sequences (see White Dwarfs) and radioactive-decay dating of old stars provide further constraints. Historically, in the 1990s, some stellar ages appeared to exceed the age implied by a matter-only universe, an early hint that a cosmological constant was needed (see Cosmological Constant).
Caveats. The quoted uncertainty of 0.02 billion years is statistical within ΛCDM; it assumes the model is correct. Models with different early-universe physics could shift the inferred value. Individual stellar age estimates have large systematic uncertainties, and a few stars have occasionally been reported with central ages slightly above the cosmological value, usually consistent with it within errors.
Concept Relationship Map
Requires: Hubble constant, cosmic microwave background
Enables: fate of the universe
Commonly confused with: the radius of the observable universe (~46 billion light-years)
Sources
- Planck Collaboration (2020). Astronomy & Astrophysics, 641, A6.
- Ryden, B. (2016). Introduction to Cosmology (2nd ed.). Cambridge University Press.