Hubble Constant
The Hubble constant (H0) is the present-day value of the Hubble parameter: the rate at which the universe is expanding, expressed as the recession velocity per unit distance of distant galaxies. It is measured in kilometres per second per megaparsec, and current measurements fall between about 67 and 73 km/s/Mpc depending on the method, a disagreement known as the Hubble tension.
Curious
In 1929 Edwin Hubble showed that galaxies are moving away from us, and that farther galaxies recede faster (see Edwin Hubble). That straight-line relationship is Hubble's law, and the slope is the Hubble constant. You can explore the law interactively in the Hubble's Law simulation.
The name is slightly misleading: H0 is constant across space at any one moment, but it changes over cosmic time as the expansion speeds up or slows down. The subscript zero means “value today”. Because the number sets the overall scale of the universe's size and age, pinning it down is one of the central goals of observational cosmology.
Exploring
| Method | Approximate H0 (km/s/Mpc) | Type |
|---|---|---|
| Planck 2018 CMB (assuming ΛCDM) | 67.4 ± 0.5 | Early universe |
| SH0ES Cepheids + Type Ia supernovae | 73.0 ± 1.0 | Late universe |
Deep Dive
Definition. The Hubble parameter is H(t) = ȧ/a, where a(t) is the cosmic scale factor; H0 is its value at the present time. Hubble's law v = H0d holds for distances small compared with the Hubble radius c/H0 (about 4,400 Mpc for 67.4 km/s/Mpc). The conversion 1 Mpc = 3.0857×1022 m gives H0 = 67.4 km/s/Mpc = 2.18×10−18 s−1. The critical density that sets the geometry is ρc = 3H02/(8πG) ≈ 8.5×10−27 kg/m³.
How it is measured. The “distance ladder” calibrates distances with geometry (parallax), then Cepheid variables, then Type Ia supernovae in distant galaxies (see Supernovae). The independent “early-universe” route infers H0 from the pattern of temperature fluctuations in the cosmic microwave background, assuming the standard ΛCDM model. Other routes include gravitational-wave “standard sirens” (see LIGO and Virgo), strong-lensing time delays, and the tip of the red giant branch.
The Hubble tension (unresolved). The distance-ladder value near 73 km/s/Mpc and the CMB-inferred value near 67 km/s/Mpc differ by more than the quoted uncertainties, at roughly the 5-sigma level in some analyses. Some recent analyses using tip-of-the-red-giant-branch and JWST data find intermediate values near 70, with their authors disagreeing on the implications. Whether the tension reflects undiscovered measurement systematics or new physics beyond ΛCDM, such as early dark energy, is an open question; this Codex does not take a side.
Concept Relationship Map
Requires: Big Bang theory, supernovae (distance ladder)
Enables: age of the universe, cosmological constant
Commonly confused with: a recession speed (H0 is a rate in km/s per Mpc)
Sources
- Hubble, E. (1929). Proceedings of the National Academy of Sciences, 15(3), 168–173.
- Planck Collaboration (2020). Astronomy & Astrophysics, 641, A6.
- Riess, A.G., et al. (2022). “A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team.” The Astrophysical Journal Letters, 934, L7.