Hubble constant

The Hubble constant, denoted H_0, is the slope of the velocity-distance relation v = H_0 d discovered by Edwin Hubble in 1929: a galaxy at distance d recedes with velocity proportional to d, and H_0 is the constant of proportionality. It is conventionally quoted in kilometers per second per megaparsec (km/s/Mpc), so a value of 70 means a galaxy one megaparsec away recedes at about 70 km/s, one two megaparsecs away at about 140 km/s, and so on. Its reciprocal, the Hubble time 1/H_0, is roughly 14 billion years and approximates the age of the universe; the Hubble length c/H_0 is about 4,400 Mpc and sets the rough scale of the observable universe.

The name is a mild misnomer. H_0 is the present value of the time-dependent Hubble parameter H(t) = a-dot/a, where a(t) is the cosmic scale factor. H is uniform across space at any one instant — every comoving observer measures the same value in every direction — but it changes over cosmic time, having been far larger in the early universe and tending toward a constant in a dark-energy-dominated future. The subscript zero marks 'today.' The Hubble constant is therefore a snapshot of an evolving quantity, not a fixed feature of the universe.

The numerical value of H_0 is the most contested number in modern cosmology. Measurements based on the cosmic distance ladder (Cepheids calibrating Type Ia supernovae) give about 73 km/s/Mpc, while measurements anchored in the early universe (fitting the cosmic microwave background within the standard model and extrapolating forward) give about 67 km/s/Mpc. Each is quoted with roughly one-percent precision, and the disagreement — the 'Hubble tension' — exceeds five standard deviations. As of the mid-2020s it is unresolved, and it is taken seriously as a possible signal of physics beyond the concordance model.

Hubble's 1929 fit gave a slope near 500 km/s/Mpc, badly inflated because his Cepheid distance calibration was too small. Successive recalibrations through the twentieth century lowered the value; by the 1990s estimates ranged across a factor of two (50 to 100 km/s/Mpc), and the Hubble Space Telescope Key Project narrowed it to about 72 km/s/Mpc in 2001. The current tension between local and early-universe determinations emerged in the 2010s as both methods reached percent-level precision and failed to agree.