Hubble Time Calculator

The Hubble time (t_H) = 1/H0 is an estimate of the age of the universe based on the current expansion rate. For H0 = 67.4 km/s/Mpc: t_H = 1 / (67.4 km/s/Mpc) = 1 / (2.184e-18 s^-1) = 4.58e17 s = 14.51 Gyr. The actual age of a flat Lambda-CDM universe (13.8 Gyr) is about 0.95 times the Hubble time, because the expansion has been decelerating (matter-dominated era) and accelerating (dark energy era).

The Hubble constant H0 is the current rate of expansion of the universe: distant galaxies recede at velocity v = H0 * d, where d is their proper distance. The Planck 2018 value (from CMB) is H0 = 67.4 +/- 0.5 km/s/Mpc. Local measurements (Cepheids, Type Ia supernovae) give H0 approximately 73 km/s/Mpc. This tension (known as the Hubble tension) is a major open problem in cosmology.

The Hubble radius (or Hubble length) = c/H0 is the distance at which recession velocity equals the speed of light. For H0 = 67.4 km/s/Mpc: c/H0 = 299,792 / 67.4 = 4,448 Mpc = 14.5 billion light-years. Objects beyond the Hubble radius are receding faster than light, yet we can still observe them because their light was emitted when they were closer. The observable universe extends much farther (~46.5 billion light-years).

Edwin Hubble first estimated H0 in 1929 (with large errors). The 2001 Hubble Space Telescope Key Project established H0 = 72 +/- 8 km/s/Mpc. Planck 2018 CMB measurement gives 67.4 km/s/Mpc. The SH0ES team using Cepheid-calibrated supernovae gives 73.04 +/- 1.04 km/s/Mpc (2022). The discrepancy (the Hubble tension) at 5-sigma significance suggests possible new physics beyond Lambda-CDM.

Hubble time (1/H0 = 14.5 Gyr for H0 = 67.4) overestimates the actual age (13.8 Gyr) because the universe has not expanded at a constant rate. During the matter-dominated era (early universe), expansion was decelerating, meaning H was larger in the past. During the dark energy era (last 5 Gyr), expansion is accelerating. The ratio (actual age)/(Hubble time) depends on the cosmological parameters Omega_m and Omega_Lambda.