Hubble Constant

The Hubble constant H₀ is the present-day value of the Hubble parameter, quantifying the current rate of cosmic expansion. It is defined as the ratio of a galaxy's recession velocity to its distance in the linear regime of Hubble's law, with units of kilometers per second per megaparsec (km/s/Mpc). Despite its name, the Hubble constant is not truly constant — it varies with cosmic time, decreasing as the universe ages. The subscript zero denotes evaluation at the present epoch.

The precise value of H₀ has been contested for nearly a century. Early estimates by Edwin Hubble himself were around 500 km/s/Mpc, implying a universe younger than the Earth — a paradox resolved only when Allan Sandage and others corrected the distance ladder in the 1950s. Modern measurements fall into two camps: late-universe methods (Cepheid variables and Type Ia supernovae) yield approximately 73 km/s/Mpc, while early-universe methods (the cosmic microwave background interpreted through the Friedmann equations) give roughly 67 km/s/Mpc. This Hubble tension — a 5σ discrepancy that persists across independent analyses — may be the most significant crisis in observational cosmology since the discovery of dark energy.

The Hubble constant is the closest thing cosmology has to a birth certificate for the universe, yet we cannot agree on what it says. The tension between early and late measurements is not a calibration problem waiting to be solved; it is a structural stress test for the standard cosmological model. If the Friedmann equations plus six free parameters cannot reconcile two independent measurements of the same quantity, the model is not fine — it is finished.