Space Telescope Key Project Completes Task of Measuring the Hubble Constant Within 10%

Five years before the Hubble Space Telescope was launched in 1990, NASA designated a “Key Project” for the orbiting telescope. The project's goal was to pin down the Hubble constant $H0$ to within 10% At the time, $H0,$ a fundamental cosmological parameter that measures the universe's present rate of expansion, was uncertain to within a factor of two. Estimates ranged from 50 to 100 km/s per megaparsec. (Hubble's law of universal expansion asserts that, at cosmological distances, recessional velocity is proportional to distance; 1 Mpc is about 3 million light‐years.)