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

AUG 01, 1999

By measuring hundreds of periodically varying stars out to 80 million light‐years, the Hubble telescope has calibrated much brighter cosmological yardsticks that we can see billions of light‐years away.

DOI: 10.1063/1.882773

Bertram M. Schwarzschild

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 $H_{0}$ to within 10% At the time, $H_{0},$ 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.)

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