The cosmic distance ladder, symbolically shown here in this artist's concept, is a series of stars and other objects within galaxies that have known distances. Image credit: NASA/JPL-Caltech.
The Hubble constant (H0) is a measure of the present expansion rate of the universe, in units of kilometers per second per megaparsec (km/s/Mpc). It relates the apparent recession velocity of a galaxy, or galaxy cluster, to its distance from the Milky Way. The larger the Hubble constant, the younger the universe.
Using data from the Spitzer Space Telescope, astronomers produced in 2012 the most accurate value yet for the Hubble constant: 74.3 plus or minus 2.1 kilometers per second per megaparsec (a megaparsec being roughly 3 million light-years). This brings the uncertainty in the value of the constant down to a mere 3 percent.
The Hubble constant is named after the American astronomer Edwin P. Hubble whose observations in the 1920s confirmed that the universe has been expanding since it exploded into being in the Big Bang (now known to have happened about 13.7 billion years ago). In the late 1990s, astronomers found that the expansion is accelerating, or speeding up over time. Determining the expansion rate, expressed as the Hubble constant, is critical for understanding the age and size of the universe.