In the 1920s astronomer Edwin Hubble discovered that galaxies are all moving away from us at a rate proportional to their distance. The farther a galaxy is, the faster it appears to be moving away. This relationship, called the Hubble constant, establishes an expansion rate that is critical for estimating the age and size of the universe. The universe is expanding because it was born in a fiery explosion, called the Big Bang, many billions of years ago.

The quest to precisely determine the Hubble constant was headed by the Key Project team, a group of astronomers who used NASA's Hubble Space Telescope to look far away for accurate "milepost markers," a special class of stars called Cepheid variables. The rhythmic pulsation of these aged, bright stars yields their intrinsic brightness, which in turn is needed to measure precise distances. Hubble's clear vision has allowed astronomers to accomplish in a few years Cepheid observations that previously required decades of painstaking work with ground-based telescopes.

Astronomers used Hubble to systematically search for Cepheids in our galactic neighborhood out to the distance of the Virgo cluster of galaxies. Cepheids were used to calibrate even more remote markers such as supernovae, which are so bright they can be seen at far greater distances, billions of light-years.

The team found that the universe is between 12 and 14 billion years old, depending on the density of matter in space.

Hubble played an important role in the discovery that the expanding universe may actually be accelerating. In other words, the galaxies are rushing away from each other ever faster as time goes on.

Hubble observations of distant supernovae were used to measure how quickly the universe was expanding long ago. Astronomers were surprised to discover that the universe was actually expanding at a slower rate billions of years ago. This implies that there may be a mysterious force, an "anti-gravity" that pushes galaxies apart at an increasingly faster rate the farther away they are from each other. This means the universe may continue expanding forever and never collapse in a fiery "Big Crunch."