Big Bang cosmology makes a number of predictions, one of which is the expansion of the universe. However, the Big Bang theory does not predict how fast that expansion is, or what the density of matter in the universe is. These quantities, which describe the fundamental, global nature of the universe, must be determined by observations. For the past 15 years, Wendy Freedman has been working on a project to measure the Hubble constant--the rate of the expansion of the universe. The Hubble constant, when combined with a value for the average density of mass/energy in the universe, yields a measure of the universe's age.

Freedman has been a principal investigator, with Robert Kennicutt of the University of Arizona and Jeremy Mould of the Australian National University, of a group of 27 astronomers located across the U.S. and in Canada, Great Britain, and Australia. For the past eight years, the group has been using the Hubble Space Telescope (HST) to improve accuracy in the measurement of the Hubble constant. This year has marked the end of this effort, with the recent completion of a paper submitted to the Astrophysical Journal presenting the final results. Using five independent methods, all calibrated by distances measured using Cepheid variables discovered with HST, Freedman and collaborators found a value of the Hubble constant of 74 +/- 4 +/- 7 km/sec/megaparsec. The two error bars reflect the statistical and the systematic uncertainties, respectively.

The current best estimates for the average density of matter in the universe suggest that it is about one-third of the so-called critical density. Other recent observations of type Ia supernovae at high redshifts indicate that there may be another component of energy, perhaps due to what is known as the cosmological constant. In addition, recent measurements of fluctuations in the cosmic microwave background spectrum provide strong evidence for a critical density universe (although these observations cannot distinguish between matter and energy contributions). Allowing for this new "standard" cosmological model, where the universe has a critical density due to contributions from both matter and vacuum energy, a Hubble constant of 74 results in an expansion age for the universe of 13 billion years. This age agrees well with recent results from the Hipparcos satellite, which give very similar ages for the oldest stars in our galaxy.

Figure 1. The galaxy Messier 100 is shown here as imaged by the Hubble Space Telescope.