The Weight, Shape, and Fate of the Universe
In order to estimate the size, luminosity and mass of galaxies we need to know the Hubble constant and the density of the universe. In recent years, these numbers have been measured to better than 1% accuracy. This is a remarkable improvement on the situation 20 years ago when the Hubble constant was not known to better than a factor of 2.
Three observations taken together have made this improvement possible. The first is the observation of the variations in brightness of the cosmic background radiation. The second is the observation of distant supernova explosions. The third is the observation of the clustering of galaxies. The attempt to better constrain the density of the universe using supernovas led to the remarkable discovery that instead of slowing down as we might, expect the expansion of the universe is in fact accelerating. This discovery implies the existence of a new force in the universe that astronomers call dark energy.