The Universe as a laboratory for gravity

Classic tests of the cosmological parameters such as Hubble's constant or the density of the Universe, ρ0, are designed to explore the spacetime geometry of the Universe. They have sometimes been ineffective because of the evolution of the galaxies used as test objects. Over the last decade, a range of new possibilities for determining the cosmological parameters has opened up. I will begin by describing recent, more precise, measurements of Hubble's constant (the expansion rate of the Universe). I will then turn to measurements employing supernovae in distant galaxies to determine the acceleration or deceleration rate of the Universe. Interestingly, the current data suggest that the Universe is being accelerated by a cosmological constant term - an addition to standard general relativity. Next, I will describe how present and future measurements of fluctuations in the cosmic microwave background will give us very precise measurements of a number of cosmological parameters. Interestingly, the combination of microwave background and supernovae results will be much more precise in defining the cosmological parameters than either technique used separately. In describing both the supernovae results and observations of the microwave background, I will spend a little time discussing observational techniques and sources of possible systematic error.