The Accelerating Universe and Dark Energy: Evidence from Type Ia Supernovae

The measured luminosity distances of Type Ia supernovae (SNe Ia) as a function of redshift have shown that the expansion of the Universe is currently accelerating, probably due to the presence of repulsive dark energy such as Einstein's cosmological constant (Lambda). From about 200 SNe Ia, we find that H_0t_0 = 0.96 +/ 0.04, and Omega_Lambda - 1.4 Omega_M = 0.35 +/- 0.14. Combining our data with the results of large-scale structure surveys, we find a best fit for Omega_M and Omega_Lambda of 0.28 and 0.72, respectively -- essentially identical to the recent WMAP results (and having comparable precision). The sum of the densities, ~1.0, agrees with extensive measurements of the cosmic microwave background radiation, including WMAP, and coincides with the value predicted by most inflationary models for the early Universe. A number of possible systematic effects (dust, SN evolution) thus far do not seem to eliminate the need for Omega_Lambda > 0. Recently, analyses of SNe Ia at z = 1.0-1.7 provide further support for current acceleration, and give tentative evidence for an early epoch of deceleration. The dynamical age of the Universe is 13.1 +/- 1.5 Gyr, consistent with the ages of globular star clusters and with the WMAP result of 13.7 +/- 0.2 Gyr. Current projects include the search for additional SNe Ia at z > 1 to confirm the early deceleration, and the measurement of a few hundred SNe Ia at z = 0.2-0.8 to determine the equation of state of the dark energy, w = P/(rho c^2).