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

I discuss the use of Type Ia supernovae (SNe Ia) for cosmological distance determinations. Low-redshift SNe Ia (z < ∼ 0.1) demonstrate that the Hubble expansion is linear, that H0 = 72 ± 8 km s−1 Mpc−1, and that the properties of dust in other galaxies are similar to those of dust in the Milky Way. The light curves of high-redshift (z = 0.3–1) SNe Ia are stretched in a manner consistent with the expansion of space; similarly, their spectra exhibit slower temporal evolution (by a factor of 1 + z) than those of nearby SNe Ia. The measured luminosity distances of 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 (Λ). From about 200 SNe Ia, we find that H0t0 = 0.96 ± 0.04, and ΩΛ − 1.4ΩM = 0.35 ± 0.14. Combining our data with the results of large-scale structure surveys, we find a best fit for ΩM and ΩΛ 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: the Universe is flat on large scales. A number of possible systematic effects (dust, supernova evolution) thus far do not seem to eliminate the need for ΩΛ > 0. However, during the past few years some very peculiar low-redshift SNe Ia have been discovered, and we must be mindful of possible systematic effects if such objects are more abundant at high redshifts. 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 estimated to be 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 more accurately determine the equation of state of the dark energy, w = P/(ρc), whose value is now constrained by SNe Ia to be in the range −1.48 < ∼ w < ∼ − 0.72 at 95% confidence.