Evidence from Type Ia Supernovae for an Accelerating Universe

We review the use of Type Ia supernovae for cosmological distance determinations. Low-redshift SNe Ia (z ∼ 0.1) demonstrate that the Hubble expansion is linear, that H0 = 65 ± 2 (statistical) km s −1 Mpc, and that the properties of dust in other galaxies are similar to those of dust in the Milky Way. We find that 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 luminosity distances of our first set of 16 high-redshift SNe Ia are, on average, 10–15% farther than expected in a low massdensity (ΩM = 0.2) universe without a cosmological constant. Preliminary analysis of our second set of 9 SNe Ia is consistent with this. Our work supports models with positive cosmological constant and a current acceleration of the expansion. We address the main potential sources of systematic error; at present, none of them appears to reconcile the data with ΩΛ = 0 and q0 ≥ 0. The dynamical age of the Universe is estimated to be 14.2± 1.7 Gyr, consistent with the ages of globular star clusters.