Evidence from Type Ia Supernovae for an Accelerating Universe and Dark Energy

Abstract

I review 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 H_0 = 65 +/- 2 (statistical) km/s/Mpc, 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&#39;s cosmological constant (Λ). Combining our data with existing measurements of the cosmic microwave background (CMB) radiation and with the results of large-scale structure surveys, we find a best fit for Ω_m and Ω_Λof about 0.3 and 0.7, respectively. Other studies (e.g., masses of clusters of galaxies) also suggest that Ω_m \approx 0.3. The sum of the densities, ~1.0, agrees 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 Ω_Λ> 0. Most 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. Current projects include the search for additional SNe Ia at z > 1 to confirm the early deceleration, and the measurement of a few hundred SNeIa at z = 0.2 - 0.8 to determine the equation of state of the dark energy, w = P/(ρc^2).