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

Abstract

� 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 (�). From about 200 SNe Ia, we find that H0t0 = 0.96 ± 0.04, and � 1.4M = 0.35 ± 0.14. Combining our data with the results of large-scale structure surveys, we find a best fit for M and � 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 � > 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/(�c 2 ), whose value is now constrained by SNe Ia to be in the range 1.48 < � w < � 0.72 at 95% confidence.