Type Ia Supernovae and Cosmology

Abstract

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 with H ₀ = 72±8 km s⁻¹ Mpc⁻¹, and that the properties of dust in other galaxies are generally 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 H ₀ t ₀ = 0.96±0.04, and Ω_Λ − 1.4Ω_M = 0.35±0.14. Combining our data with the results of large-scale structure surveys, we find a best fit for Ω_M and Ω_Λ 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 Ω_Λ > 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. According to the most recent data sets, the SN Ia rate at z > 1 is several times greater than that at low redshifts, presumably because of higher star formation rates long ago. Moreover, the typical delay time from progenitor star formation to SN Ia explosion appears to be substantial, ∼ 3 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 parameter of the dark energy, w = {tiP}/(ρ c ²), whose value is now constrained by SNe Ia to be in the range −1.48 ≲ w ≲ – 0.72 at 95% confidence.