2000ApJ...536..531W

Because of flux conservation, flux-averaging justifies the use of the distance-redshift relation for a smooth universe in the analysis of Type Ia supernova (SN Ia) data. We have combined the SN Ia data from the High-z SN Search and the Supernova Cosmology Project and binned the combined data by flux-averaging in redshift intervals of Δz=0.05 and Δz=0.1. We find that the unbinned data yield a Hubble constant of H₀=65±1 km.s^–1.Mpc^–1 (statistical error only), a matter density fraction of Ω_m=0.7±0.4, and a vacuum energy density fraction of Ω_Λ=1.2±0.5. The binned data for Δz=0.1 yield H₀=65±1 km.s^–1.Mpc^–1 (statistical error only), Ω_m=0.3±0.6, and Ω_Λ=0.7±0.7. Our results are not sensitive to the redshift bin size. Flux-averaging leads to less biased estimates of the cosmological parameters by reducing the bias due to systematic effects such as weak lensing.

Comparison of the data of 18 SNe Ia published by both groups yields a mean SN Ia peak absolute magnitude of M_B=-19.33±0.25. The internal dispersion of each data set is about 0.20 mag in the calibrated SN Ia peak absolute magnitudes. The difference in analysis techniques introduces an additional uncertainty of about 0.15 mag.

If the SNe Ia peak absolute luminosity changes with redshift due to evolution, our ability to measure the cosmological parameters from SN Ia data will be significantly diminished. Assuming power-law evolution in the peak absolute luminosity, (1+z)^β, we find a strong degeneracy between the evolution power-law index β and the matter density fraction Ω_m. For Ω_m=0.3, we find that the unbinned data yield H₀=65±1 km.s^–1.Mpc^–1 (statistical error only), Ω_Λ=1.4±1.1, and β=0.5±1.6, and the binned data (with Δz=0.1) yield H₀=65±1 km.s^–1.Mpc^–1 (statistical error only), Ω_Λ=0.6±1.4, and β=0.0±1.0.