Supernova Constraints and Systematic Uncertainties from the First 3 Years of the Supernova Legacy Survey

TL;DR

This study combines supernova data from multiple surveys over three years to confirm cosmic acceleration and constrain dark energy properties, thoroughly analyzing systematic uncertainties to improve future measurements.

Contribution

It provides a comprehensive analysis of systematic uncertainties in supernova cosmology, including their impact on dark energy constraints, and demonstrates the importance of calibration in reducing these uncertainties.

Findings

Supernova data confirm cosmic acceleration at >99.9% confidence.

The dark energy equation of state parameter w is consistent with a cosmological constant.

Calibration uncertainties dominate the total systematic error.

Abstract

We combine high redshift Type Ia supernovae from the first 3 years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-$z$, 93 SDSS, 242 SNLS, and 14 {\it Hubble Space Telescope}). SN data alone require cosmic acceleration at >99.9% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least $z=1.4$) in a flat universe, we find $w = -0.91^{+0.16}_{-0.20}(\mathrm{stat}) ^{+0.07}_{-0.14} (\mathrm{sys})$ from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between host-galaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematics covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems.