Supernova Constraints and Systematic Uncertainties From the First Three Years of the Supernova Legacy Survey

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 Hubble Space Telescope). SN data alone require cosmic acceleration at > 99.999% 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 (stat) +0.07 −0.14 (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.Recently K09 ( §10.2.3) have presented evidence for a strong decrease in β with redshift

Observational probes of cosmic acceleration

“…Prominent examples include the supernova and weak lensing programs of the CFHT Legacy Survey (CFHTLS; Conley et al 2011;Semboloni et al 2006a;Heymans et al 2012b), the ESSENCE supernova survey (Wood-Vasey et al, 2007), BAO measurements from the Sloan Digital Sky Survey (SDSS; Eisenstein et al 2005;Percival et al 2010;Padmanabhan et al 2012), and the SDSS-II supernova survey . These have been complemented by extensive multi-wavelength studies of local and high-redshift supernovae such as the Carnegie Supernova Project (Hamuy et al, 2006;Freedman et al, 2009), by systematic searches for z > 1 supernovae with Hubble Space Telescope Suzuki et al, 2012), by dark energy constraints from the evolution of X-ray or optically selected clusters (Henry et al, 2009;Vikhlinin et al, 2009;Rozo et al, 2010), by improved measurements of the Hubble constant (Riess et al, , 2011Freedman et al, 2012), and by CMB data from the WMAP satellite (Bennett et al, 2003;Larson et al, 2011) and from ground-based experiments that probe smaller angular scales.…”

“…The largest high-redshift (z ≈ 0.4 − 1.0) data sets are those from the ESSENCE survey (Wood-Vasey et al 2007; Narayan et al, in prep. ; ∼200 spectroscopically confirmed Type Ia SNe) and the CFHT Supernova Legacy Survey (SNLS; Astier et al 2006;Conley et al 2011;Sullivan et al 2011; ∼500 spectroscopically confirmed Type Ia SNe in the three-year data set SNLS3). At very high redshifts, HST surveys (Riess et al, 2004Suzuki et al, 2012) have yielded ∼ 25 Type Ia SNe at z > 1.0, which confirm the expectation that the universe was decelerating at high redshift and limit possible systematic effects from evolution of the supernova population or intergalactic dust extinction.…”

“…While any such "survey of surveys" is not unique and may not be optimal for a specific application, these compilations are popular because of their ease of use. Recent examples include the "Gold" sample (Riess et al, 2004, the "Union" and "Union2" samples (Kowalski et al, 2008;Amanullah et al, 2010), the "Constitution" sample (Hicken et al, 2009a), and the compilation of local, SDSS-II, SNLS3, and HST supernovae analyzed by Conley et al (2011). Figure 5 Luminosity distance vs. redshift for our fiducial cosmological model (solid curves), superposed on supernova measurements from the Union2 compilation (Amanullah et al, 2010).…”

“…The most up-to-date analysis is that of Conley et al (2011), who find w = −0.91 +0.16 −0.20 (stat) +0.07 −0.14 (sys) for SNe alone, assuming a flat universe with constant w and marginalizing over Ω m . Combining this measurement with other data sets, Sullivan et al (2011) find w = −1.016 +0.077 −0.079 in combination with 7-year WMAP CMB constraints (similar to the value and error bar quoted above), and w = −1.061 +0.069 −0.068 after adding BAO and H 0 measurements.…”

“…22 Conley et al (2011) provide further evidence for an error in the local U -band calibration, and they omit these data from their cosmological analysis.…”