IMPROVED DARK ENERGY CONSTRAINTS FROM ∼100 NEW CfA SUPERNOVA TYPE Ia LIGHT CURVES

Abstract

We combine the CfA3 supernova Type Ia (SN Ia) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. (2008) to form the Constitution set and, combined with a BAO prior, produces 1 + w = 0.013 +0.066 −0.068 (0.11 syst), consistent with the cosmological constant. The CfA3 addition makes the cosmologically-useful sample of nearby SN Ia between 2.6 and 2.9 times larger than before, reducing the statistical uncertainty to the point where systematics play the largest role. We use four light curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2 (R V = 3.1), and MLCS2k2 (R V = 1.7). SALT produces high-redshift Hubble residuals with systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2 overestimates the intrinsic luminosity of SN Ia with 0.7 < ∆ < 1.2. MLCS2k2 with R V = 3.1 overestimates host-galaxy extinction while R V ≈ 1.7 does not. Our investigation is consistent with no Hubble bubble. We also find that, after lightcurve correction, SN Ia in Scd/Sd/Irr hosts are intrinsically fainter than those in E/S0 hosts by 2σ, suggesting that they may come from different populations. We also find that SN Ia in Scd/Sd/Irr hosts have low scatter (0.1 mag) and equation of state, p = wρ, where the equation of state parameter, w, relates the dark energy density, ρ, to the dark energy pressure, p. In a Friedman universe, ρ depends on 1 + w and the scale factor of the universe, a, as ρ ∼ a −3(1+w) . The first question that arises is whether the dark energy density is constant (1 + w = 0, a cosmological constant) or not. We choose to use the notation, 1 + w, since it is then easier to think about values of w larger than −1 (1 + w > 0) or more negative than −1 (1 + w < 0). In the case of 1 + w < 0 the dark energy grows in density as the universe expands! The second question is whether the dark energy properties, as described by w, are constant in time or not.The first study on the equation of state produced a 95%-confidence limit of 1 + w < 0.3, assuming Ω M ∼ 0.2 and zero possibility of 1 + w < 0 (Garnavich et al. 1998). Knop et al. (2003) found 1 + w = −0.05 +0.15 −0.20 . Riess et al. (2005) reported 1 + w = −0.02 +0.13 −0.19 . The SNLS and ESSENCE surveys were designed to narrow the constraints on 1+w and their first reports showed significant improvement in statistical uncertainty over the previous values, bringing them down to the range where systematic uncertainties,which they try to reduce as well, are of roughly equal importance. Astier et al. (2006, A06, hereafter) found 1 + w = −0.02 ± 0.09 while Wood-Vasey et al. (2007, WV07, hereafter) found 1+w = −0.07±0.09. Most recently, Kowalski et al. (2008) (K08, hereafter) made a compilation of the literature SN Ia, plus several new nearby ones that they present, and found 1 + w = −0.01 ± 0.08 when using the same priors as A06 and WV07. All of these studies are consistent with a cosmological constant. On the time-evolution...

Observational probes of cosmic acceleration

“…In the analysis of Wood-Vasey et al (2007), different priors about host galaxy extinction change the inferred value of w by amounts comparable to the statistical error. When the ratio of extinction to reddening is treated as a free parameter in the cosmological fits, the derived values are typically quite far from those measured for Galactic interstellar dust, e.g., R V ≡ A V /E(B − V ) = 1.5 − 2.5 (Hicken et al 2009b;Kessler et al 2009;Sullivan et al 2011) instead of the mean R V = 3.1 found in the diffuse interstellar medium of the Milky Way (Cardelli et al, 1989). This difference could be a reflection of different kinds of dust along the line of sight to the supernova (e.g., circumstellar dust), but it could also arise from intrinsic color differences among SNe Ia with similar light curve shapes, which would reduce the inferred R V if they are assumed to arise from reddening.…”

“…With these corrections, the dispersion in well measured optical band peak magnitudes is only ∼ 0.12 magnitudes (Hicken et al 2009b;Folatelli et al 2010), allowing each well measured supernova to provide a luminosity-distance estimate with ∼ 6% uncertainty. The diversity of SN Ia light curves is not fully understood, and peculiar SNe Ia appear to produce ∼ 5% non-Gaussian tails in the SN Ia distribution .…”

Allan Sandage and the distance scale

“…Different procedures have been proposed. We use here the m corr V magnitudes of Reindl et al (2005) because they define a Hubble diagram with a particularly small scatter of 0.14 mag (see extended to z > 1 with overlapping data from several SN Ia collections (e.g., Hicken et al 2009;Kessler et al 2009). The SN HST Project provided Cepheids in eight SNe Ia-hosting galaxies.…”

Constraints on anisotropic cosmic expansion from supernovae

“…Besides, we infer from SN data typical fluctuations of the expansion rate and compare them with expectations for the ΛCDM model. The use of SN data fitted with 4 different light-curve fitters by Hicken et al (2009) allows us to study systematic effects.…”