Browsing by Author "Bassett, B A"

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    How flat is our Universe really?
    (2013) Okouma, PM; Fantaye, Y; Bassett, B A
    Distance measurement provide no constraints on curvature independent of assumptions about the dark energy, raising the question, how flat is our Universe if we make no such assumptions? Allowing for general evolution of the dark energy equation of state with 20 free parameters that are allowed to cross the phantom divide, w(z) = -1, we show that while it is indeed possible to match the first peak in the Cosmic Microwave Background with non-flat models and arbitrary Hubble constant, H_0, the full WMAP7 and supernova data alone imply -0.12 < Omega_k < 0.01 (2sigma). If we add an H_0 prior, this tightens significantly to Omega_k = 0.002 pm 0.009 . These constitute the most conservative and model-independent constraints on curvature available today, and illustrate that the curvature-dynamics degeneracy is broken by current data, with a key role played by the Integrated Sachs Wolfe effect rather than the distance to the surface of last scattering. If one imposes a quintessence prior on the dark energy (-1 leq w(z) leq 1) then just the WMAP7 and supernova data alone force the Universe to near flatness: Omega_k = 0.013 pm 0.012. Finally, allowing for curvature, we find that all datasets are consistent with a Harrison-Zel'dovich spectral index, n_s = 1, at 2sigma, illustrating the interplay between early and late-universe constraints.
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    Improved cosmological constraints from a joint analysis of the SDSS-II and SNLS supernova samples
    (2014) Betoule, M; Kessler, R; Guy, J; Mosher, J; Hardin, D; Biswas, R; Astier, P; El-Hage, P; Konig, M; Kuhlmann, S; Marriner, J; Pain, R; Regnault, N; Balland, C; Bassett, B A; Brown, P J; Campbell, H; Carlberg, R G; Cellier-Holzem, F; Cinabro, D; Conley, A; D’Andrea, C B; DePoy, D L; Doi, M; Ellis, R S; Fabbro, S; Filippenko, A V; Foley, R J; Frieman, J A; Fouchez, D; Galbany, L; Goobar, A
    Aims. We present cosmological constraints from a joint analysis of type Ia supernova (SN Ia) observations obtained by the SDSS-II and SNLS collaborations. The dataset includes several low-redshift samples (z < 0:1), all three seasons from the SDSS-II (0:05 < z < 0:4), and three years from SNLS (0:2 < z < 1), and it totals 740 spectroscopically confirmed type Ia supernovae with high-quality light curves. Methods. We followed the methods and assumptions of the SNLS three-year data analysis except for the following important improvements: 1) the addition of the full SDSS-II spectroscopically-confirmed SN Ia sample in both the training of the SALT2 light-curve model and in the Hubble diagram analysis (374 SNe); 2) intercalibration of the SNLS and SDSS surveys and reduced systematic uncertainties in the photometric calibration, performed blindly with respect to the cosmology analysis; and 3) a thorough investigation of systematic errors associated with the SALT2 modeling of SN Ia light curves. Results. We produce recalibrated SN Ia light curves and associated distances for the SDSS-II and SNLS samples. The large SDSS-II sample provides an eective, independent, low-z anchor for the Hubble diagram and reduces the systematic error from calibration systematics in the low-z SN sample. For a flat CDM cosmology, we find m = 0:295 0:034 (stat+sys), a value consistent with the most recent cosmic microwave background (CMB) measurement from the Planck and WMAP experiments. Our result is 1:8 (stat+sys) dierent than the previously published result of SNLS three-year data. The change is due primarily to improvements in the SNLS photometric calibration. When combined with CMB constraints, we measure a constant dark-energy equation of state parameter w = ?1:018 0:057 (stat+sys) for a flat universe. Adding baryon acoustic oscillation distance measurements gives similar constraints: w = ?1:0270:055. Our supernova measurements provide the most stringent constraints to date on the nature of dark energy.