The recent robust and homogeneous analysis of the world's supernova distance-redshift data, together with cosmic microwave background and baryon acoustic oscillation data-provides a powerful tool for constraining cosmological models. Here we examine particular classes of scalar field, modified gravity, and phenomenological models to assess whether they are consistent with observations even when their behavior deviates from the cosmological constant ?. Some models have tension with the data, while others survive only by approaching the cosmological constant, and a couple are statistically favored over ? cold dark matter. Dark energy described by two equation-of-state parameters has considerable phase space to avoid ? and next-generation data will be required to constrain such physics, with the level of complementarity between probes varying with cosmology.

We present measurements of Ωm and Ω Λ from a blind analysis of 21 high-redshift supemovae using a new technique (CMAGIC) for fitting the multicolor light curves of Type la supernovae, first introduced by Wang and coworkers. CMAGIC takes advantage of the remarkably simple behavior of Type la supernovae on color-magnitude diagrams and has several advantages over current techniques based on maximum magnitudes. Among these are a reduced sensitivity to host galaxy dust extinction, a shallower luminosity-width relation, and the relative simplicity of the fitting procedure. This allows us to provide a cross-check of previous supernova cosmology results, despite the fact that current data sets were not observed in a manner optimized for CMAGIC. We describe the details of our novel blindness procedure, which is designed to prevent experimenter bias. The data are broadly consistent with the picture of an accelerating universe and agree with a flat universe within 1.7 σ, including systematics. We also compare the CMAGIC results directly with those of a maximum magnitude fit to the same supernovae, finding that CMAGIC favors more acceleration at the 1.6 σ level, including systematics and the correlation between the two measurements. A fit for w assuming a flat universe yields a value that is consistent with a cosmological constant within 1.2 σ. © 2006. The American Astronomical Society. All rights reserved.

We estimate systematic errors due to K-corrections in standard photometric analyses of high-redshift Type Iasupernovae. Errors due to K-correction occur when the spectral template model underlying the light curve fitterpoorly represents the actual supernova spectral energy distribution, meaning that the distance modulus cannot berecovered accurately. In order to quantify this effect, synthetic photometry is performed on artificially redshiftedspectrophotometric data from 119 low-redshift supernovae from the Nearby Supernova Factory, and the resultinglight curves are fit with a conventional light curve fitter. We measure the variation in the standardized magnitudethat would be fit for a given supernova if located at a range of redshifts and observed with various filter setscorresponding to current and future supernova surveys. We find significant variation in the measurements of thesame supernovae placed at different redshifts regardless of filters used, which causes dispersion greater than?0.05 mag for measurements of photometry using the Sloan-like filters and a bias that corresponds to a 0.03 shift inw when applied to an outside data set. To test the result of a shift in supernova population or environment at higherredshifts, we repeat our calculations with the addition of a reweighting of the supernovae as a function of redshiftand find that this strongly affects the results and would have repercussions for cosmology. We discuss possiblemethods to reduce the contribution of the K-correction bias and uncertainty.

We present spectra for 14 high-redshift (0.17 < z < 0.83) supernovae, which were discovered by the Supernova Cosmology Project as part of a campaign to measure cosmological parameters. The spectra are used to determine the redshift and classify the supernova type, essential information if the supernova are to be used for cosmological studies. Redshifts were derived either from the spectrum of the host galaxy or from the spectrum of the supernova itself. We present evidence that these supernovae are of Type Ia (SNe Ia) by matching to spectra of nearby supernovae. We find that the dates of the spectra relative to maximum light determined from this fitting process are consistent with the dates determined from the photometric light curves, and, moreover, the spectral time sequences for SNe Ia at low and high redshift are indistinguishable. We also show that the expansion velocities measured from blueshifted Ca H and K are consistent with those measured for low-redshift SNe Ia. From these first-level quantitative comparisons we find no evidence for evolution in SN Ia properties between these low- and high-redshift samples. Thus, even though our samples may not be complete, we conclude that there is a population of SNe Ia at high redshift whose spectral properties match those at low redshift. © 2005. The American Astronomical Society. All rights reserved.

We present a progress report on a project to derive the evolution of the volumetric supernova Type Ia rate from the Supernova Legacy Survey. Our preliminary estimate of the rate evolution divides the sample from Neill et al. into two redshift bins: 0.2 < z < 0.4, and 0.4 < z < 0.6. We extend this by adding a bin from the sample analyzed in Sullivan et al. in the range 0.6 < z < 0.75 from the same time period. We compare the derived trend with previously published rates and a supernova Type Ia production model having two components: one component associated closely with star formation and an additional component associated with host galaxy mass. Our observed trend is consistent with this model, which predicts a rising SN Ia rate out to at least z = 2. © 2007 American Institute of Physics.

We present the discovery and measurements of a gravitationally lensed supernova (SN) behind the galaxy cluster MOO J1014+0038. Based on multi-band Hubble Space Telescope and Very Large Telescope (VLT) photometry of the supernova, and VLT spectroscopy of the host galaxy, we find a 97.5% probability that this SN is a SN Ia, and a 2.5% chance of a CC SN. Our typing algorithm combines the shape and color of the light curve with the expected rates of each SN type in the host galaxy. With a redshift of 2.2216, this is the highest redshift SN Ia discovered with a spectroscopic host-galaxy redshift. A further distinguishing feature is that the lensing cluster, at redshift 1.23, is the most distant to date to have an amplified SN. The SN lies in the middle of the color and light-curve shape distributions found at lower redshift, disfavoring strong evolution to z = 2.22. We estimate an amplification due to gravitational lensing of (1.10 0.23 mag) - compatible with the value estimated from the weak-lensing-derived mass and the mass-concentration relation from ΛCDM simulations - making it the most amplified SN Ia discovered behind a galaxy cluster.