We report measurements of the mass density, Ω M , and cosmological-constant energy density, Ω Λ , of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these supernovae, at redshifts between 0.18 and 0.83, are fit jointly with a set of supernovae from the Calán/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All supernova peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Ω M − 0.6 Ω Λ ≈ −0.2 ± 0.1 in the region of interest (Ω M ∼ < 1.5). For a flat (Ω M + Ω Λ = 1) cosmology we find Ω flat M = 0.28 +0.09 −0.08 (1σ statistical) +0.05 −0.04 (identified systematics). The data are strongly inconsistent with a Λ = 0 flat cosmology, the simplest inflationary universe model. An open, Λ = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Λ > 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t flat 0 = 14.9 +1.4 −1.1 (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calán/Tololo sample and our high-redshift sample. Excluding those few supernovae which are outliers in color excess or fit residual does not significantly change the results. The conclusions are also robust whether or not a width-luminosity relation is used to standardize the supernova peak magnitudes. We discuss, and constrain where possible, hypothetical alternatives to a cosmological constant.

The ultimate fate of the universe, infinite expansion or a big crunch, can be determined by measuring the redshifts, apparent brightnesses, and intrinsic luminosities of very distant supernovae. Recent developments have provided tools that make such a program practicable: (1) Studies of relatively nearby

We report on work to increase the number of well-measured Type Ia supernovae (SNe Ia) at high redshifts. Light curves, including high signal-to-noise HST data, and spectra of six SNe Ia that were discovered during 2001 are presented. Additionally, for the two SNe with z > 1, we present groundbased J-band photometry from Gemini and the VLT. These are among the most distant SNe Ia for which ground based near-IR observations have been obtained. We add these six SNe Ia together with other data sets that have recently become available in the literature to the Union compilation (Kowalski et al. 2008). We have made a number of refinements to the Union analysis chain, the most important ones being the refitting of all light curves with the SALT2 fitter and an improved handling of systematic errors. We call this new compilation, consisting of 557 supernovae, the Union2 compilation. The flat concordance ΛCDM model remains an excellent fit to the Union2 data with the best fit constant equation of state parameter w = −0.997 +0.050 −0.054 (stat)+0.077 −0.082 (stat + sys together) for a flat universe, or w = −1.035 +0.055 −0.059 (stat)+0.093 −0.097 (stat + sys together) with curvature. We also present improved constraints on w(z). While no significant change in w with redshift is detected, there is still considerable room for evolution in w. The strength of the constraints depend strongly on redshift. In particular, at z 1, the existence and nature of dark energy are only weakly constrained by the data.

We present a new compilation of Type Ia supernovae (SNe Ia), a new data set of low-redshift nearby-Hubble-flow SNe, and new analysis procedures to work with these heterogeneous compilations. This ''Union'' compilation of 414 SNe Ia, which reduces to 307 SNe after selection cuts, includes the recent large samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the older data sets, as well as the recently extended data set of distant supernovae observed with the Hubble Space Telescope (HST ). A single, consistent, and blind analysis procedure is used for all the various SN Ia subsamples, and a new procedure is implemented that consistently weights the heterogeneous data sets and rejects outliers. We present the latest results from this Union compilation and discuss the cosmological constraints from this new compilation and its combination with other cosmological measurements (CMB and BAO). The constraint we obtain from supernovae on the dark energy density is à ¼ 0:713 þ0:027 À0:029 (stat) þ0:036 À0:039 (sys), for a flat, ÃCDM universe. Assuming a constant equation of state parameter, w, the combined constraints from SNe, BAO, and A CMB give w ¼ À0:969 þ0:059 À0:063 (stat) þ0:063 À0:066 (sys). While our results are consistent with a cosmological constant, we obtain only relatively weak constraints on a w that varies with redshift. In particular, the current SN data do not yet significantly constrain w at z > 1. With the addition of our new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are currently the tightest available.

We report measurements of M , à , and w from 11 supernovae (SNe) at z ¼ 0:36 0:86 with high-quality light curves measured using WFPC2 on the Hubble Space Telescope (HST). This is an independent set of high-redshift SNe that confirms previous SN evidence for an accelerating universe. The high-quality light curves available from photometry on WFPC2 make it possible for these 11 SNe alone to provide measurements of the cosmological parameters comparable in statistical weight to the previous results. Combined with earlier Supernova Cosmology Project data, the new SNe yield a measurement of the mass density M ¼ 0:25 þ0:07 À0:06 ðstatisticalÞ AE 0:04 (identified systematics), or equivalently, a cosmological constant of à ¼ 0:75 þ0:06 À0:07 ðstatisticalÞ AE 0:04 (identified systematics), under the assumptions of a flat universe and that the dark energy equation-of-state parameter has a constant value w ¼ À1. When the SN results are combined with independent flat-universe measurements of M from cosmic microwave background and galaxy redshift distortion data, they provide a measurement of w ¼ À1:05 þ0:15 À0:20 ðstatisticalÞ AE 0:09 (identified systematic), if w is assumed to be constant in time. In addition to high-precision light-curve measurements, the new data offer greatly improved color measurements of the high-redshift SNe and hence improved host galaxy extinction estimates. These extinction measurements show no anomalous negative E(BÀV ) at high redshift. The precision of the measurements is such that it is possible to perform a host galaxy extinction correction directly for individual SNe without any assumptions or priors on the parent E(BÀV ) distribution. Our cosmological fits using full extinction corrections confirm that dark energy is required with Pð à > 0Þ > 0:99, a result consistent with previous and current SN analyses that rely on the identification of a low-extinction subset or prior assumptions concerning the intrinsic extinction distribution. , and the National Optical Astronomy Observatory. Based in part on observations made with the European Southern Observatory telescopes (ESO programs 60.A-0586 and 265.A-5721). Based in part on observations made with the Canada-France-Hawaii Telescope, operated by the National

We construct a semiclassical Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological model assuming a running cosmological constant (CC). It turns out that the CC becomes variable at arbitrarily low energies due to the remnant quantum effects of the heaviest particles, e.g. the Planck scale physics. These effects are universal in the sense that they lead to a low-energy structure common to a large class of high-energy theories. Remarkably, the uncertainty concerning the unknown high-energy dynamics is accumulated into a single parameter ν, such that the model has an essential predictive power. Future Type Ia supernovae experiments (like SNAP) can verify whether this framework is correct. For the flat FLRW case and a moderate value ν ∼ 10 −2 , we predict an increase of 10 − 20% in the value of Ω Λ at redshifts z = 1 − 1.5 perfectly reachable by SNAP.

The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5• wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for the discovery of new objects. Supernova imaging observations are being acquired between September 1 and November 30 of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

R-band intensity measurements along the light curve of Type Ia supernovae discovered by the Supernova Cosmology Project (SCP) are fitted in brightness to templates allowing a free parameter the time-axis width factor w ≡ s × (1 + 2 z). The data points are then individually aligned in the time-axis, normalized and K-corrected back to the rest frame, after which the nearly 1300 normalized intensity measurements are found to lie on a well-determined common rest-frame B-band curve which we call the "composite curve". The same procedure is applied to 18 low-redshift Calán/Tololo SNe with z < 0.11; these nearly 300 B-band photometry points are found to lie on the composite curve equally well. The SCP search technique produces several measurements before maximum light for each supernova. We demonstrate that the linear stretch factor, s, which parameterizes the light-curve timescale appears independent of z, and applies equally well to the declining and rising parts of the light curve. In fact, the B band template that best fits this composite curve fits the individual supernova photometry data when stretched by a factor s with χ 2 /DoF ≈ 1, thus as well as any parameterization can, given the current data sets. The measurement of the date of explosion, however, is model dependent and not tightly constrained by the current data. We also demonstrate the 1 + z light-curve time-axis broadening expected from cosmological expansion. This argues strongly against alternative explanations, such as tired light, for the redshift of distant objects.