140756
The expansion of the universe observed with supernovae.
Over the last 20 years, supernovae have become a key tool to constrain the expansion history of the Universe through the construction of Hubble diagrams, using luminosity distances to supernovae belonging to the 'Ia' subtype. This technique was key for the discovery that the expansion of the Universe is now accelerating. We review the principle and difficulties of the measurements, the classification and diversity of supernovae, and the physics of explosion. We discuss the systematic uncertainties affecting the cosmological conclusions with some emphasis on photometric calibration. We describe the major supernova cosmology surveys, the presented analyses and their conclusions, together with the present status of the field. We conclude on the expectations for the near future.
59468119
Photometry of supernovae in an image series: methods and application to the SuperNova Legacy Survey (SNLS)
Aims. We present a technique to measure lightcurves of time-variable point sources on a spatially structured background from imaging data. The technique was developed to measure lightcurves of SNLS supernovae in order to infer their distances. This photometry technique performs simultaneous point spread function (PSF) photometry at the same sky position on an image series. Methods. We describe two implementations of the method: one that resamples images before measuring fluxes, and one which does not. In both instances, we sketch the key algorithms involved and present the validation using semi-artificial sources introduced in real images in order to assess the accuracy of the supernova flux measurements relative to that of surrounding stars. We describe the methods required to anchor these PSF fluxes to calibrated aperture catalogs, in order to derive SN magnitudes. Results. We find a marginally significant bias of 2 mmag of the after-resampling method, and no bias at the mmag accuracy for the non-resampling method. Given surrounding star magnitudes, we determine the systematic uncertainty of SN magnitudes to be less than 1.5 mmag, which represents about one third of the current photometric calibration uncertainty a ecting SN measurements. The SN photometry delivers several by-products: bright star PSF flux measurements which have a repeatability of about 0.6%, as for aperture measurements; we measure relative astrometric positions with a noise floor of 2.4 mas for a single-image bright star measurement; we show that in all bands of the MegaCam instrument, stars exhibit a profile linearly broadening with flux by about 0.5% over the whole brightness range.
121654518
Cosmology and dark energy
Global cosmological parameters are now known to an accuracy totally unexpected ten years ago, and considerable improvements are expected in the next decade both from space missions and ground-based facilities. Although there were some early indications, convincing evidences of the existence of dark energy appeared in the late 90's. This definitely makes the measurement of cosmological parameters more complicated, because the nature of this component of the universe is totally unknown, and present data only constraints the simplest scenarios. We will review here how dark energy existence was established and confirmed, and the current observational constraints we can place. We will also describe the observational approaches expected to provide new cosmological constrains over approximately the next decade.
18389855
Can luminosity distance measurements probe the equation of state of dark energy
Abstract Distance measurements to type Ia supernovae (SNe Ia) at cosmological distances indicate that the Universe is accelerating and that a large fraction of the critical energy density exists in a component with negative pressure. Various hypotheses on the nature of this “dark energy” can be tested via their prediction for the equation of state of this component. If the dark energy is due to a scalar field, its equation of state will in general vary with time and is related to the potential of the field. We review the intrinsic degeneracies of luminosity distance measurements and compute the expected accuracies that can be obtained for the equation of state parameter from a realistic high statistic SNe Ia experiment.
42424969
Cosmological Parameter Uncertainties from SALT-II Type Ia Supernova Light Curve Models
We use simulated type Ia supernova (SN Ia) samples, including both photometry and spectra, to perform the first direct validation of cosmology analysis using the SALT-II light curve model. This validation includes residuals from the light curve training process, systematic biases in SN Ia distance measurements, and a bias on the dark energy equation of state parameter w. Using the SN-analysis package SNANA, we simulate and analyze realistic samples corresponding to the data samples used in the SNLS3 analysis: ∼120 low-redshift (z < 0.1) SNe Ia, ∼255 Sloan Digital Sky Survey SNe Ia (z < 0.4), and ∼290 SNLS SNe Ia (z ≤ 1). To probe systematic uncertainties in detail, we vary the input spectral model, the model of intrinsic scatter, and the smoothing (i.e., regularization) parameters used during the SALT-II model training. Using realistic intrinsic scatter models results in a slight bias in the ultraviolet portion of the trained SALT-II model, and w biases (w {sub input} – w {sub recovered}) ranging from –0.005 ± 0.012 to –0.024 ± 0.010. These biases are indistinguishable from each other within the uncertainty; the average bias on w is –0.014 ± 0.007.
53466158
Experimental constraints on the uncoupled Galileon model from SNLS3 data and other cosmological probes
Aims. The Galileon model is a modified gravity theory that may provide
 an explanation for the accelerated expansion of the Universe. This model does not suffer
 from instabilities or ghost problems (normally associated with higher-order derivative
 theories), restores local General Relativity – thanks to the Vainshtein screening effect –
 and predicts late-time acceleration of the expansion.Methods. We derive a new definition of the Galileon parameters that
 allows us to avoid having to choose initial conditions for the Galileon field. We tested
 this model against precise measurements of the cosmological distances and the rate of
 growth of cosmic structures.Results. We observe a weak tension between the constraints set by growth
 data and those from distances. However, we find that the Galileon model remains consistent
 with current observations and is still competitive with the ΛCDM model, contrary to what
 was concluded in recent publications.
6507259
Kalman filter track fits and track breakpoint analysis
We give an overview of track fitting using the Kalman filter method in the NOMAD detector at CERN, and emphasize how the wealth of by-product information can be used to analyze track breakpoints (discontinuities in track parameters caused by scattering, decay, etc.). After reviewing how this information has been previously exploited by others, we describe extensions which add power to breakpoint detection and characterization. We show how complete fits to the entire track, with breakpoint parameters added, can be easily obtained from the information from unbroken fits. Tests inspired by the Fisher F-test can then be used to judge breakpoints. Signed quantities (such as change in momentum at the breakpoint) can supplement unsigned quantities such as the various chisquares. We illustrate the method with electrons from real data, and with Monte Carlo simulations of pion decays.