The acceleration of the expansion of the universe, and theneed for Dark Energy, were inferred from the observations of Type Iasupernovae (SNe Ia) 1;2. There is consensus that SNeIa are thermonuclearexplosions that destroy carbon-oxygen white dwarf stars that accretematter from a companion star3, although the nature of this companionremains uncertain. SNe Ia are thought to be reliable distance indicatorsbecause they have a standard amount of fuel and a uniform trigger theyare predicted to explode when the mass of the white dwarf nears theChandrasekhar mass 4 - 1.4 solar masses. Here we show that the highredshift supernova SNLS-03D3bb has an exceptionally high luminosity andlow kinetic energy that both imply a super-Chandrasekhar mass progenitor.Super-Chandrasekhar mass SNeIa shouldpreferentially occur in a youngstellar population, so this may provide an explanation for the observedtrend that overluminous SNe Ia only occur in young environments5;6. Sincethis supernova does not obey the relations that allow them to becalibrated as standard candles, and since no counterparts have been foundat low redshift, future cosmology studies will have to considercontamination from such events.

/pdf/the-type-ia-supernova-snls-03d3bb-from-a-super-chandrasekhar-1hg1yc3r8g.pdf

The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star

The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein’s theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (“morphon field”) modeling Dark Energy. The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation. The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard model of cosmology; exact solutions and perturbative results modeling this instability lie in the right sector to explain Dark Energy from inhomogeneities. It is fair to say that, even if backreaction effects turn out to be less important than anticipated by some researchers, the concordance high-precision cosmology, the architecture of current N-body simulations, as well as standard perturbative approaches may all fall short in correctly describing the Late Universe.

Dark Energy from structure: a status report

We revisit Lyman-alpha bounds on the dark matter mass in Lambda Warm Dark Matter (Lambda-WDM) models, and derive new bounds in the case of mixed Cold plus Warm models (Lambda-CWDM), using a set up which is a good approximation for several theoretically well-motivated dark matter models. We combine WMAP5 results with two different Lyman-alpha data sets, including observations from the Sloan Digital Sky Survey. We pay a special attention to systematics, test various possible sources of error, and compare the results of different statistical approaches. Expressed in terms of the mass of a non-resonantly produced sterile neutrino, our bounds read m_NRP > 8 keV (frequentist 99.7% confidence limit) or m_NRP > 12.1 keV (Bayesian 95% credible interval) in the pure Lambda-WDM limit. For the mixed model, we obtain limits on the mass as a function of the warm dark matter fraction F_WDM. Within the mass range studied here (5 keV < m_NRP < infinity), we find that any mass value is allowed when F_WDM < 0.6 (frequentist 99.7% confidence limit); similarly, the Bayesian joint probability on (F_WDM, 1/m_NRP) allows any value of the mass at the 95% confidence level, provided that F_WDM < 0.35.

/pdf/lyman-alpha-constraints-on-warm-and-on-warm-plus-cold-dark-3tezmerzgd.pdf

Lyman-alpha constraints on warm and on warm-plus-cold dark matter models

The near uniformity of the light curves of type Ia supernovae makes them good standard candles for measuring cosmic expansion, but an observational correction (whose underlying physics has been unknown) must be applied to account for the fact that the brighter ones have 'broader' light curves. Kasen et al. report multi-dimensional modelling of the explosion physics and radiative transfer of type Ia supernovae, which reveals that the breaking of spherical symmetry is a critical factor in determining both the width luminosity relation and the observed scatter about it. The slope and normalization of the width–luminosity relation has a weak dependence on trace abundances of elements other than carbon and oxygen. Failing to correct for this effect could lead to systematic overestimates of up to 2% in the distance to remote supernovae. This work is a step towards explaining both the nature of type Ia supernova events and the physics behind their suitability as cosmological probes. The near uniformity of the light curves of type Ia supernovae makes them good 'standard candles' for measuring cosmic expansion, but a correction must be applied to account for the fact that the brighter ones have broader light curves. Here, multi-dimensional modelling of the explosion physics and radioactive transfer reveals that failing to correct for an effect on the slope and normalization of the width–luminosity relation could lead to systematic overestimates of up to 2% of the distance to remote supernovae. Type Ia supernovae result when carbon-oxygen white dwarfs in binary systems accrete mass from companion stars, reach a critical mass and explode. The near uniformity of their light curves makes these supernovae good ‘standard candles’ for measuring cosmic expansion1,2,3,4, but a correction must be applied to account for the fact that the brighter ones have broader light curves5. One-dimensional modelling, with a certain choice of parameters, can reproduce this general trend in the width–luminosity relation6,7,8; but the processes of ignition and detonation have recently been shown to be intrinsically asymmetric9,10,11,12,13, so parameterization must have its limits. Here we report multi-dimensional modelling of the explosion physics and radiative transfer, which reveals that the breaking of spherical symmetry is a critical factor in determining both the width–luminosity relation and the observed scatter about it. The deviation from spherical symmetry can also explain the finite polarization detected in the light from some supernovae14. The slope and normalization of the width–luminosity relation has a weak dependence on certain properties of the white dwarf progenitor, in particular the trace abundances of elements other than carbon and oxygen. Failing to correct for this effect could lead to systematic overestimates of up to 2 per cent in the distance to remote supernovae.

The diversity of type Ia supernovae from broken symmetries

The holographic dark energy model is proposed by Li as an attempt for probing the nature of dark energy within the framework of quantum gravity. The main characteristic of holographic dark energy is governed by a numerical parameter $c$ in the model. The parameter $c$ can only be determined by observations. Thus, in order to characterize the evolving feature of dark energy and to predict the fate of the Universe, it is of extraordinary importance to constrain the parameter $c$ by using the currently available observational data. In this paper, we derive constraints on the holographic dark energy model from the latest observational data including the gold sample of 182 type Ia supernovae (SNIa), the shift parameter of the cosmic microwave background (CMB) given by the three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations, and the baryon acoustic oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS). The joint analysis gives the fit results in $1\mathrm{\text{\ensuremath{-}}}\ensuremath{\sigma}$: $c={0.91}_{\ensuremath{-}0.18}^{+0.26}$ and ${\ensuremath{\Omega}}_{\mathrm{m}0}=0.29\ifmmode\pm\else\textpm\fi{}0.03$. That is to say, though the possibility of $cl1$ is more favored, the possibility of $cg1$ cannot be excluded in one-sigma error range, which is somewhat different from the result derived from previous investigations using earlier data. So, according to the new data, the evidence for the quintom feature in the holographic dark energy model is not as strong as before.

https://arxiv.org/pdf/astro-ph/0701405.pdf

Constraints on holographic dark energy from the latest supernovae, galaxy clustering, and cosmic microwave background anisotropy observations

We present distance measurements to 71 high redshift type Ia supernovae discovered during the first year of the 5-year Supernova Legacy Survey (SNLS). These events were detected and their multi-color light-curves measured using the MegaPrime/MegaCam instrument at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in four bands, as part of the CFHT Legacy Survey (CFHTLS). Follow-up spectroscopy was performed at the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and to measure their redshift. With this data set, we have built a Hubble diagram extending to z = 1, with all distance measurements involving at least two bands. Systematic uncertainties are evaluated making use of the multi-band photometry obtained at CFHT. Cosmological fits to this first year SNLS Hubble diagram give the following results: ! M = 0.263 ± 0.042 (stat) ± 0.032 (sys) for a flat " CDM model; and w = " 1.023 ± 0.090 (stat) ± 0.054 (sys) for a flat cosmology with constant equation of state w when combined with the constraint from the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations.

/pdf/the-supernova-legacy-survey-measurement-of-m-and-w-from-the-1wc3zmqruw.pdf

The Supernova Legacy Survey: measurement of ! M, ! ! and w from the first year data set ! ,! !

We present a novel technique for fitting restframe I-band light curves on a data set of 42 type Ia supernovae (SNe Ia). Using the result of the fit, we construct a Hubble diagram with 26 SNe from the subset at 0.01 < z < 0.1. Adding two SNe at z ~; 0.5 yields results consistent with a flat Lambda-dominated concordance universe (OmegaM, Omega Lambda) = (0.25, 0.75). For one of these, SN 2000fr, new near infrared data are presented. The high redshift supernova NIR data are also used to test for systematic effects in the use of SNe Ia as distance estimators. A flat, Lambda = 0, universe where the faintness of supernovae at z ~; 0.5 is due to grey dust homogeneously distributed in the intergalactic medium is disfavoured based on the high-z Hubble diagram using this small data-set. However, the uncertainties are large and no firm conclusion may be drawn. We explore the possibility of setting limits on intergalactic dust based on B - I and B - V colour measurements, and conclude that about 20 well measured SNe are needed to give statistically significant results. We also show that the high redshift restframe I-band data points are better fit by light curve templates that show a prominent second peak, suggesting that they are not intrinsically underluminous.

/pdf/restframe-i-band-hubble-diagram-for-type-ia-supernovae-up-to-51pdtlxwlh.pdf

Restframe I-band Hubble diagram for type Ia supernovae up to redshift z ~; 0.5 - eScholarship

We present distance measurements to 71 high redshift type Ia supernovae discovered during the first year of the 5-year Supernova Legacy Survey (SNLS). These events were detected and their multi-color light-curves measured using t he MegaPrime/MegaCam instrument at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in four bands. Follow-up spectroscopy was perf ormed at the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and to measure their redshift. With this data set, we have built a Hubble diagram extending to z = 1, with all distance measurements involving at least two bands. Systematic uncertainties are evaluated making use of the multi- band photometry obtained at CFHT. Cosmological fits to this fi rst year SNLS Hubble diagram give the following results : M = 0.263± 0.042 (stat)± 0.032 (sys) for a flatCDM model; and w = −1.023± 0.090 (stat)± 0.054 (sys) for a flat cosmology with constant equation of state w when combined with the constraint from the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations.

P. Antilogus

Papers

The Supernova Legacy Survey: measurement of ! M, ! ! and w from the first year data set ! ,! !

Restframe I-band Hubble diagram for type Ia supernovae up to redshift z ~; 0.5 - eScholarship

The Supernova Legacy Survey: Measurement ofM, and w from the First Year Data Set ⋆