10 research outputs found
The supernova cosmology cookbook: Bayesian numerical recipes
Theoretical and observational cosmology have enjoyed a number of significant
successes over the last two decades. Cosmic microwave background measurements
from the Wilkinson Microwave Anisotropy Probe and Planck, together with
large-scale structure and supernova (SN) searches, have put very tight
constraints on cosmological parameters. Type Ia supernovae (SNIa) played a
central role in the discovery of the accelerated expansion of the Universe,
recognised by the Nobel Prize in Physics in 2011. The last decade has seen an
enormous increase in the amount of high quality SN observations, with SN
catalogues now containing hundreds of objects. This number is expected to
increase to thousands in the next few years, as data from next-generation
missions, such as the Dark Energy Survey and Large Synoptic Survey Telescope
become available. In order to exploit the vast amount of forthcoming high
quality data, it is extremely important to develop robust and efficient
statistical analysis methods to answer cosmological questions, most notably
determining the nature of dark energy. To address these problems my work is
based on nested-sampling approaches to parameter estimation and model selection
and neural networks for machine-learning. Using advanced Bayesian techniques, I
constrain the properties of dark-matter haloes along the SN lines-of-sight via
their weak gravitational lensing effects, develop methods for classifying SNe
photometrically from their lightcurves, and present results on more general
issues associated with constraining cosmological parameters and testing the
consistency of different SN compilations.Comment: 119 pages, 29 figures, Doctoral Thesis in Theoretical Physics, ISBN
978-91-7447-953-
Testing the mutual consistency of different supernovae surveys
It is now common practice to constrain cosmological parameters using
supernovae (SNe) catalogues constructed from several different surveys. Before
performing such a joint analysis, however, one should check that parameter
constraints derived from the individual SNe surveys that make up the catalogue
are mutually consistent. We describe a statistically-robust mutual consistency
test, which we calibrate using simulations, and apply it to each pairwise
combination of the surveys making up, respectively, the UNION2 catalogue and
the very recent JLA compilation by Betoule et al. We find no inconsistencies in
the latter case, but conclusive evidence for inconsistency between some survey
pairs in the UNION2 catalogue.Comment: 8 pages, 9 figures, submitted to MNRA
Comparison of cosmological parameter inference methods applied to supernovae lightcurves fitted with SALT2
We present a comparison of two methods for cosmological parameter inference
from supernovae Ia lightcurves fitted with the SALT2 technique. The standard
chi-square methodology and the recently proposed Bayesian hierarchical method
(BHM) are each applied to identical sets of simulations based on the 3-year
data release from the Supernova Legacy Survey (SNLS3), and also data from the
Sloan Digital Sky Survey (SDSS), the Low Redshift sample and the Hubble Space
Telescope (HST), assuming a concordance LCDM cosmology. For both methods, we
find that the recovered values of the cosmological parameters, and the global
nuisance parameters controlling the stretch and colour corrections to the
supernovae lightcurves, suffer from small biasses. The magnitude of the biasses
is similar in both cases, with the BHM yielding slightly more accurate results,
in particular for cosmological parameters when applied to just the SNLS3 single
survey data sets. Most notably, in this case, the biasses in the recovered
matter density are in opposite directions for the two
methods. For any given realisation of the SNLS3-type data, this can result in a
discrepancy in the estimated value of
between the two methods, which we find to be the case for real SNLS3 data. As
more higher and lower redshift SNIa samples are included, however, the
cosmological parameter estimates of the two methods converge.Comment: 10 pages, 7 figures, submitted to MNRA
Bayesian constraints on dark matter halo properties using gravitationally-lensed supernovae
A hierarchical Bayesian method is applied to the analysis of Type-Ia
supernovae (SNIa) observations to constrain the properties of the dark matter
haloes of galaxies along the SNIa lines-of-sight via their gravitational
lensing effect. The full joint posterior distribution of the dark matter halo
parameters is explored using the nested sampling algorithm {\sc MultiNest},
which also efficiently calculates the Bayesian evidence, thereby facilitating
robust model comparison. We first demonstrate the capabilities of the method by
applying it to realistic simulated SNIa data, based on the real 3-year data
release from the Supernova Legacy Survey (SNLS3). Assuming typical values for
the halo parameters in our simulations, we find that a catalogue analogous to
the existing SNLS3 data set is incapable of detecting the lensing signal, but a
catalogue containing approximately three times as many SNIa does produce robust
and accurate parameter constraints and model selection results for two halo
models: a truncated singular isothermal sphere (SIS) and a
Navarro--Frenk--White (NFW) profile, thereby validating our analysis
methodology. In the analysis of the real SNLS3 data, contrary to previous
studies, we obtain only a very marginal detection of a lensing signal and weak
constraints on the halo parameters for the truncated SIS model, although these
constraints are tighter than those obtained from the equivalent simulated SNIa
data set. This difference is driven by a preferred value of in
the assumed scaling-law between velocity dispersion and
luminosity, which is somewhat higher than the canonical values of and for early and late-type galaxies,
respectively, and leads to a stronger lensing effect by the halo. No detection
of a lensing signal is made for the NFW model.Comment: Revised version matches manuscript accepted by MNRAS: Manuscript ID:
MN-12-1716-MJ.R
DES14X3taz: a type I superluminous supernova showing a luminous, rapidly cooling initial pre-peak bump
We present DES14X3taz, a new hydrogen-poor superluminous supernova (SLSN-I) discovered by the Dark Energy Survey (DES) supernova program, with additional photometric data provided by the Survey Using DECam for Superluminous Supernovae. Spectra obtained using Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy on the Gran Telescopio CANARIAS show DES14X3taz is an SLSN-I at z = 0.608. Multi-color photometry reveals a double-peaked light curve: a blue and relatively bright initial peak that fades rapidly prior to the slower rise of the main light curve. Our multi-color photometry allows us, for the first time, to show that the initial peak cools from 22,000 to 8000 K over 15 rest-frame days, and is faster and brighter than any published core-collapse supernova, reaching 30% of the bolometric luminosity of the main peak. No physical 56Ni-powered model can fit this initial peak. We show that a shock-cooling model followed by a magnetar driving the second phase of the light curve can adequately explain the entire light curve of DES14X3taz. Models involving the shock-cooling of extended circumstellar material at a distance of ~=400 {\text{}}{R}&sun; are preferred over the cooling of shock-heated surface layers of a stellar envelope. We compare DES14X3taz to the few double-peaked SLSN-I events in the literature. Although the rise times and characteristics of these initial peaks differ, there exists the tantalizing possibility that they can be explained by one physical interpretation
Bahamas: new analysis of type la supernovae reveals inconsistencies with standard cosmology
We present results obtained by applying our Bayesian HierArchical Modeling for the Analysis of Supernova cosmology (Bahamas) software package to the 740 spectroscopically confirmed supernovae of type Ia (SNe Ia) from the "Joint Light-curve Analysis" (JLA) data set. We simultaneously determine cosmological parameters and standardization parameters, including corrections for host galaxy mass, residual scatter, and object-by-object intrinsic magnitudes. Combining JLA and Planck data on the cosmic microwave background, we find significant discrepancies in cosmological parameter constraints with respect to the standard analysis: we find \u3a9m = 0.399 \ub1 0.027, 2.8\u3c3, higher than previously reported, and w = -0.910 \ub1 0.045, 1.6\u3c3, higher than the standard analysis. We determine the residual scatter to be \u3c3res = 0.104 \ub1 0.005. We confirm (at the 95% probability level) the existence of two subpopulations segregated by host galaxy mass, separated at log10 (M/M 99) = 10, differing in mean intrinsic magnitude by 0.055 \ub1 0.022 mag, lower than previously reported. Cosmological parameter constraints, however, are unaffected by the inclusion of corrections for host galaxy mass. We find 3c4\u3c3 evidence for a sharp drop in the value of the color correction parameter, \u3b2 (z), at a redshift zt = 0.662 \ub1 0.055. We rule out some possible explanations for this behavior, which remains unexplained