1,960 research outputs found
Should we doubt the cosmological constant?
While Bayesian model selection is a useful tool to discriminate between
competing cosmological models, it only gives a relative rather than an absolute
measure of how good a model is. Bayesian doubt introduces an unknown benchmark
model against which the known models are compared, thereby obtaining an
absolute measure of model performance in a Bayesian framework. We apply this
new methodology to the problem of the dark energy equation of state, comparing
an absolute upper bound on the Bayesian evidence for a presently unknown dark
energy model against a collection of known models including a flat LambdaCDM
scenario. We find a strong absolute upper bound to the Bayes factor B between
the unknown model and LambdaCDM, giving B < 3. The posterior probability for
doubt is found to be less than 6% (with a 1% prior doubt) while the probability
for LambdaCDM rises from an initial 25% to just over 50% in light of the data.
We conclude that LambdaCDM remains a sufficient phenomenological description of
currently available observations and that there is little statistical room for
model improvement.Comment: 10 pages, 2 figure
Pippi - painless parsing, post-processing and plotting of posterior and likelihood samples
Interpreting samples from likelihood or posterior probability density
functions is rarely as straightforward as it seems it should be. Producing
publication-quality graphics of these distributions is often similarly painful.
In this short note I describe pippi, a simple, publicly-available package for
parsing and post-processing such samples, as well as generating high-quality
PDF graphics of the results. Pippi is easily and extensively configurable and
customisable, both in its options for parsing and post-processing samples, and
in the visual aspects of the figures it produces. I illustrate some of these
using an existing supersymmetric global fit, performed in the context of a
gamma-ray search for dark matter. Pippi can be downloaded and followed at
http://github.com/patscott/pippi .Comment: 4 pages, 1 figure. v3: Updated for pippi 2.0. New features include
hdf5 support, out-of-core processing, inline post-processing with arbitrary
Python code in the input file, and observable-specific data cuts. Pippi can
be downloaded from http://github.com/patscott/pipp
The impact of priors and observables on parameter inferences in the Constrained MSSM
We use a newly released version of the SuperBayeS code to analyze the impact
of the choice of priors and the influence of various constraints on the
statistical conclusions for the preferred values of the parameters of the
Constrained MSSM. We assess the effect in a Bayesian framework and compare it
with an alternative likelihood-based measure of a profile likelihood. We employ
a new scanning algorithm (MultiNest) which increases the computational
efficiency by a factor ~200 with respect to previously used techniques. We
demonstrate that the currently available data are not yet sufficiently
constraining to allow one to determine the preferred values of CMSSM parameters
in a way that is completely independent of the choice of priors and statistical
measures. While b->s gamma generally favors large m_0, this is in some contrast
with the preference for low values of m_0 and m_1/2 that is almost entirely a
consequence of a combination of prior effects and a single constraint coming
from the anomalous magnetic moment of the muon, which remains somewhat
controversial. Using an information-theoretical measure, we find that the
cosmological dark matter abundance determination provides at least 80% of the
total constraining power of all available observables. Despite the remaining
uncertainties, prospects for direct detection in the CMSSM remain excellent,
with the spin-independent neutralino-proton cross section almost guaranteed
above sigma_SI ~ 10^{-10} pb, independently of the choice of priors or
statistics. Likewise, gluino and lightest Higgs discovery at the LHC remain
highly encouraging. While in this work we have used the CMSSM as particle
physics model, our formalism and scanning technique can be readily applied to a
wider class of models with several free parameters.Comment: Minor changes, extended discussion of profile likelihood. Matches
JHEP accepted version. SuperBayeS code with MultiNest algorithm available at
http://www.superbayes.or
Effects of exposures to repeated heat stress on the survival of the pea aphid Acyrthosiphon pisum and its endoparasitoid Aphidius ervi
Organisms could be exposed to several heat waves during their life, and their ability to survive a heat wave strongly depends on the effects of the previous one. Exposure to extreme temperatures can have important effects on the outcome of host-parasitoid interactions, as the ability of the parasitoid to survive depends on the ability of its host to cope successfully with these stresses. In the present study we address the impact of repeated exposure to heat stress on the survival of the pea aphid Acyrthosiphon pisum (Harris)
(Hemiptera Aphididae) and its endoparasitoid Aphidius ervi Haliday Hymenoptera Braconidae). The first treatment consisted of a heat stress of 35 °C for 30 minutes performed on 4 days old aphids, the second and third heat stresses of 39 °C were performed on 5 days old and on adult aphids, espectively. The three treatments were applied alone or in all their ombinations. We found that aphid thermal tolerance is positively influenced by heat hardening if a severe stress occurs a few days after the first event. Adult
parasitized aphids show significantly higher survival than unparasitized ones; however, the effects of parasitization and hardening on host survival after heat shock are not additive. We also found that A. ervi has a lower thermotolerance capacity than its host and does not show apparent hardening effects. In addition, parasitoid survival after mummification is not affected by the previously experienced heat shock. The possible explanations of the observed phenomena are discussed
Fast Acceleration of Transrelativistic Electrons in Astrophysical Turbulence
Highly energetic, relativistic electrons are commonly present in many
astrophysical systems, from solar flares to the intra-cluster medium, as
indicated by observed electromagnetic radiation. However, open questions remain
about the mechanisms responsible for their acceleration, and possible
re-acceleration. Ubiquitous plasma turbulence is one of the possible universal
mechanisms. We study the energization of transrelativistic electrons in
turbulence using hybrid particle-in-cell, which provide a realistic model of
Alfv\'{e}nic turbulence from MHD to sub-ion scales, and test particle
simulations for electrons. We find that, depending on the electron initial
energy and turbulence strength, electrons may undergo a fast and efficient
phase of energization due to the magnetic curvature drift during the time they
are trapped in dynamic magnetic structures. In addition, electrons are
accelerated stochastically which is a slower process that yields lower maximum
energies. The combined effect of these two processes determines the overall
electron acceleration. With appropriate turbulence parameters, we find that
superthermal electrons can be accelerated up to relativistic energies. For
example, with heliospheric parameters and a relatively high turbulence level,
rapid energization to MeV energies is possible.Comment: Accepted for publication in The Astrophysical Journa
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