1,399 research outputs found
Applications of Bayesian model selection to cosmological parameters
Bayesian model selection is a tool to decide whether the introduction of a
new parameter is warranted by data. I argue that the usual sampling statistic
significance tests for a null hypothesis can be misleading, since they do not
take into account the information gained through the data, when updating the
prior distribution to the posterior. On the contrary, Bayesian model selection
offers a quantitative implementation of Occam's razor.
I introduce the Savage-Dickey density ratio, a computationally quick method
to determine the Bayes factor of two nested models and hence perform model
selection. As an illustration, I consider three key parameters for our
understanding of the cosmological concordance model. By using WMAP 3-year data
complemented by other cosmological measurements, I show that a non-scale
invariant spectral index of perturbations is favoured for any sensible choice
of prior. It is also found that a flat Universe is favoured with odds of 29:1
over non--flat models, and that there is strong evidence against a CDM
isocurvature component to the initial conditions which is totally
(anti)correlated with the adiabatic mode (odds of about 2000:1), but that this
is strongly dependent on the prior adopted.
These results are contrasted with the analysis of WMAP 1-year data, which
were not informative enough to allow a conclusion as to the status of the
spectral index. In a companion paper, a new technique to forecast the Bayes
factor of a future observation is presented.Comment: v2 to v3: minor changes, matches accepted version by MNRAS. v1 to v2:
major revision. New results using WMAP 3-yr data, scale-invariant spectrum
now disfavoured with moderate evidence. New benchmark test for the accuracy
of the method. Bayes factor forecast methodology (PPOD, formerly called ExPO)
expanded and now presented in a companion paper (astro-ph/0703063
Hunting Down the Best Model of Inflation with Bayesian Evidence
We present the first calculation of the Bayesian evidence for different
prototypical single field inflationary scenarios, including representative
classes of small field and large field models. This approach allows us to
compare inflationary models in a well-defined statistical way and to determine
the current "best model of inflation". The calculation is performed numerically
by interfacing the inflationary code FieldInf with MultiNest. We find that
small field models are currently preferred, while large field models having a
self-interacting potential of power p>4 are strongly disfavoured. The class of
small field models as a whole has posterior odds of approximately 3:1 when
compared with the large field class. The methodology and results presented in
this article are an additional step toward the construction of a full numerical
pipeline to constrain the physics of the early Universe with astrophysical
observations. More accurate data (such as the Planck data) and the techniques
introduced here should allow us to identify conclusively the best inflationary
model.Comment: 12 pages, 2 figures, uses RevTeX. Misprint corrected, references
added. Matches published versio
Phonon-Assisted Two-Photon Interference from Remote Quantum Emitters
Photonic quantum technologies are on the verge offinding applications in everyday life with quantum cryptography andquantum simulators on the horizon. Extensive research has beencarried out to identify suitable quantum emitters and single epitaxialquantum dots have emerged as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangledphoton-pairs. In order to build up quantum networks, it is essentialto interface remote quantum emitters. However, this is still anoutstanding challenge, as the quantum states of dissimilar“artificialatoms”have to be prepared on-demand with highfidelity and thegenerated photons have to be made indistinguishable in all possibledegrees of freedom. Here, we overcome this major obstacle and show an unprecedented two-photon interference (visibility of 51±5%) from remote strain-tunable GaAs quantum dots emitting on-demand photon-pairs. We achieve this result by exploiting forthefirst time the full potential of a novel phonon-assisted two-photon excitation scheme, which allows for the generation ofhighly indistinguishable (visibility of 71±9%) entangled photon-pairs (fidelity of 90±2%), enables push-button biexciton statepreparation (fidelity of 80±2%) and outperforms conventional resonant two-photon excitation schemes in terms of robustnessagainst environmental decoherence. Our results mark an important milestone for the practical realization of quantum repeatersand complex multiphoton entanglement experiments involving dissimilar artificial atom
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
Effects of Trichoderma harzianum Strain T22 on the Arthropod Community Associated with Tomato Plants and on the Crop Performance in an Experimental Field
Fungi belonging to the genus Trichoderma have received much attention in recent years due to their beneficial effects on crop health and their use as pest control agents. Trichoderma activates direct plant defenses against phytophagous arthropods and reinforces indirect plant defense through the attraction of predators. Although the plant defenses against insect herbivores were demonstrated in laboratory experiments, little attention has been paid to the use of Trichoderma spp. in open field conditions. In the present study, we investigated the effects of the inoculation of the commercial Trichoderma harzianum strain T22 on the arthropod community associated with tomato plants and on the crop performance in an experimental field located in South Italy. Our results showed that inoculation with T. harzianum could alter the arthropod community and reduce the abundance of specific pests under field conditions with respect to the sampling period. The present study also confirmed the beneficial effect of T. harzianum against plant pathogens and on tomato fruit. The complex tomato–arthropod–microorganism interactions that occurred in the field are discussed to enrich our current information on the possibilities of using Trichoderma as a green alternative agent in agricultur
Flat Tree-level Inflationary Potentials in Light of CMB and LSS Data
We use cosmic microwave background and large scale structure data to test a
broad and physically well-motivated class of inflationary models: those with
flat tree-level potentials (typical in supersymmetry). The non-trivial features
of the potential arise from radiative corrections which give a simple
logarithmic dependence on the inflaton field, making the models very
predictive. We also consider a modified scenario with new physics beyond a
certain high-energy cut-off showing up as non-renormalizable operators (NRO) in
the inflaton field. We find that both kinds of models fit remarkably well CMB
and LSS data, with very few free parameters. Besides, a large part of these
models naturally predict a reasonable number of e-folds. A robust feature of
these scenarios is the smallness of tensor perturbations (r < 10^{-3}). The NRO
case can give a sizeable running of the spectral index while achieving a
sufficient number of e-folds. We use Bayesian model comparison tools to assess
the relative performance of the models. We believe that these scenarios can be
considered as a standard physical class of inflationary models, on a similar
footing with monomial potentials.Comment: 42 LaTeX pages, 8 figure
A robust estimate of the Milky Way mass from rotation curve data
We present a new estimate of the mass of the Milky Way, inferred via a Bayesian approach by making use of tracers of the circular velocity in the disk plane and stars in the stellar halo, as from the publicly available galkin compilation. We use the rotation curve method to determine the dark matter distribution and total mass under different assumptions for the dark matter profile, while the total stellar mass is constrained by surface stellar density and microlensing measurements. We also include uncertainties on the baryonic morphology via Bayesian model averaging, thus converting a potential source of systematic error into a more manageable statistical uncertainty. We evaluate the robustness of our result against various possible systematics, including rotation curve data selection, uncertainty on the Sun's velocity V0, dependence on the dark matter profile assumptions, and choice of priors. We find the Milky Way's dark matter virial mass to be log10M200DM/ Mo\u2d9 = 11.92+0.06-0.05(stat)\ub10.28\ub10.27(syst) (M200DM=8.3+1.2-0.9(stat)
71011 Mo\u2d9). We also apply our framework to Gaia DR2 rotation curve data and find good statistical agreement with the above results
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