429 research outputs found
Why anthropic reasoning cannot predict Lambda
We revisit anthropic arguments purporting to explain the measured value of
the cosmological constant. We argue that different ways of assigning
probabilities to candidate universes lead to totally different anthropic
predictions. As an explicit example, we show that weighting different universes
by the total number of possible observations leads to an extremely small
probability for observing a value of Lambda equal to or greater than what we
now measure. We conclude that anthropic reasoning within the framework of
probability as frequency is ill-defined and that in the absence of a
fundamental motivation for selecting one weighting scheme over another the
anthropic principle cannot be used to explain the value of Lambda, nor, likely,
any other physical parameters.Comment: 4 pages, 1 figure. Discussion slighlty expanded, refs added,
conclusions unchanged. Matches published versio
Monolithic or hierarchical star formation? A new statistical analysis
We consider an analytic model of cosmic star formation which incorporates
supernova feedback, gas accretion and enriched outflows, reproducing the
history of cosmic star formation, metallicity, supernovae type II rates and the
fraction of baryons allocated to structures. We present a new statistical
treatment of the available observational data on the star formation rate and
metallicity that accounts for the presence of possible systematics. We then
employ a Bayesian Markov Chain Monte Carlo method to compare the predictions of
our model with observations and derive constraints on the 7 free parameters of
the model. We find that the dust correction scheme one chooses to adopt for the
star formation data is critical in determining which scenario is favoured
between a hierarchical star formation model, where star formation is prolonged
by accretion, infall and merging, and a monolithic scenario, where star
formation is rapid and efficient. We distinguish between these modes by
defining a characteristic minimum mass, M > 10^{11} M_solar, in our fiducial
model, for early type galaxies where star formation occurs efficiently. Our
results indicate that the hierarchical star formation model can achieve better
agreement with the data, but that this requires a high efficiency of
supernova-driven outflows. In a monolithic model, our analysis points to the
need for a mechanism that drives metal-poor winds, perhaps in the form of
supermassive black hole-induced outflows. Furthermore, the relative absence of
star formation beyond z ~ 5 in the monolithic scenario requires an alternative
mechanism to dwarf galaxies for reionizing the universe at z ~ 11, as required
by observations of the microwave background. While the monolithic scenario is
less favoured in terms of its quality-of-fit, it cannot yet be excluded.Comment: Expanded discussion on the role of mergers and on reionization in the
monolithic scenario, refs added, main results unchanged. Matches version to
appear in MNRA
Monolithic or hierarchical star formation? A new statistical analysis
We consider an analytic model of cosmic star formation which incorporates
supernova feedback, gas accretion and enriched outflows, reproducing the
history of cosmic star formation, metallicity, supernovae type II rates and the
fraction of baryons allocated to structures. We present a new statistical
treatment of the available observational data on the star formation rate and
metallicity that accounts for the presence of possible systematics. We then
employ a Bayesian Markov Chain Monte Carlo method to compare the predictions of
our model with observations and derive constraints on the 7 free parameters of
the model. We find that the dust correction scheme one chooses to adopt for the
star formation data is critical in determining which scenario is favoured
between a hierarchical star formation model, where star formation is prolonged
by accretion, infall and merging, and a monolithic scenario, where star
formation is rapid and efficient. We distinguish between these modes by
defining a characteristic minimum mass, M > 10^{11} M_solar, in our fiducial
model, for early type galaxies where star formation occurs efficiently. Our
results indicate that the hierarchical star formation model can achieve better
agreement with the data, but that this requires a high efficiency of
supernova-driven outflows. In a monolithic model, our analysis points to the
need for a mechanism that drives metal-poor winds, perhaps in the form of
supermassive black hole-induced outflows. Furthermore, the relative absence of
star formation beyond z ~ 5 in the monolithic scenario requires an alternative
mechanism to dwarf galaxies for reionizing the universe at z ~ 11, as required
by observations of the microwave background. While the monolithic scenario is
less favoured in terms of its quality-of-fit, it cannot yet be excluded.Comment: Expanded discussion on the role of mergers and on reionization in the
monolithic scenario, refs added, main results unchanged. Matches version to
appear in MNRA
The cosmological constant and the paradigm of adiabaticity
We discuss the value of the cosmological constant as recovered from CMB and
LSS data and the robustness of the results when general isocurvature initial
conditions are allowed for, as opposed to purely adiabatic perturbations. The
Bayesian and frequentist statistical approaches are compared. It is shown that
pre-WMAP CMB and LSS data tend to be incompatible with a non-zero cosmological
constant, regardless of the type of initial conditions and of the statistical
approach. The non-adiabatic contribution is constrained to be < 40% (2sigma
c.l.).Comment: 9 pages, 5 figures, to appear in New Astronomy Reviews, Proceedings
of the 2nd CMBNET Meeting, 20-21 February 2003, Oxford, U
Applications of Bayesian model selection to cosmological parameters
Bayesian model selection is a tool for deciding whether the introduction of a new parameter is warranted by the 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. In contrast, 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 Wilkinson Microwave Anisotropy Probe (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 cold dark matter 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 presente
A Global Analysis of Dark Matter Signals from 27 Dwarf Spheroidal Galaxies using 11 Years of Fermi-LAT Observations
We search for a dark matter signal in 11 years of Fermi-LAT gamma-ray data
from 27 Milky Way dwarf spheroidal galaxies with spectroscopically measured
-factors. Our analysis includes uncertainties in -factors and background
normalisations and compares results from a Bayesian and a frequentist
perspective. We revisit the dwarf spheroidal galaxy Reticulum II, confirming
that the purported gamma-ray excess seen in Pass 7 data is much weaker in Pass
8, independently of the statistical approach adopted. We introduce for the
first time posterior predictive distributions to quantify the probability of a
dark matter detection from another dwarf galaxy given a tentative excess. A
global analysis including all 27 dwarfs shows no indication for a signal in
nine annihilation channels. We present stringent new Bayesian and frequentist
upper limits on the dark matter cross section as a function of dark matter
mass. The best-fit dark matter parameters associated with the Galactic Centre
excess are excluded by at least 95% confidence level/posterior probability in
the frequentist/Bayesian framework in all cases. However, from a Bayesian model
comparison perspective, dark matter annihilation within the dwarfs is not
strongly disfavoured compared to a background-only model. These results
constitute the highest exposure analysis on the most complete sample of dwarfs
to date. Posterior samples and likelihood maps from this study are publicly
available.Comment: 27+5 pages, 10 figures. Version 2 corresponds to the Accepted
Manuscript version of the JCAP article; the analysis has been updated to Pass
8 R3 data plus 4FGL catalogue, with one more year of data and more
annihilation channels. Supplementary Material (tabulated limits, likelihoods,
and posteriors) is available on Zenodo at
https://doi.org/10.5281/zenodo.261226
Cosmic Microwave Background Anisotropies: Beyond Standard Parameters
In the first part of this work, I review the theoretical framework of
cosmological perturbation theory necessary to understand the generation and
evolution of cosmic microwave background (CMB) anisotropies. Using analytical
and numerical techniques, in the second part I describe the impact on the CMB
power spectra of the standard cosmological parameters (such as the
matter-energy budget of the Universe, its curvature, the amplitude and spectral
properties of the primordial fluctuations, etc.). I introduce the most general
type of initial conditions for the primordial perturbations, deriving a new
analytical approximation for the neutrino isocurvature modes.
In the third part, I discuss the issue of extracting constraints on the
parameters of interest from the recent, high-quality CMB measurements,
presenting the relevant statistical tools and focusing on the Fisher matrix
analysis as a technique to produce reliable forecasts for the performance of
future observations. I then apply those tools to the study of several possible
extensions of the (currently) standard Lambda CDM model: the presence of extra
relativistic particles, possible time variations of the fine structure
constant, and the value of the primordial Helium mass fraction. I also use the
CMB as a tool to study the very early Universe, via its dependence on the type
of initial conditions: I relax the assumption of purely adiabatic initial
conditions and discuss the observational consequences and constraints on the
presence of general isocurvature modes.Comment: PhD thesis, 231 pages, 50+ low resolution figures to comply with
arXiv restrictions. Higher resolution version available from
http://mpej.unige.ch/~trotta/html/thesis.ht
Bayesian Calibrated Significance Levels Applied to the Spectral Tilt and Hemispherical Asymmetry
Bayesian model selection provides a formal method of determining the level of
support for new parameters in a model. However, if there is not a specific
enough underlying physical motivation for the new parameters it can be hard to
assign them meaningful priors, an essential ingredient of Bayesian model
selection. Here we look at methods maximizing the prior so as to work out what
is the maximum support the data could give for the new parameters. If the
maximum support is not high enough then one can confidently conclude that the
new parameters are unnecessary without needing to worry that some other prior
may make them significant. We discuss a computationally efficient means of
doing this which involves mapping p-values onto upper bounds of the Bayes
factor (or odds) for the new parameters. A p-value of 0.05 ()
corresponds to odds less than or equal to 5:2 which is below the `weak' support
at best threshold. A p-value of 0.0003 () corresponds to odds of
less than or equal to 150:1 which is the `strong' support at best threshold.
Applying this method we find that the odds on the scalar spectral index being
different from one are 49:1 at best. We also find that the odds that there is
primordial hemispherical asymmetry in the cosmic microwave background are 9:1
at best.Comment: 5 pages. V2: clarifying comments added in response to referee report.
Matches version to appear in MNRA
Quantifying the tension between the Higgs mass and (g-2)_mu in the CMSSM
Supersymmetry has been often invoqued as the new physics that might reconcile
the experimental muon magnetic anomaly, a_mu, with the theoretical prediction
(basing the computation of the hadronic contribution on e^+ e^- data). However,
in the context of the CMSSM, the required supersymmetric contributions (which
grow with decreasing supersymmetric masses) are in potential tension with a
possibly large Higgs mass (which requires large stop masses). In the limit of
very large m_h supersymmetry gets decoupled, and the CMSSM must show the same
discrepancy as the SM with a_mu . But it is much less clear for which size of
m_h does the tension start to be unbearable. In this paper, we quantify this
tension with the help of Bayesian techniques. We find that for m_h > 125 GeV
the maximum level of discrepancy given current data (~ 3.3 sigma) is already
achieved. Requiring less than 3 sigma discrepancy, implies m_h < 120 GeV. For a
larger Higgs mass we should give up either the CMSSM model or the computation
of a_mu based on e^+ e^-; or accept living with such inconsistency
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