57 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
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
Observational signatures of Jordan-Brans-Dicke theories of gravity
We analyze the Jordan-Brans-Dicke model (JBD) of gravity, where deviations
from General Relativity (GR) are described by a scalar field non-minimally
coupled to gravity. The theory is characterized by a constant coupling
parameter, ; GR is recovered in the limit . In such theories, gravity modifications manifest at early times,
so that one cannot rely on the usual approach of looking for inconsistencies in
the expansion history and perturbations growth in order to discriminate between
JBD and GR. However, we show that a similar technique can be successfully
applied to early and late times observables instead. Cosmological parameters
inferred extrapolating early-time observations to the present will match those
recovered from direct late-time observations only if the correct gravity theory
is used. We use the primary CMB, as will be seen by the Planck satellite, as
the early-time observable; and forthcoming and planned Supernov{\ae}, Baryonic
Acoustic Oscillations and Weak Lensing experiments as late-time observables. We
find that detection of values of as large as 500 and 1000 is
within reach of the upcoming (2010) and next-generation (2020) experiments,
respectively.Comment: minor revision, references added, matching version published in JCA
Gauge-Invariant Initial Conditions and Early Time Perturbations in Quintessence Universes
We present a systematic treatment of the initial conditions and evolution of
cosmological perturbations in a universe containing photons, baryons,
neutrinos, cold dark matter, and a scalar quintessence field. By formulating
the evolution in terms of a differential equation involving a matrix acting on
a vector comprised of the perturbation variables, we can use the familiar
language of eigenvalues and eigenvectors. As the largest eigenvalue of the
evolution matrix is fourfold degenerate, it follows that there are four
dominant modes with non-diverging gravitational potential at early times,
corresponding to adiabatic, cold dark matter isocurvature, baryon isocurvature
and neutrino isocurvature perturbations. We conclude that quintessence does not
lead to an additional independent mode.Comment: Replaced with published version, 12 pages, 2 figure
Observational constraints on the curvaton model of inflation
Simple curvaton models can generate a mixture of of correlated primordial
adiabatic and isocurvature perturbations. The baryon and cold dark matter
isocurvature modes differ only by an observationally null mode in which the two
perturbations almost exactly compensate, and therefore have proportional
effects at linear order. We discuss the CMB anisotropy in general mixed models,
and give a simple approximate analytic result for the large scale CMB
anisotropy. Working numerically we use the latest WMAP observations and a
variety of other data to constrain the curvaton model. We find that models with
an isocurvature contribution are not favored relative to simple purely
adiabatic models. However a significant primordial totally correlated baryon
isocurvature perturbation is not ruled out. Certain classes of curvaton model
are thereby ruled out, other classes predict enough non-Gaussianity to be
detectable by the Planck satellite. In the appendices we review the relevant
equations in the covariant formulation and give series solutions for the
radiation dominated era.Comment: Minor changes and corrections to match version accepted by PR
Adiabatic and entropy perturbations propagation in a bouncing Universe
By studying some bouncing universe models dominated by a specific class of
hydrodynamical fluids, we show that the primordial cosmological perturbations
may propagate smoothly through a general relativistic bounce. We also find that
the purely adiabatic modes, although almost always fruitfully investigated in
all other contexts in cosmology, are meaningless in the bounce or null energy
condition (NEC) violation cases since the entropy modes can never be neglected
in these situations: the adiabatic modes exhibit a fake divergence that is
compensated in the total Bardeen gravitational potential by inclusion of the
entropy perturbations.Comment: 25 pages, no figure, LaTe
Suppression of Entropy Perturbations in Multi--Field Inflation on the Brane
At energies higher than the brane tension, the dynamics of a scalar field
rolling down a potential are modified relative to the predictions of General
Relativity. The modifications imply, among other things, that steeper
potentials can be used to drive an epoch of slow--roll inflation. We
investigate the evolution of entropy and adiabatic modes during inflation
driven by two scalar fields confined on the brane. We show that the amount of
entropy perturbations produced during inflation is suppressed compared to the
predictions made by General Relativity. As a consequence, the initial
conditions do not matter in multiple field inflation in brane worlds if
inflation is driven at energies much higher than the brane tension.Comment: 4 pages, 1 figur
An Observational Test of Two-field Inflation
We study adiabatic and isocurvature perturbation spectra produced by a period
of cosmological inflation driven by two scalar fields. We show that there
exists a model-independent consistency condition for all two-field models of
slow-roll inflation, despite allowing for model-dependent linear processing of
curvature and isocurvature perturbations during and after inflation on
super-horizon scales. The scale-dependence of all spectra are determined solely
in terms of slow-roll parameters during inflation and the dimensionless
cross-correlation between curvature and isocurvature perturbations. We present
additional model-dependent consistency relations that may be derived in
specific two-field models, such as the curvaton scenario.Comment: 6 pages, latex with revtex, no figures; v2, minor changes, to appear
in Physical Review
Correlated perturbations from inflation and the cosmic microwave background
We compare the latest cosmic microwave background data with theoretical
predictions including correlated adiabatic and CDM isocurvature perturbations
with a simple power-law dependence. We find that there is a degeneracy between
the amplitude of correlated isocurvature perturbations and the spectral tilt. A
negative (red) tilt is found to be compatible with a larger isocurvature
contribution. Estimates of the baryon and CDM densities are found to be almost
independent of the isocurvature amplitude. The main result is that current
microwave background data do not exclude a dominant contribution from CDM
isocurvature fluctuations on large scales.Comment: 5 pages, revtex, 3 figures. V3 - DASI data added and reionization
taken into account. New figure 2. Matches version to appear in PR
Implications for the Constrained MSSM from a new prediction for b to s gamma
We re-examine the properties of the Constrained MSSM in light of updated
constraints, paying particular attention to the impact of the recent
substantial shift in the Standard Model prediction for BR(B to X_s gamma). With
the help of a Markov Chain Monte Carlo scanning technique, we vary all relevant
parameters simultaneously and derive Bayesian posterior probability maps. We
find that the case of \mu>0 remains favored, and that for \mu<0 it is
considerably more difficult to find a good global fit to current constraints.
In both cases we find a strong preference for a focus point region. This leads
to improved prospects for detecting neutralino dark matter in direct searches,
while superpartner searches at the LHC become more problematic, especially when
\mu<0. In contrast, prospects for exploring the whole mass range of the
lightest Higgs boson at the Tevatron and the LHC remain very good, which
should, along with dark matter searches, allow one to gain access to the
otherwise experimentally challenging focus point region. An alternative measure
of the mean quality-of-fit which we also employ implies that present data are
not yet constraining enough to draw more definite conclusions. We also comment
on the dependence of our results on the choice of priors and on some other
assumptions.Comment: JHEP versio
- …