426 research outputs found
Fingerprinting dark energy
Dark energy perturbations are normally either neglected or else included in a
purely numerical way, obscuring their dependence on underlying parameters like
the equation of state or the sound speed. However, while many different
explanations for the dark energy can have the same equation of state, they
usually differ in their perturbations so that these provide a fingerprint for
distinguishing between different models with the same equation of state. In
this paper we derive simple yet accurate approximations that are able to
characterize a specific class of models (encompassing most scalar-field models)
which is often generically called "dark energy". We then use the approximate
solutions to look at the impact of the dark energy perturbations on the dark
matter power spectrum and on the integrated Sachs-Wolfe effect in the cosmic
microwave background radiation.Comment: 11 pages, 5 figures, minor changes to match published versio
Parametrization for the Scale Dependent Growth in Modified Gravity
We propose a scale dependent analytic approximation to the exact linear
growth of density perturbations in Scalar-Tensor (ST) cosmologies. In
particular, we show that on large subhorizon scales, in the Newtonian gauge,
the usual scale independent subhorizon growth equation does not describe the
growth of perturbations accurately, as a result of scale-dependent relativistic
corrections to the Poisson equation. A comparison with exact linear numerical
analysis indicates that our approximation is a significant improvement over the
standard subhorizon scale independent result on large subhorizon scales. A
comparison with the corresponding results in the Synchronous gauge demonstrates
the validity and consistency of our analysis.Comment: 10 pages, 5 figures. Minor modifications and references added to
match published versio
A parametrization for the growth index of linear matter perturbations
We propose a parametrization for the growth index of the linear matter
perturbations, . The growth factor of
the perturbations parameterized as is analyzed for both the
CDM model and the DGP model with our proposed form for . We find
that is negative for the CDM model but is positive for the DGP
model. Thus it provides another signature to discriminate them. We demonstrate
that with taking our proposed form approximates
the growth factor very well both at low and high redshfits for both kinds of
models. In fact, the error is below 0.03% for the CDM model and 0.18%
for the DGP model for all redshifts when . Therefore, our
parametrization may be robustly used to constrain the growth index of different
models with the observational data which include points for redshifts ranging
from 0.15 to 3.8, thus providing discriminative signatures for different
models.Comment: 14 pages, 6 figures; Added reference
Measuring the dark side (with weak lensing)
We introduce a convenient parametrization of dark energy models that is
general enough to include several modified gravity models and generalized forms
of dark energy. In particular we take into account the linear perturbation
growth factor, the anisotropic stress and the modified Poisson equation. We
discuss the sensitivity of large scale weak lensing surveys like the proposed
DUNE satellite to these parameters. We find that a large-scale weak-lensing
tomographic survey is able to easily distinguish the Dvali-Gabadadze-Porrati
model from LCDM and to determine the perturbation growth index to an absolute
error of 0.02-0.03.Comment: 19 pages, 11 figure
Consistency of LCDM with Geometric and Dynamical Probes
The LCDM cosmological model assumes the existence of a small cosmological
constant in order to explain the observed accelerating cosmic expansion.
Despite the dramatic improvement of the quality of cosmological data during the
last decade it remains the simplest model that fits remarkably well (almost)
all cosmological observations. In this talk I review the increasingly
successful fits provided by LCDM on recent geometric probe data of the cosmic
expansion. I also briefly discuss some emerging shortcomings of the model in
attempting to fit specific classes of data (eg cosmic velocity dipole flows and
cluster halo profiles). Finally, I summarize recent results on the
theoretically predicted matter overdensity () evolution (a dynamical probe of the cosmic expansion),
emphasizing its scale and gauge dependence on large cosmological scales in the
context of general relativity. A new scale dependent parametrization which
describes accurately the growth rate of perturbations even on scales larger
than 100h^{-1}Mpc is shown to be a straightforward generalization of the well
known scale independent parametrization f(a)=\omms(a)^\gamma valid on smaller
cosmological scales.Comment: 20 pages, 6 figures. Invited review at the 1st Mediterranean
Conference on Classical and Quantum Gravity (MCCQG). To appear in the
proceeding
Cosmology with Interaction between Phantom Dark Energy and Dark Matter and the Coincidence Problem
We study a cosmological model in which phantom dark energy is coupled to dark
matter by phenomenologically introducing a coupled term to the equations of
motion of dark energy and dark matter. This term is parameterized by a
dimensionless coupling function , Hubble parameter and the energy
density of dark matter, and it describes an energy flow between the dark energy
and dark matter. We discuss two cases: one is the case where the
equation-of-state of the dark energy is a constant; the other is
that the dimensionless coupling function is a constant. We investigate
the effect of the interaction on the evolution of the universe, the total
lifetime of the universe, and the ratio of the period when the universe is in
the coincidence state to its total lifetime. It turns out that the interaction
will produce significant deviation from the case without the interaction.Comment: Latex, 17 pages including 14 figures, minor change
Crossing the Phantom Divide: Theoretical Implications and Observational Status
If the dark energy equation of state parameter w(z) crosses the phantom
divide line w=-1 (or equivalently if the expression d(H^2(z))/dz - 3\Omega_m
H_0^2 (1+z)^2 changes sign) at recent redshifts, then there are two possible
cosmological implications: Either the dark energy consists of multiple
components with at least one non-canonical phantom component or general
relativity needs to be extended to a more general theory on cosmological
scales. The former possibility requires the existence of a phantom component
which has been shown to suffer from serious theoretical problems and
instabilities. Therefore, the later possibility is the simplest realistic
theoretical framework in which such a crossing can be realized. After providing
a pedagogical description of various dark energy observational probes, we use a
set of such probes (including the Gold SnIa sample, the first year SNLS
dataset, the 3-year WMAP CMB shift parameter, the SDSS baryon acoustic
oscillations peak (BAO), the X-ray gas mass fraction in clusters and the linear
growth rate of perturbations at z=0.15 as obtained from the 2dF galaxy redshift
survey) to investigate the priors required for cosmological observations to
favor crossing of the phantom divide. We find that a low \Omega_m prior
(0.2<\Omega_m <0.25) leads, for most observational probes (except of the SNLS
data), to an increased probability (mild trend) for phantom divide crossing. An
interesting degeneracy of the ISW effect in the CMB perturbation spectrum is
also pointed out.Comment: Accepted in JCAP (to appear). Comments added, typos corrected. 19
pages (revtex), 8 figures. The numerical analysis files (Mathematica +
Fortran) with instructions are available at
http://leandros.physics.uoi.gr/pdl-cross/pdl-cross.htm . The ppt file of a
relevant talk may be downloaded from
http://leandros.physics.uoi.gr/pdl-cross/pdl2006.pp
Comparison of Standard Ruler and Standard Candle constraints on Dark Energy Models
We compare the dark energy model constraints obtained by using recent
standard ruler data (Baryon Acoustic Oscillations (BAO) at z=0.2 and z=0.35 and
Cosmic Microwave Background (CMB) shift parameters R and l_a) with the
corresponding constraints obtained by using recent Type Ia Supernovae (SnIa)
standard candle data (ESSENCE+SNLS+HST from Davis et. al.). We find that, even
though both classes of data are consistent with LCDM at the 2\sigma level,
there is a systematic difference between the two classes of data. In
particular, we find that for practically all values of the parameters
(\Omega_0m,\Omega_b) in the 2\sigma range of the the 3-year WMAP data (WMAP3)
best fit, LCDM is significantly more consistent with the SnIa data than with
the CMB+BAO data. For example for (\Omega_0m,\Omega_b)=(0.24,0.042)
corresponding to the best fit values of WMAP3, the dark energy equation of
state parametrization w(z)=w_0 + w_1 (z/(1+z)) best fit is at a 0.5\sigma
distance from LCDM (w_0=-1,w_1=0) using the SnIa data and 1.7\sigma away from
LCDM using the CMB+BAO data. There is a similar trend in the earlier data (SNLS
vs CMB+BAO at z=0.35). This trend is such that the standard ruler CMB+BAO data
show a mild preference for crossing of the phantom divide line w=-1, while the
recent SnIa data favor LCDM. Despite of this mild difference in trends, we find
no statistically significant evidence for violation of the cosmic distance
duality relation \eta \equiv d_L(z)/(d_A(z) (1+z)^2)=1. For example, using a
prior of \Omega_0m=0.24, we find \eta=0.95 \pm 0.025 in the redshift range
0<z<2, which is consistent with distance duality at the 2\sigma level.Comment: References added. 9 pages, 7 figures. The Mathematica files with the
numerical analysis of the paper can be found at
http://leandros.physics.uoi.gr/rulcand/rulcand.ht
Non-Celiac Gluten Sensitivity: The New Frontier of Gluten Related Disorders
Non Celiac Gluten sensitivity (NCGS) was originally described in the 1980s and recently a “re-discovered” disorder characterized by intestinal and extra-intestinal symptoms related to the ingestion of gluten-containing food, in subjects that are not affected with either celiac disease (CD) or wheat allergy (WA). Although NCGS frequency is still unclear, epidemiological data have been generated that can help establishing the magnitude of the problem. Clinical studies further defined the identity of NCGS and its implications in human disease. An overlap between the irritable bowel syndrome (IBS) and NCGS has been detected, requiring even more stringent diagnostic criteria. Several studies suggested a relationship between NCGS and neuropsychiatric disorders, particularly autism and schizophrenia. The first case reports of NCGS in children have been described. Lack of biomarkers is still a major limitation of clinical studies, making it difficult to differentiate NCGS from other gluten related disorders. Recent studies raised the possibility that, beside gluten, wheat amylase-trypsin inhibitors and low-fermentable, poorly-absorbed, short-chain carbohydrates can contribute to symptoms (at least those related to IBS) experienced by NCGS patients. In this paper we report the major advances and current trends on NCG
Euclid:Validation of the MontePython forecasting tools
The Euclid mission of the European Space Agency will perform a survey of weak lensing cosmic shear and galaxy clustering in order to constrain cosmological models and fundamental physics. We expand and adjust the mock Euclid likelihoods of the MontePython software in order to match the exact recipes used in previous Euclid Fisher matrix forecasts for several probes: weak lensing cosmic shear, photometric galaxy clustering, the cross-correlation between the latter observables, and spectroscopic galaxy clustering. We also establish which precision settings are required when running the Einstein-Boltzmann solvers CLASS and CAMB in the context of Euclid. For the minimal cosmological model, extended to include dynamical dark energy, we perform Fisher matrix forecasts based directly on a numerical evaluation of second derivatives of the likelihood with respect to model parameters. We compare our results with those of other forecasting methods and tools. We show that such MontePython forecasts agree very well with previous Fisher forecasts published by the Euclid Collaboration, and also, with new forecasts produced by the CosmicFish code, now interfaced directly with the two Einstein-Boltzmann solvers CAMB and CLASS. Moreover, to establish the validity of the Gaussian approximation, we show that the Fisher matrix marginal error contours coincide with the credible regions obtained when running Monte Carlo Markov Chains with MontePython while using the exact same mock likelihoods. The new Euclid forecast pipelines presented here are ready for use with additional cosmological parameters, in order to explore extended cosmological models
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