73 research outputs found
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
Anisotropies in the Cosmic Neutrino Background after WMAP 5-year Data
We search for the presence of cosmological neutrino background (CNB)
anisotropies in recent WMAP 5-year data using their signature imprinted on
modifications to cosmic microwave background (CMB) anisotropy power spectrum.
By parametrizing the neutrino background anisotropies with the speed viscosity
parameter c_\vis, we find that the WMAP 5-year data alone provide only a weak
indication for CNB anisotropies with c_\vis^2 > 0.06 at the 95% confidence
level. When we combine CMB anisotropy data with measurements of galaxy
clustering, SN-Ia Hubble diagram, and other cosmological information, the
detection increases to c_\vis^2 > 0.16 at the same 95% confidence level.
Future data from Planck, combined with a weak lensing survey such as the one
expected with DUNE from space, will be able to measure the CNB anisotropy
parameter at about 10% accuracy. We discuss the degeneracy between neutrino
background ansiotropies and other cosmological para meters such as the number
of effective neutrinos species and the dark energy equation of state.Comment: 7 Pages, 7 Figure
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
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
Planck 2015 results. XIV. Dark energy and modified gravity
We study the implications of Planck data for models of dark energy (DE) and modified gravity (MG), beyond the cosmological constant scenario. We start with cases where the DE only directly affects the background evolution, considering Taylor expansions of the equation of state, principal component analysis and parameterizations related to the potential of a minimally coupled DE scalar field. When estimating the density of DE at early times, we significantly improve present constraints. We then move to general parameterizations of the DE or MG perturbations that encompass both effective field theories and the phenomenology of gravitational potentials in MG models. Lastly, we test a range of specific models, such as k-essence, f(R) theories and coupled DE. In addition to the latest Planck data, for our main analyses we use baryonic acoustic oscillations, type-Ia supernovae and local measurements of the Hubble constant. We further show the impact of measurements of the cosmological perturbations, such as redshift-space distortions and weak gravitational lensing. These additional probes are important tools for testing MG models and for breaking degeneracies that are still present in the combination of Planck and background data sets. All results that include only background parameterizations are in agreement with LCDM. When testing models that also change perturbations (even when the background is fixed to LCDM), some tensions appear in a few scenarios: the maximum one found is \sim 2 sigma for Planck TT+lowP when parameterizing observables related to the gravitational potentials with a chosen time dependence; the tension increases to at most 3 sigma when external data sets are included. It however disappears when including CMB lensing
Euclid preparation: XXIV. Calibration of the halo mass function in (?)CDM cosmologies
Euclid s photometric galaxy cluster survey has the potential to be a very competitive cosmological probe. The main cosmological probe with observations of clusters is their number count, within which the halo mass function (HMF) is a key theoretical quantity. We present a new calibration of the analytic HMF, at the level of accuracy and precision required for the uncertainty in this quantity to be subdominant with respect to other sources of uncertainty in recovering cosmological parameters from Euclid cluster counts. Our model is calibrated against a suite of N-body simulations using a Bayesian approach taking into account systematic errors arising from numerical effects in the simulation. First, we test the convergence of HMF predictions from different N-body codes, by using initial conditions generated with different orders of Lagrangian Perturbation theory, and adopting different simulation box sizes and mass resolution. Then, we quantify the effect of using different halo finder algorithms, and how the resulting differences propagate to the cosmological constraints. In order to trace the violation of universality in the HMF, we also analyse simulations based on initial conditions characterised by scale-free power spectra with different spectral indexes, assuming both Einsteinde Sitter and standard CDM expansion histories. Based on these results, we construct a fitting function for the HMF that we demonstrate to be sub-percent accurate in reproducing results from 9 different variants of the CDM model including massive neutrinos cosmologies. The calibration systematic uncertainty is largely sub-dominant with respect to the expected precision of future massobservation relations; with the only notable exception of the effect due to the halo finder, that could lead to biased cosmological inference
Euclid: Modelling massive neutrinos in cosmology -- a code comparison
The measurement of the absolute neutrino mass scale from cosmological
large-scale clustering data is one of the key science goals of the Euclid
mission. Such a measurement relies on precise modelling of the impact of
neutrinos on structure formation, which can be studied with -body
simulations. Here we present the results from a major code comparison effort to
establish the maturity and reliability of numerical methods for treating
massive neutrinos. The comparison includes eleven full -body implementations
(not all of them independent), two -body schemes with approximate time
integration, and four additional codes that directly predict or emulate the
matter power spectrum. Using a common set of initial data we quantify the
relative agreement on the nonlinear power spectrum of cold dark matter and
baryons and, for the -body codes, also the relative agreement on the
bispectrum, halo mass function, and halo bias. We find that the different
numerical implementations produce fully consistent results. We can therefore be
confident that we can model the impact of massive neutrinos at the sub-percent
level in the most common summary statistics. We also provide a code validation
pipeline for future reference.Comment: 43 pages, 17 figures, 2 tables; published on behalf of the Euclid
Consortium; data available at https://doi.org/10.5281/zenodo.729797
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