Domenico Sapone

Measuring the dark side (with weak lensing)

We introduce a convenient parameterization 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 (assuming systematic errors can be controlled). We find that a large-scale weak lensing tomographic survey is able to easily distinguish the Dvali–Gabadadze–Porrati model from ΛCDM and to determine the perturbation growth index to an absolute error of 0.02–0.04.

A parametrization of the growth index of matter perturbations in various Dark Energy models and observational prospects using a Euclid-like survey

We provide exact solutions to the cosmological matter perturbation equation in a homogeneous FLRW universe with a vacuum energy that can be parametrized by a constant equation of state parameter w and a very accurate approximation for the Ansatz w(a) = w0+wa(1−a). We compute the growth index γ = log f(a)/log Ωm(a), and its redshift dependence, using the exact and approximate solutions in terms of Legendre polynomials and show that it can be parametrized as γ(a) = γ0+γa(1−a) in most cases. We then compare four different types of dark energy (DE) models: wΛCDM, DGP, f(R) and a LTB-large-void model, which have very different behaviors at z1. This allows us to study the possibility to differentiate between different DE alternatives using wide and deep surveys like Euclid, which will measure both photometric and spectroscopic redshifts for several hundreds of millions of galaxies up to redshift z 2. We do a Fisher matrix analysis for the prospects of differentiating among the different DE models in terms of the growth index, taken as a given function of redshift or with a principal component analysis, with a value for each redshift bin for a Euclid-like survey. We use as observables the complete and marginalized power spectrum of galaxies P(k) and the Weak Lensing (WL) power spectrum. We find that, using P(k), one can reach (2%, 5%) errors in (w0,wa), and (4%, 12%) errors in (γ0,γa), while using WL we get errors at least twice as large. These estimates allow us to differentiate easily between DGP, f(R) models and ΛCDM, while it would be more difficult to distinguish the latter from a variable equation of state parameter or LTB models using only the growth index.

DARK ENERGY IN PRACTICE

In this paper we review a part of the approaches that have been considered to explain the extraordinary discovery of the late time acceleration of the Universe. We discuss the arguments that have led physicists and astronomers to accept dark energy as the current preferable candidate to explain the acceleration. We highlight the problems and the attempts to overcome the difficulties related to such a component. We also consider alternative theories capable of explaining the acceleration of the Universe, such as modification of gravity. We compare the two approaches and point out the observational consequences, reaching the sad but foresightful conclusion that we will not be able to distinguish between a Universe filled by dark energy or a Universe where gravity is different from General Relativity. We review the present observations and discuss the future experiments that will help us to learn more about our Universe. This is not intended to be a complete list of all the dark energy models but this paper should be seen as a review on the phenomena responsible for the acceleration. Moreover, in a landscape of hardly compelling theories, it is an important task to build simple measurable parameters useful for future experiments that will help us to understand more about the evolution of the Universe.

Constraints on early dark energy from CMB lensing and weak lensing tomography

Dark energy can be studied by its influence on the expansion of the Universe as well as on the growth history of the large-scale structure. In this paper, we follow the growth of the cosmic density field in early dark energy cosmologies by combining observations of the primary CMB temperature and polarisation power spectra at high redshift, of the CMB lensing deflection field at intermediate redshift and of weak cosmic shear at low redshifts for constraining the allowed amount of early dark energy. We present these forecasts using the Fisher matrix formalism and consider the combination of Planck data with the weak lensing survey of Euclid. We find that combining these data sets gives powerful constraints on early dark energy and is able to break degeneracies in the parameter set inherent to the various observational channels. The derived statistical 1σ-bound on the early dark energy density parameter is σ(Ωed) = 0.0022 which suggests that early dark energy models can be well examined in our approach. In addition, we derive the dark energy figure of merit for the considered dark energy parameterisation and comment on the applicability of the growth index to early dark energy cosmologies.

Fingerprinting dark energy. III. Distinctive marks of viscosity

D. S. acknowledges support from the JAEDoc program with ref. JAEDoc074 and the Spanish MICINN under the project AYA2009-13936-C06-06. D. S. also acknowledges financial support from the Madrid Regional Government (CAM) under the program HEPHACOS P-ESP-00346, Consolider-Ingenio 2010 PAU (CSD2007-00060), as well as in the European Union Marie Curie Network ‘‘UniverseNet’’ under Contract No.MRTNCT- 2006-035863. E.M. was supported by the Spanish MICINNs Juan de la Cierva program (JCI-2010-08112), by CICYT through the project FPA-2009 09017, by the Community of Madrid through the project HEPHACOS (S2009/ESP-1473) under Grant No. P-ESP-00346, and by the European Union ITN project (FP7-PEOPLE-2011-ITN, PITN-GA-2011-289442-INVISIBLES)

Novel null-test for the Λ cold dark matter model with growth-rate data

Current and upcoming surveys will measure the cosmological parameters with an extremely high accuracy. The primary goal of these observations is to eliminate some of the currently viable cosmological models created to explain the late-time accelerated expansion (either real or only inferred). However, most of the statistical tests used in cosmology have a strong requirement: the use of a model to fit the data. Recently there has been an increased interest on finding tests that are model independent, i.e. to have a function that depends entirely on observed quantities and not on the model, see for instance [C. Clarkson, B. Bassett and T. H. C. Lu, Phys. Rev. Lett.101 (2008) 011301, arXiv:0712.3457 [astro-ph]]. In this paper, we present an alternative consistency check at the perturbative level for a homogeneous and isotropic Universe filled with a dark energy component. This test makes use of the growth of matter perturbations data and it is able to detect a deviation from the standard cosmological model, which could later be attributed to a clustering dark energy component, a tension in the data or a modification of gravity, within the framework of a Friedmann–Lemaitre–Robertson–Walker (FLRW) universe.

Can dark energy viscosity be detected with the Euclid survey

D. S. acknowledges support from the JAEDoc program with Grant No. JAEDoc074 and the Spanish MICINN under Project No. AYA2009-13936-C06-06. D. S. also acknowledges financial support from the Madrid Regional Government (CAM) under the program HEPHACOS P-ESP-00346, Consolider-Ingenio 2010 PAU (CSD2007-00060), as well as the European Union Marie Curie Network ‘‘UniverseNet’’ under Contract No. MRTN-CT-2006-035863. E. M. was supported by the Spanish MICINNs Juan de la Cierva programme (Grant No. JCI-2010-08112), by CICYT through Project No. FPA-2009 09017, by the Community of Madrid through the project HEPHACOS (Grant No. S2009/ESP- 1473) under Grant No. P-ESP-00346 and by the European Union FP7 ITN INVISIBLES (Marie Curie Actions, PITNGA-2011-289442). M. K. acknowledges funding by the Swiss NSF

Accuracy of the growth index in the presence of dark energy perturbations

We present the analytical solutions for the evolution of matter density perturbations, for a model with a constant dark energy equation of state

Comparison of piecewise-constant methods for dark energy

w

Reconstruction of the null-test for the matter density perturbations

but when the effects of the dark energy perturbations are properly taken into account. We consider two cases, the first when the sound speed of the perturbations is zero