arXiv subjects

Vincenzo F. Cardone

Publications and source records attributed to Vincenzo F. Cardone.

28 records · Page 2Linked to original sources

arXiv2012

An updated analysis of two classes of f(R) theories of gravity

The observed accelerated cosmic expansion can be a signature of fourth\,-\,order gravity theories, where the acceleration of the Universe is a consequence of departures from Einstein General Relativity, rather than the sign of the existence of a fluid with negative pressure. In the fourth\,-\,order gravity theories, the gravity Lagrangian is described by an analytic function $f(R)$ of the scalar curvature $R$ subject to the demanding conditions that no detectable deviations from standard GR is observed on the Solar System scale. Here we consider two classes of $f(R)$ theories able to pass Solar System tests and investigate their viability on cosmological scales. To this end, we fit the theories to a large dataset including the combined Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter $H(z)$ data from passively evolving red galaxies, Baryon Acoustic Oscillations extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS) and the distance priors from the Wilkinson Microwave Anisotropy Probe seven years (WMAP7) data. We find that both classes of $f(R)$ fit very well this large dataset with the present\,-\,day values of the matter density, Hubble constant and deceleration parameter in agreement with previous estimates; however, the strong degeneracy among the $f(R)$ parameters prevents us from strongly constraining their values. We also derive the growth factor $g = d\lnδ/d\ln{a}$, with $δ= δρ_M/ρ_M$ the matter density perturbation, and show that it can still be well approximated by $g(z) \propto Ω_M(z)^γ$. We finally constrain $γ$ (on some representative scales) and investigate its redshift dependence to see whether future data can discriminate between these classes of $f(R)$ theories and standard dark energy models.

astro-ph.CO↗

arXiv2011

Gamma-ray bursts as cosmological probes: LambdaCDM vs. conformal gravity

LambdaCDM, for the currently preferred cosmological density Omega_0 and cosmological constant Omega_Lambda, predicts that the Universe expansion decelerates from early times to redshift z~0.9 and accelerates at later times. On the contrary, the cosmological model based on conformal gravity predicts that the cosmic expansion has always been accelerating. To distinguish between these two very different cosmologies, we resort to gamma-ray bursts (GRBs), which have been suggested to probe the Universe expansion history at z>1, where identified type Ia supernovae (SNe) are rare. We use the full Bayesian approach to infer the cosmological parameters and the additional parameters required to describe the GRB data available in the literature. For the first time, we use GRBs as cosmological probes without any prior information from other data. In addition, when we combine the GRB samples with SNe, our approach neatly avoids all the inconsistencies of most numerous previous methods that are plagued by the so-called circularity problem. In fact, when analyzed properly, current data are consistent with distance moduli of GRBs and SNe that can respectively be, in a variant of conformal gravity, ~15 and ~3 magnitudes fainter than in LambdaCDM. Our results indicate that the currently available SN and GRB samples are accommodated equally well by both LambdaCDM and conformal gravity and do not exclude a continuous accelerated expansion. We conclude that GRBs are currently far from being effective cosmological probes, as they are unable to distinguish between these two very different expansion histories.

astro-ph.CO↗

arXiv2011

Tomography from the Next Generation of Cosmic Shear Experiments for Viable f(R) Models

We present the cosmic shear signal predicted by two viable cosmological models in the framework of modified-action f(R) theories. We use f(R) models where the current accelerated expansion of the Universe is a direct consequence of the modified gravitational Lagrangian rather than Dark Energy (DE), either in the form of vacuum energy/cosmological constant or of a dynamical scalar field (e.g. quintessence). We choose Starobinsky's (St) and Hu & Sawicki's (HS) f(R) models, which are carefully designed to pass the Solar System gravity tests. In order to further support - or rule out - f(R) theories as alternative candidates to the DE hypothesis, we exploit the power of weak gravitational lensing, specifically of cosmic shear. We calculate the tomographic shear matrix as it would be measured by the upcoming ESA Cosmic Vision Euclid satellite. We find that in the St model the cosmic shear signal is almost completely degenerate with LCDM, but it is easily distinguishable in the HS model. Moreover, we compute the corresponding Fisher matrix for both the St and HS models, thus obtaining forecasts for their cosmological parameters. Finally, we show that the Bayes factor for cosmic shear will definitely favour the HS model over LCDM if Euclid measures a value larger than ~0.02 for the extra HS parameter n_HS.

astro-ph.CO↗

arXiv2011

Testing a Phenomenologically Extended DGP Model with Upcoming Weak Lensing Surveys

A phenomenological extension of the well-known brane-world cosmology of Dvali, Gabadadze and Porrati (eDGP) has recently been proposed. In this model, a cosmological-constant-like term is explicitly present as a non-vanishing tension sigma on the brane, and an extra parameter alpha tunes the cross-over scale r_c, the scale at which higher dimensional gravity effects become non negligible. Since the Hubble parameter in this cosmology reproduces the same LCDM expansion history, we study how upcoming weak lensing surveys, such as Euclid and DES (Dark Energy Survey), can confirm or rule out this class of models. We perform Markov Chain Monte Carlo simulations to determine the parameters of the model, using Type Ia Supernovæ, H(z) data, Gamma Ray Bursts and Baryon Acoustic Oscillations. We also fit the power spectrum of the temperature anisotropies of the Cosmic Microwave Background to obtain the correct normalisation for the density perturbation power spectrum. Then, we compute the matter and the cosmic shear power spectra, both in the linear and non-linear regimes. The latter is calculated with the two different approaches of Hu and Sawicki (2007) (HS) and Khoury and Wyman (2009) (KW). With the eDGP parameters coming from the Markov Chains, KW reproduces the LCDM matter power spectrum at both linear and non-linear scales and the LCDM and eDGP shear signals are degenerate. This result does not hold with the HS prescription: Euclid can distinguish the eDGP model from LCDM because their expected power spectra roughly differ by the 3sigma uncertainty in the angular scale range 700<l<3000; on the contrary, the two models differ at most by the 1sigma uncertainty over the range 500<l<3000 in the DES experiment and they are virtually indistinguishable.

astro-ph.CO↗

arXiv2010

Cosmography of f(R) - brane cosmology

Cosmography is a useful tool to constrain cosmological models, in particular dark energy models. In the case of modified theories of gravity, where the equations of motion are generally quite complicated, cosmography can contribute to select realistic models without imposing arbitrary choices a priori. Indeed, its reliability is based on the assumptions that the universe is homogeneous and isotropic on large scale and luminosity distance can be "tracked" by the derivative series of the scale factor a(t). We apply this approach to induced gravity brane-world models where an f(R)-term is present in the brane effective action. The virtue of the model is to self-accelerate the normal and healthy DGP branch once the f(R)-term deviates from the Hilbert-Einstein action. We show that the model, coming from a fundamental theory, is consistent with the LCDM scenario at low redshift. We finally estimate the cosmographic parameters fitting the Union2 Type Ia Supernovae (SNeIa) dataset and the distance priors from Baryon Acoustic Oscillations (BAO) and then provide constraints on the present day values of f(R) and its second and third derivatives.

gr-qc↗

arXiv2009

Can MONDian vector theories explain the cosmic speed up ?

Generalized Einstein - Aether vector field models have been shown to provide, in the weak field regime, modifications to gravity which can be reconciled with the successfull MOND proposal. Very little is known, however, on the function F(K) defining the vector field Lagrangian so that an analysis of the viability of such theories at the cosmological scales has never been performed. As a first step along this route, we rely on the relation between F(K) and the MOND interpolating function $μ(a/a_0)$ to assign the vector field Lagrangian thus obtaining what we refer to as "MONDian vector models". Since they are able by construction to recover the MOND successes on galaxy scales, we investigate whether they can also drive the observed accelerated expansion by fitting the models to the Type Ia Supernovae data. Should be this the case, we have a unified framework where both dark energy and dark matter can be seen as different manifestations of a single vector field. It turns out that both MONDian vector models are able to well fit the low redshift data on Type Ia Supernovae, while some tension could be present in the high z regime.

astro-ph.CO↗

arXiv2009

Constraining f(R) theories with Type Ia Supernovae and Gamma Ray Bursts

Fourth - order gravity theories have received much interest in recent years thanks to their ability to provide an accelerated cosmic expansion in a matter only universe. In these theories, the Lagrangian density of the gravitational field has the form R + f(R), and the explicit choice of the arbitrary function f(R) must meet the local tests of gravity and the constraints from the primordial abundance of the light elements. Two popular classes of f(R) models, which are expected to fulfill all the above requirements, have recently been proposed. However, neither of these models has ever been quantitatively tested against the available astrophysical data. Here, by combining Type Ia Supernovae and Gamma Ray Bursts, we investigate the ability of these models to reproduce the observed Hubble diagram over the redshift range (0, 7). We find that both models fit very well this dataset with the present day values of the matter density and deceleration parameters which agree with previous estimates. However, the strong degeneracy among the f(R) parameters prevents us from putting strong constraints on the values of these parameters; nevertheless, we can identify the regions of the parameter space that should, in principle, be carefully explored with future data and dynamical probes in order to discriminate among f(R) theories and standard dark energy models.

astro-ph.CO↗

arXiv2008

Averaging procedure in variable-G cosmologies

Previous work in the literature had built a formalism for spatially averaged equations for the scale factor, giving rise to an averaged Raychaudhuri equation and averaged Hamiltonian constraint, which involve a backreaction source term. The present paper extends these equations to include models with variable Newton parameter and variable cosmological term, motivated by the nonperturbative renormalization program for quantum gravity based upon the Einstein-Hilbert action. We focus on the Brans-Dicke form of the renormalization-group improved action functional. The coupling between backreaction and spatially averaged three-dimensional scalar curvature is found to survive, and a variable-G cosmic quintet is found to emerge. Interestingly, under suitable assumptions, an approximate solution can be found where the early universe tends to a FLRW model, while keeping track of the original inhomogeneities through three effective fluids. The resulting qualitative picture is that of a universe consisting of baryons only, while inhomogeneities average out to give rise to the full dark-side phenomenology.

gr-qc↗

arXiv2008

Halting eternal acceleration with an effective negative cosmological constant

In order to solve the problem of eternal acceleration, a model has been recently proposed including both a negative cosmological constant $Λ$ and a scalar field evolving under the action of an exponential potential. We further explore this model by contrasting it against the Hubble diagram of Type Ia supernovae, the gas mass fraction in galaxy clusters and the acoustic peak and shift parameters. It turns out that the model is able to fit quite well this large dataset so that we conclude that a negative $Λ$ is indeed allowed and could represent a viable mechanism to halt eternal acceleration. In order to avoid problems with theoretical motivations for both a negative $Λ$ term and the scalar field, we reconstruct the gravity Lagrangian $f(R)$ of a fourth order theory of gravity predicting the same dynamics (scale factor and Hubble parameter) as the starting model. We thus end up with a $f(R)$ theory able to both fit the data and solve the problem of eternal acceleration without the need of unusual negative $Λ$ and ad hoc scalar fields.

astro-ph↗

arXiv2003

The lensing properties of the Sersic model

It is well known that the surface brightness of elliptical galaxies and of bulges of spiral galaxies is best fitted by the Sersic r^{1/n} profile. It is thus interesting to explore the lensing properties of the Sersic model because of its wide range of applicability. To this aim, we evaluate the lensing potential, the deflection angle, the time delay between the images and the a mplification for a circularly symmetric lens whose surface density is described by the Sersic profile. We estimate the same quantities also adding a shear term to the deflection potential in order to study (at the lowest order) the impact of deviations from radial symmetry or the contribution from other nearby lenses. Moreover, we investigate the systematic errors due to the use of a de Vaucouleurs profile instead of the correct Sersic model also taking into account the presence of a dark halo.

astro-ph↗