arXiv subjects

Orlando Luongo

Publications and source records attributed to Orlando Luongo.

At least 163 records · Page 9Linked to original sources

arXiv2017

Model-independent reconstruction of $f(T)$ teleparallel cosmology

We propose a model-independent formalism to numerically solve the modified Friedmann equations in the framework of $f(T)$ teleparallel cosmology. Our strategy is to expand the Hubble parameter around the redshift $z=0$ up to a given order and to adopt cosmographic bounds as initial settings to determine the corresponding $f(z)\equiv f(T(H(z)))$ function. In this perspective, we distinguish two cases: the first expansion is up to the jerk parameter, the second expansion is up to the snap parameter. We show that inside the observed redshift domain $z\leq1$, only the net strength of $f(z)$ is modified passing from jerk to snap, whereas its functional behavior and shape turn out to be identical. As first step, we set the cosmographic parameters by means of the most recent observations. Afterwards, we calibrate our numerical solutions with the concordance $Λ$CDM model. In both cases, there is a good agreement with the cosmological standard model around $z\leq 1$, with severe discrepancies outer of this limit. We demonstrate that the effective dark energy term evolves following the test-function: $f(z)=\mathcal A+\mathcal Bz^2e^{\mathcal Cz}$. Bounds over the set $\left\{\mathcal A, \mathcal B, \mathcal C\right\}$ are also fixed by statistical considerations, comparing discrepancies between $f(z)$ with data. The approach opens the possibility to get a wide class of test-functions able to frame the dynamics of $f(T)$ without postulating any model \emph{a priori}. We thus re-obtain the $f(T)$ function through a back-scattering procedure once $f(z)$ is known. We figure out the properties of our $f(T)$ function at the level of background cosmology, to check the goodness of our numerical results. Finally, a comparison with previous cosmographic approaches is carried out giving results compatible with theoretical expectations.

gr-qc↗

arXiv2017

Information entropy and dark energy evolution

The information entropy is here investigated in the context of early and late cosmology under the hypothesis that distinct phases of universe evolution are entangled between them. The approach is based on the \emph{entangled state ansatz}, representing a coarse-grained definition of primordial \emph{dark temperature} associated to an \emph{effective entangled energy density}. The dark temperature definition comes from assuming either Von Neumann or linear entropy as sources of cosmological thermodynamics. We interpret the involved information entropies by means of probabilities of forming structures during cosmic evolution. Following this recipe, we propose that quantum entropy is simply associated to the thermodynamical entropy and we investigate the consequences of our approach using the adiabatic sound speed. As byproducts, we analyze two phases of universe evolution: the late and early stages. To do so, we first recover that dark energy reduces to a pure cosmological constant, as zero-order entanglement contribution, and second that inflation is well-described by means of an effective potential. In both cases, we infer numerical limits which are compatible with current observations.

gr-qc↗

arXiv2016

Model-independent limits and constraints on extended theories of gravity from cosmic reconstruction techniques

The onset of dark energy domination depends on the particular gravitational theory driving the cosmic evolution. Model independent techniques are crucial to test both the present $Λ$CDM cosmological paradigm and alternative theories, making the least possible number of assumptions about the Universe. In this paper we investigate whether cosmography is able to distinguish between different gravitational theories, by determining bounds on model parameters for three different extensions of General Relativity, i.e. $k-$essence, $F(T)$ and $f(R)$ theories. We expand each class of theories in powers of redshift $z$ around the present time, making no additional assumptions. This procedure is an extension of previous work and can be seen as the most general approach for testing extended theories of gravity with cosmography. In the case of $F(T)$ and $f(R)$ theories, we show that some assumptions on model parameters often made in previous works are superfluous or unjustified. We use data from the Union2.1 SN catalogue, BAO data and $H(z)$ differential age compilations, which probe cosmology on different scales. We perform a Monte Carlo analysis using a Metropolis-Hastings algorithm with a Gelman-Rubin convergence criterion, reporting 1-$σ$ and 2-$σ$ confidence levels. We perform two distinct fits, first taking only data at $z<1$ and then for all $z$. We obtain the corresponding parameter intervals, and find that the data is compatible the $Λ$CDM limit of all three theories at the 1-$σ$ level, while still compatible with quite a large portion of parameter space. We compare our results to the truncated $Λ$CDM paradigm, showing that the permitted regions of coefficients are significantly modified and in general widened compared to values reported in the existing literature. Finally, we test the extended theories through the Bayesian selection criteria AIC and BIC.

gr-qc↗

arXiv2016

Geometric approaches to the thermodynamics of black holes

In this summary, we present the main topics of the talks presented in the parallel session "Black holes - 5" of the 14th Marcel Grossmann Meeting held in Rome, Italy in July 2015. We first present a short review of the main approaches used to understand thermodynamics by using differential geometry. Then, we present a brief summary of each presentation, including some general remarks and comments.

gr-qc↗

arXiv2016

Dark Energy and Dark Matter from an additional adiabatic fluid

The Dark Sector is described by an additional barotropic fluid which evolves adiabatically during the universe's history and whose adiabatic exponent $γ$ is derived from the standard definitions of specific heats. Although in general $γ$ is a function of the redshift, the Hubble parameter and its derivatives, we find that our assumptions lead necessarily to solutions with $γ= $ constant in a FLRW universe. The adiabatic fluid acts effectively as the sum of two distinct components, one evolving like non-relativistic matter and the other depending on the value of the adiabatic index. This makes the model particularly interesting as a way of simultaneously explaining the nature of both Dark Energy and Dark Matter, at least at the level of the background cosmology. The $Λ$CDM model is included in this family of theories when $γ= 0$. We fit our model to SNIa, $H(z)$ and BAO data, discussing the model selection criteria. The implications for the early-universe and the growth of small perturbations in this model are also discussed.

gr-qc↗

arXiv2015

Cosmological degeneracy versus cosmography: a cosmographic dark energy model

In this work we use cosmography to alleviate the degeneracy among cosmological models, proposing a way to parameterize matter and dark energy in terms of cosmokinematics quantities. The recipe of using cosmography allows to expand observable quantities in Taylor series and to directly compare those expansions with data. We adopt this strategy and we propose a fully self-consistent parametrization of the total energy density driving the late time universe speed up. Afterwards, we describe a feasible \emph{cosmographic dark energy model}, in which matter is fixed whereas dark energy evolves by means of the cosmographic series. Our technique provides robust constraints on cosmokinematic parameters, permitting one to separately bound matter from dark energy densities. Our cosmographic dark energy model turns out to be one parameter only, but differently from the $Λ$CDM paradigm, it does not contain ansatz on the dark energy form. In addition, we even determine the free parameter of our model in suitable $1σ$ intervals through Monte Carlo analyses based on the Metropolis algorithm. We compare our results with the standard concordance model and we find that our treatment seems to indicate that dark energy slightly evolves in time, reducing to a pure cosmological constant only as $z\rightarrow0$.

gr-qc↗

arXiv2015

Cardy-Verlinde entropy in Hořava-Lifshitz gravity

We investigate homogeneous cosmological models with perfect-fluid sources in the framework of the Ho\v rava-Lifshitz model for quantum gravity. We show that the Hamiltonian constraint of such spacetimes can be rewritten as the Cardy formula for the entropy in conformal field theory. The Cardy entropy is shown to depend explicitly on the value of the Ho\v rava parameter $λ$ so that it can be interpreted as determining the entropy and the gravitational interaction of the theory. Moreover, we show that Verlinde's Pythagorean representation of the Hamiltonian constraint is also valid in the case of homogeneous Ho\v rava-Lifshitz spacetimes. We interpret these results as a further indication of a deep relationship between gravity, thermodynamics and holography in the quantum regime.

gr-qc↗

arXiv2015

On the theory and applications of modern cosmography

Cosmography represents an important branch of cosmology which aims to describe the universe without the need of postulating \emph{a priori} any particular cosmological model. All quantities of interest are expanded as a Taylor series around here and now, providing in principle, a way of directly matching with cosmological data. In this way, cosmography can be regarded a model-independent technique, able to fix cosmic bounds, although several issues limit its use in various model reconstructions. The main purpose of this review is to focus on the key features of cosmography, emphasising both the strategy for obtaining the observable cosmographic series and pointing out any drawbacks which might plague the standard cosmographic treatment. In doing so, we relate cosmography to the most relevant cosmological quantities and to several dark energy models. We also investigate whether cosmography is able to provide information about the form of the cosmological expansion history, discussing how to reproduce the dark fluid from the cosmographic sound speed. Following this, we discuss limits on cosmographic priors and focus on how to experimentally treat cosmographic expansions. Finally, we present some of the latest developments of the cosmographic method, reviewing the use of rational approximations, based on cosmographic Padé polynomials. Future prospects leading to more accurate cosmographic results, able to better reproduce the expansion history of the universe are also discussed in detail.

gr-qc↗

arXiv2015

Self-accelerated universe induced by repulsive effects as an alternative to dark energy and modified gravities

The existence of current-time universe's acceleration is usually modeled by means of two main strategies. The first makes use of a dark energy barotropic fluid entering \emph{by hand} the energy-momentum tensor of Einstein's theory. The second lies on extending the Hilbert-Einstein action giving rise to the class of extended theories of gravity. In this work, we propose a third approach, derived as an intrinsic geometrical effect of space-time, which provides repulsive regions under certain circumstances. We demonstrate that the effects of repulsive gravity naturally emerge in the field of a homogeneous and isotropic universe. To this end, we use an invariant definition of repulsive gravity based upon the behavior of the curvature eigenvalues. Moreover, we show that repulsive gravity counterbalances the standard gravitational attraction influencing both late and early times of the universe evolution. This phenomenon leads to the present speed up and to the fast expansion due to the inflationary epoch. In so doing, we are able to unify both dark energy and inflation in a single scheme, showing that the universe changes its dynamics when ${\ddot H\over H}=-2\dot H$, at the repulsion onset time where this condition is satisfied. Further, we argue that the spatial scalar curvature can be taken as vanishing because it does not affect at all the emergence of repulsive gravity. We check the goodness of our approach through two cosmological fits involving the most recent union 2.1 supernova compilation.

gr-qc↗

arXiv2015

Transition redshift in $f(T)$ cosmology and observational constraints

We extract constraints on the transition redshift $z_{tr}$, determining the onset of cosmic acceleration, predicted by an effective cosmographic construction, in the framework of $f(T)$ gravity. In particular, employing cosmography we obtain bounds on the viable $f(T)$ forms and their derivatives. Since this procedure is model independent, as long as the scalar curvature is fixed, we are able to determine intervals for $z_{tr}$. In this way we guarantee that the Solar-System constraints are preserved and moreover we extract bounds on the transition time and the free parameters of the scenario. We find that the transition redshifts predicted by $f(T)$ cosmology, although compatible with the standard $Λ$CDM predictions, are slightly smaller. Finally, in order to obtain observational constraints on $f(T)$ cosmology, we perform a Monte Carlo fitting using supernova data, involving the most recent union 2.1 data set.

gr-qc↗

arXiv2014

Emerging the dark sector from thermodynamics of cosmological systems with constant pressure

We investigate the thermodynamics of general fluids that have the constriction that their pressure is constant. For example, this happens in the case of pure dust matter, for which the pressure vanishes and also in the case of standard dark matter phenomenology. Assuming a finite non-zero pressure, the corresponding dynamics is richer than one naively would expect. In particular, it can be considered as a unified description of dark energy and dark matter. We first consider the more general thermodynamic properties of this class of fluids finding the important result that for them adiabatic and isothermal processes should coincide. We therefore study their behaviors in curved space-times where local thermal equilibrium can be appealed. Thus, we show that this dark fluid degenerates with the dark sector of the LCDM model only in the case of adiabatic evolution. We demonstrate that, adding dissipative processes, a phantom behavior can occur and finally we further highlight that an arbitrary decomposition of the dark sector, into ad hoc dark matter and dark energy terms, may give rise to phantom dark energy, whereas the whole dark sector remains non-phantom.

gr-qc↗

arXiv2014

Cosmographic transition redshift in $f(R)$ gravity

We propose a strategy to infer the transition redshift $z_{da}$, which characterizes the passage through the universe decelerated to accelerated phases, in the framework $f(R)$ gravities. To this end, we numerically reconstruct $f(z)$, i.e. the corresponding $f(R)$ function re-expressed in terms of the redshift $z$ and we show how to match $f(z)$ with cosmography. In particular, we relate $f(z)$ and its derivatives to the cosmographic coefficients, i.e. $H_0, q_0$ and $j_0$ and demonstrate that its corresponding evolution may be framed by means of an effective logarithmic dark energy term $Ω_X$, slightly departing from the case of a pure cosmological constant. Afterwards, we show that our model predicts viable transition redshift constraints, which agree with $Λ$CDM. To do so, we compute the corresponding $z_{da}$ in terms of cosmographic outcomes and find that $z_{da}\leq1$. Finally, we reproduce an effective $f(z)$ and show that this class of models is fairly well compatible with present-time data. To do so, we get numerical constraints employing Monte Carlo fits with the Union 2.1 supernova survey and with the Hubble measurement data set.

astro-ph.CO↗

arXiv2014

Connecting early and late universe by $f(R)$ gravity

Inflation and dark energy are two of the most relevant aspects of modern cosmology. These different epochs provide the universe is passing through accelerated phases soon after the Big-Bang and at present stage of its evolution. In this review paper, we discuss that both eras can be, in principle, described by a geometric picture, under the standard of $f(R)$ gravity. We give the fundamental physics motivations and outline the main ingredients of $f(R)$ inflation, quintessence and cosmography. This wants to be a quick summary of $f(R)$ paradigm without claiming of completeness.

gr-qc↗

arXiv2014

Precision cosmology with Padé rational approximations: theoretical predictions versus observational limits

We propose a novel approach for parameterizing the luminosity distance, based on the use of rational "Padé" approximations. This new technique extends standard Taylor treatments, overcoming possible convergence issues at high redshifts plaguing standard cosmography. Indeed, we show that Padé expansions enable us to confidently use data over a larger interval with respect to the usual Taylor series. To show this property in detail, we propose several Padé expansions and we compare these approximations with cosmic data, thus obtaining cosmographic bounds from the observable universe for all cases. In particular, we fit Padé luminosity distances with observational data from different uncorrelated surveys. We employ union 2.1 supernova data, baryonic acoustic oscillation, Hubble space telescope measurements and differential age data. In so doing, we also demonstrate that the use of Padé approximants can improve the analyses carried out by introducing cosmographic auxiliary variables, i.e. a standard technique usually employed in cosmography in order to overcome the divergence problem. Moreover, for any drawback related to standard cosmography, we emphasize possible resolutions in the framework of Padé approximants. In particular, we investigate how to reduce systematics, how to overcome the degeneracy between cosmological coefficients, how to treat divergences and so forth. As a result, we show that cosmic bounds are actually refined through the use of Padé treatments and the thus derived best values of the cosmographic parameters show slight departures from the standard cosmological paradigm. Although all our results are perfectly consistent with the $Λ$CDM model, evolving dark energy components different from a pure cosmological constant are not definitively ruled out.

gr-qc↗

arXiv2014

Characterizing repulsive gravity with curvature eigenvalues

Repulsive gravity has been investigated in several scenarios near compact objects by using different intuitive approaches. Here, we propose an invariant method to characterize regions of repulsive gravity, associated to black holes and naked singularities. Our method is based upon the behavior of the curvature tensor eigenvalues, and leads to an invariant definition of a \emph{repulsion radius}. The repulsion radius determines a physical region, which can be interpreted as a repulsion sphere, where the effects due to repulsive gravity naturally arise. Further, we show that the use of effective masses to characterize repulsive regions can lead to coordinate-dependent results whereas, in our approach, repulsion emerges as a consequence of the spacetime geometry in a completely invariant way. Our definition is tested in the spacetime of an electrically charged Kerr naked singularity and in all its limiting cases. We show that a positive mass can generate repulsive gravity if it is equipped with an electric charge or an angular momentum. We obtain reasonable results for the spacetime regions contained inside the repulsion sphere whose size and shape depend on the value of the mass, charge and angular momentum. Consequently, we define repulsive gravity as a classical relativistic effect by using the geometry of spacetime only.

gr-qc↗

arXiv2014

A Newtonian approach to the cosmological dark fluids

We review the hydrodynamics of the dark sector components in Cosmology. For this purpose we use the approach of Newtonian gravitational instability, and thereafter we add corrections to arrive to a full relativistic description. In Cosmology and Astrophysics, it is usual to decompose the dark sector into two species, dark matter and dark energy. We will use instead a unified approach by describing a single unified dark fluid with very simple assumptions, namely the dark fluid is barotropic and its sound speed vanishes.

physics.gen-ph↗

arXiv2014

Curvature dark energy reconstruction through different cosmographic distance definitions

In the context of $f(\mathcal{R})$ gravity, dark energy is a geometrical fluid with negative equation of state. Since the function $f(\mathcal{R})$ is not known \emph{a priori}, the need of a model independent reconstruction of its shape represents a relevant technique to determine which $f(\mathcal{R})$ model is really favored with respect to others. To this aim, we relate cosmography to a generic $f(\mathcal R)$ and its derivatives in order to provide a model independent investigation at redshift $z \sim 0$. Our analysis is based on the use of three different cosmological distance definitions, in order to alleviate the duality problem, i.e. the problem of which cosmological distance to use with specific cosmic data sets. We therefore consider the luminosity, $d_L$, flux, $d_F$, and angular, $d_A$, distances and we find numerical constraints by the Union 2.1 supernovae compilation and measurement of baryonic acoustic oscillations, at $z_{BAO}=0.35$. We notice that all distances reduce to the same expression, i.e. $d_{L;F;A}\sim\frac{1}{\mathcal H_0}z$, at first order. Thus, to fix the cosmographic series of observables, we impose the initial value of $H_0$ by fitting $\mathcal H_0$ through supernovae only, in the redshift regime $z<0.4$. We find that the pressure of curvature dark energy fluid is slightly lower than the one related to the cosmological constant. This indicates that a possible evolving curvature dark energy realistically fills the current universe. Moreover, the combined use of $d_L,d_F$ and $d_A$ shows that the sign of the acceleration parameter agrees with theoretical bounds, while its variation, namely the jerk parameter, is compatible with $j_0>1$. Finally, we infer the functional form of $f(\mathcal{R})$ by means of a truncated polynomial approximation, in terms of fourth order scale factor $a(t)$.

gr-qc↗

arXiv2014

Cosmographic bounds on the cosmological deceleration-acceleration transition redshift in $f(\mathcal{R})$ gravity

We examine the observational viability of a class of $f(\mathcal{R})$ gravity cosmological models. Particular attention is devoted to constraints from the recent observational determination of the redshift of the cosmological deceleration-acceleration transition. Making use of the fact that the Ricci scalar is a function of redshift $z$ in these models, $\mathcal {R=R}(z)$, and so is $f(z)$, we use cosmography to relate a $f(z)$ test function evaluated at higher $z$ to late-time cosmographic bounds. First, we consider a model independent procedure to build up a numerical $f(z)$ by requiring that at $z=0$ the corresponding cosmological model reduces to standard $Λ$CDM. We then infer late-time observational constraints on $f(z)$ in terms of bounds on the Taylor expansion cosmographic coefficients. In doing so we parameterize possible departures from the standard $Λ$CDM model in terms of a two-parameter logarithmic correction. The physical meaning of the two parameters is also discussed in terms of the post Newtonian approximation. Second, we provide numerical estimates of the cosmographic series terms by using Type Ia supernova apparent magnitude data and Hubble parameter measurements. Finally, we use these estimates to bound the two parameters of the logarithmic correction. We find that the deceleration parameter in our model changes sign at a redshift consistent with what is observed.

gr-qc↗