arXiv subjects

Orlando Luongo

Publications and source records attributed to Orlando Luongo.

At least 145 records · Page 8Linked to original sources

arXiv2019

Kinematic reconstructions of extended theories of gravity at small and intermediate redshifts

In the last few decades, extensions of General Relativity have reached always more attention especially in view of possible breakdowns of the standard $Λ$CDM paradigm at intermediate and high redshift regimes. If General Relativity would not be the ultimate theory of gravity, modifying Einstein's gravity in the homogeneous and isotropic universe may likely represent a viable path toward the description of current universe acceleration. We here focus our attention on two classes of extended theories, i.e. the $f(R)$ and $f(R,G)$-gravity. We parameterize the so-obtained Hubble function by means of effective barotropic fluids, by calibrating the shapes of our curves through some of the most suitable dark energy parameterizations, XCDM, CPL, WP. Afterwards, by virtue of the correspondence between the Ricci scalar and the Gauss-Bonnet topological invariant with the redshift $z$, we rewrite $f(R,G)$ in terms of corresponding $f(z)$ auxiliary functions. This scheme enables one to get numerical shapes for $f(R,G)$ and $f(R)$ models, through a coarse-grained inverse scattering procedure. Although our procedure agrees with the simplest extensions of general relativity, it leaves open the possibility that the most suitable forms of $f(R)$ and $f(R,G)$ are rational Padé polynomials of first orders. These approximations seem to be compatible with numerical reconstructions within intermediate redshift domains and match fairly well small redshift tests.

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arXiv2019

Entanglement in model independent cosmological scenario

We propose a \emph{model-independent} approach to study entanglement creation due to the dynamics between two asymptotic quantum regimes in the framework of homogeneous and isotropic universe. We realize it by Padé expansion to reconstruct the functional form of scale factor, rather than postulating it a priori.This amounts to fix the Padé approximants constraining the free parameters in terms of current cosmic observations. Assuming fermions, we solve the Dirac equation for massive particles and we investigate entanglement entropy in terms of modes $k$ and mass $m$. We consider two rational approximations of (1,1) and (1,2) orders, which turn out to be the most suitable choices for guaranteeing cosmic bounds. Our results show qualitative agreement with those known in literature and arising from toy models, but with sensible quantitative discrepancies. Moreover, our outcomes are model independent reconstructions which show that any higher orders departing from (1,1) do not significantly modify particle-antiparticle production (hence entanglement), if cosmic bounds are taken into account.

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arXiv2019

Addressing the circularity problem in the $E_\text{p}-E_\text{iso}$ correlation of Gamma-Ray Bursts

We here propose a new model-independent technique to overcome the circularity problem affecting the use of Gamma-Ray Bursts (GRBs) as distance indicators through the use of $E_{\rm p}$--$E_{\rm iso}$ correlation. We calibrate the $E_{\rm p}$--$E_{\rm iso}$ correlation and find the GRB distance moduli that can be used to constrain dark energy models. We use observational Hubble data to approximate the cosmic evolution through Bézier parametric curve obtained through the linear combination of Bernstein basis polynomials. In so doing, we build up a new data set consisting of 193 GRB distance moduli. We combine this sample with the supernova JLA data set to test the standard $Λ$CDM model and its $w$CDM extension. We place observational constraints on the cosmological parameters through Markov Chain Monte Carlo numerical technique. Moreover, we compare the theoretical scenarios by performing the AIC and DIC statistics. For the $Λ$CDM model we find $Ω_m=0.397^{+0.040}_{-0.039}$ at the $2σ$ level, while for the $w$CDM model we obtain $Ω_m=0.34^{+0.13}_{-0.15}$ and $w=-0.86^{+0.36}_{-0.38}$ at the $2σ$ level. Our analysis suggests that $Λ$CDM model is statistically favoured over the $w$CDM scenario. No evidence for extension of the $Λ$CDM model is found.

astro-ph.HE↗

arXiv2019

Extended Gravity Cosmography

Cosmography can be considered as a sort of a model-independent approach to tackle the dark energy/modified gravity problem. In this review, the success and the shortcomings of the $Λ$CDM model, based on General Relativity and standard model of particles, are discussed in view of the most recent observational constraints. The motivations for considering extensions and modifications of General Relativity are taken into account, with particular attention to $f(R)$ and $f(T)$ theories of gravity where dynamics is represented by curvature or torsion field respectively. The features of $f(R)$ models are explored in metric and Palatini formalisms. We discuss the connection between $f(R)$ gravity and scalar-tensor theories highlighting the role of conformal transformations in the Einstein and Jordan frames. Cosmological dynamics of $f(R)$ models is investigated through the corresponding viability criteria. Afterwards, the equivalent formulation of General Relativity (Teleparallel Equivalent General Relativity) in terms of torsion and its extension to $f(T)$ gravity is considered. Finally, the cosmographic method is adopted to break the degeneracy among dark energy models. A novel approach, built upon rational Padé and Chebyshev polynomials, is proposed to overcome limits of standard cosmography based on Taylor expansion. The approach provides accurate model-independent approximations of the Hubble flow. Numerical analyses, based on Monte Carlo Markov Chain integration of cosmic data, are presented to bound coefficients of the cosmographic series. These techniques are thus applied to reconstruct $f(R)$ and $f(T)$ functions and to frame the late-time expansion history of the universe with no \emph{a priori} assumptions on its equation of state. A comparison between the $Λ$CDM cosmological model with $f(R)$ and $f(T)$ models is reported.

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arXiv2019

Effective field description of the Anton-Schmidt cosmic fluid

The effective theory of the Anton-Schmidt cosmic fluid within the Debye approximation is investigated. In this picture, the universe is modeled out by means of a medium without cosmological constant. In particular, the Anton-Schmidt representation of matter describes the pressure of crystalline solids under deformations imposed by isotropic stresses. The approach scheme is related to the fact that the universe deforms under the action of the cosmic expansion itself. Thus, we frame the dark energy term as a function of scalar fields and obtain the corresponding dark energy potential $V(φ)$. Different epochs of the universe evolution are investigated in terms of the evolution of $φ$. We show how the Anton-Schmidt equation of state is capable of describing both late and early epochs of cosmic evolution. Finally, numerical bounds on the Anton-Schmidt model with $n=-1$ are derived through a Markov Chain Monte Carlo analysis on the combination of data coming from type Ia Supernovae, observations of Hubble parameter and baryon acoustic oscillations. Statistical comparison with the $Λ$CDM model is performed by the AIC and BIC selection criteria. Results are in excellent agreement with the low-redshift data. A further generalization of the model is presented to satisfy the theoretical predictions at early-stage cosmology.

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arXiv2019

Extended logotropic fluids as unified dark energy models

We here study extended classes of logotropic fluids as \textit{unified dark energy models}. Under the hypothesis of the Anton-Schmidt scenario, we consider the universe obeying a single fluid whose pressure evolves through a logarithmic equation of state. This result is in analogy with crystals under isotropic stresses. Thus, we investigate thermodynamic and dynamical consequences by integrating the speed of sound to obtain the pressure in terms of the density, leading to an extended version of the Anton-Schmidt cosmic fluids. Within this picture, we get significant outcomes expanding the Anton-Schmidt pressure in the infrared regime. The low-energy case becomes relevant for the universe to accelerate without any cosmological constant. We therefore derive the effective representation of our fluid in terms of a Lagrangian $\mathcal{L}=\mathcal{L}(X)$, depending on the kinetic term $X$ only. We analyze both the relativistic and non-relativistic limits. In the non-relativistic limit we construct both the Hamiltonian and Lagrangian in terms of density $ρ$ and scalar field $\vartheta$, whereas in the relativistic case no analytical expression for the Lagrangian can be found. Thus, we obtain the potential as a function of $ρ$, under the hypothesis of irrotational perfect fluid. We demonstrate that the model represents a natural generalization of \emph{logotropic dark energy models}. Finally, we analyze an extended class of generalized Chaplygin gas models with one extra parameter $β$. Interestingly, we find that the Lagrangians of this scenario and the pure logotropic one coincide in the non-relativistic regime.

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arXiv2018

Growth of matter perturbations in non-minimal teleparallel dark energy

We study the growth rate of matter perturbations in the context of teleparallel dark energy in a flat universe. We investigate the dynamics of different theoretical scenarios based on specific forms of the scalar field potential. Allowing for non-minimal coupling between torsion scalar and scalar field, we perform a phase-space analysis of the autonomous systems of equations through the study of critical points. We thus analyze the stability of the critical points, and discuss the cosmological implications searching for possible attractor solutions at late times. Furthermore, combing the growth rate data and the Hubble rate measurements, we place observational constraints on the cosmological parameters of the models through Monte Carlo numerical method. We find that the scenario with a non-minimal coupling is favoured with respect to the standard quintessence case. Adopting the best-fit results, we show that the dark energy equation of state parameter can cross the phantom divide. Finally, we compare our results with the predictions of the concordance $Λ$CDM paradigm by performing Bayesian model selection.

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arXiv2018

Speeding up the universe using dust with pressure

We revise the cosmological standard model presuming that matter, i.e. baryons and cold dark matter, exhibits a non-vanishing pressure mimicking the cosmological constant effects. In particular, we propose a scalar field Lagrangian $\mathcal L_1$ for matter with the introduction of a Lagrange multiplier as constraint. We also add a symmetry breaking effective potential accounting for the classical cosmological constant problem, by adding a second Lagrangian $\mathcal{L}_2$. Investigating the Noether current due to the shift symmetry on the scalar field, $φ\rightarrowφ+c^0$, we show that $\mathcal{L}_1$ turns out to be independent from the scalar field $φ$. Further we find that a positive Helmotz free-energy naturally leads to a negative pressure without introducing by hand any dark energy term. To face out the fine-tuning problem, we investigate two phases: before and after transition due to the symmetry breaking. We propose that during transition dark matter cancels out the quantum field vacuum energy effects. This process leads to a negative and constant pressure whose magnitude is determined by baryons only. The numerical bounds over the pressure and matter densities are in agreement with current observations, alleviating the coincidence problem. Finally assuming a thermal equilibrium between the bath and our effective fluid, we estimate the mass of the dark matter candidate. Our numerical outcomes seem to be compatible with recent predictions on WIMP masses, for fixed spin and temperature. In particular, we predict possible candidates whose masses span in the range $0.5-1.7$ TeV.

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arXiv2018

Kinematic and statistical inconsistencies of Ho$\check{\textrm{r}}$ava-Lifshitz cosmology

We investigate the validity of a \emph{minimal} cosmological model derived from the renormalizable Ho$\check{\textrm{r}}$ava action at low redshift scales by using different cosmological and statistical tests. Assuming pure attractive gravity, i.e., $λ>1/3$ in the Ho$\check{\textrm{r}}$ava action, we compare the Union 2.1 supernova type Ia data with the kinematics following from a model-independent approach. The two approaches, although compatible, lead to explicit cosmographic constraints on the free parameters of the Ho$\check{\textrm{r}}$ava action, which turn out to be in strong disagreement with the $Λ$CDM, $w$CDM and Chevallier-Polarski-Linder scenarios. To show this, we use standard diagnostic tools of regression models, namely the Akaike and the Bayesian Information Criteria. Using such model-independent statistical methods, we show that Ho$\check{\textrm{r}}$ava-Lifshitz cosmology differs from the standard dark energy scenarios, \emph{independently} of the number of free parameters involved in the model. Since this result is valid at small redshift domains, it indicates the presence of inconsistencies in the minimal version of Ho$\check{\textrm{r}}$ava-Lifshitz cosmology even at the level of background cosmology.

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arXiv2018

Kinematic model-independent reconstruction of Palatini $f(R)$ cosmology

A kinematic treatment to trace out the form of $f(R)$ cosmology, within the Palatini formalism, is discussed by only postulating the universe homogeneity and isotropy. To figure this out we build model-independent approximations of the luminosity distance through rational expansions. These approximants extend the Taylor convergence radii computed for usual cosmographic series. We thus consider both Padé and the rational Chebyshev polynomials. They can be used to accurately describe the universe late-time expansion history, providing further information on the thermal properties of all effective cosmic fluids entering the energy momentum tensor of Palatini's gravity. To perform our numerical analysis, we relate the Palatini's Ricci scalar with the Hubble parameter $H$ and thus we write down a single differential equation in terms of the redshift $z$. Therefore, to bound $f(R)$, we make use of the most recent outcomes over the cosmographic parameters obtained from combined data surveys. In particular our clue is to select two scenarios, i.e. $(2,2)$ Padé and $(2,1)$ Chebyshev approximations, since they well approximate the luminosity distance at the lowest possible order. We find that best analytical matches to the numerical solutions lead to $f(R)=a+bR^n$ with free parameters given by the set $(a, b, n)=(-1.627, 0.866, 1.074)$ for $(2,2)$ Padé approximation, whereas $f(R)=α+βR^m$ with $(α, β, m)=(-1.332, 0.749, 1.124)$ for $(2,1)$ rational Chebyshev approximation. Finally, our results are compared with the $Λ$CDM predictions and with previous studies in the literature. Slight departures from General Relativity are also discussed.

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arXiv2018

Maximum turnaround radius in $f(R)$ gravity

The accelerating behavior of cosmic fluid opposes to the gravitational attraction, at present epoch, whereas standard gravity is dominant at small scales. As a consequence, there exists a \emph{point} where the effects are counterbalanced, dubbed \emph{turnaround radius}, $r_{\text{ta}}$. By construction, it provides a bound on maximum structure sizes of the observed universe. Once an upper bound on $r_{\text{ta}}$ is provided, i.e. $R_{\text{TA,max}}$, one can check whether cosmological models guarantee structure formation. Here, we focus on $f(R)$ gravity, without imposing \emph{a priori} the form of $f(R)$. We thus provide an analytic expression for the turnaround radius in the framework of $f(R)$ models. To figure this out, we compute the turnaround radius in two distinct cases: 1) under the hypothesis of static and spherically symmetric space-time, and 2) by using the cosmological perturbation theory. We thus find a criterion to enable large scale structures to be stable in $f(R)$ models, circumscribing the class of $f(R)$ theories as suitable alternative to dark energy. In particular, we get that for constant curvature, the viability condition becomes $R_{\text{dS}}f'(R_{\text{dS}}) \leq 5.48 Λ\Rightarrow f'(R_{\text{dS}}) \leq 1.37$, with $Λ$ and $R_{\text{dS}}$ respectively the observed cosmological constant and the Ricci curvature. This prescription rules out models which do not pass the aforementioned $R_{\text{TA,max}}$ limit.

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arXiv2018

Rational approximations of $f(R)$ cosmography through Padé polynomials

We consider high-redshift $f(R)$ cosmography adopting the technique of polynomial reconstruction. In lieu of considering Taylor treatments, which turn out to be non-predictive as soon as $z>1$, we take into account the Padé rational approximations which consist in performing expansions converging at high redshift domains. Particularly, our strategy is to reconstruct $f(z)$ functions first, assuming the Ricci scalar to be invertible with respect to the redshift $z$. Having the thus-obtained $f(z)$ functions, we invert them and we easily obtain the corresponding $f(R)$ terms. We minimize error propagation, assuming no errors upon redshift data. The treatment we follow naturally leads to evaluating curvature pressure, density and equation of state, characterizing the universe evolution at redshift much higher than standard cosmographic approaches. We therefore match these outcomes with small redshift constraints got by framing the $f(R)$ cosmology through Taylor series around $z\simeq 0$. This gives rise to a calibration procedure with small redshift that enables the definitions of polynomial approximations up to $z\simeq 10$. Last but not least, we show discrepancies with the standard cosmological model which go towards an extension of the $Λ$CDM paradigm, indicating an effective dark energy term evolving in time. We finally describe the evolution of our effective dark energy term by means of basic techniques of data mining.

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arXiv2018

Cosmic acceleration in non-flat $f(T)$ cosmology

We study $f(T)$ cosmological models inserting a non-vanishing spatial curvature and discuss its consequences on cosmological dynamics. To figure this out, a polynomial $f(T)$ model and a double torsion model are considered. We first analyze those models with cosmic data, employing the recent surveys of Union 2.1, baryonic acoustic oscillation and cosmic microwave background measurements. We then emphasize that the two popular $f(T)$ models enable the crossing of the phantom divide line due to dark torsion. Afterwards, we compute numerical bounds up to 3-$σ$ confidence level, emphasizing the fact that $Ω_{k0}$ turns out to be non-compatible with zero at least at 1$σ$. Moreover, we underline that, even increasing the accuracy, one cannot remove the degeneracy between our models and the $Λ$CDM paradigm. So that, we show that our treatments contain the concordance paradigm and we analyze the equation of state behaviors at different redshift domains. We also take into account gamma ray bursts and we describe the evolution of both the $f(T)$ models with high redshift data. We calibrate the gamma ray burst measurements through small redshift surveys of data and we thus compare the main differences between non-flat and flat $f(T)$ cosmology at different redshift ranges. We finally match the corresponding outcomes with small redshift bounds provided by cosmography. To do so, we analyze the deceleration parameters and their variations, proportional to the jerk term. Even though the two models well fit late-time data, we notice that the polynomial $f(T)$ approach provides an effective de-Sitter phase, whereas the second $f(T)$ framework shows analogous results compared with the $Λ$CDM predictions.

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arXiv2018

Effective gravitational coupling in modified teleparallel theories

In the present study, we consider an extended form of teleparallel Lagrangian $f(T,ϕ,X)$, as function of a scalar field $ϕ$, its kinetic term $X$ and the torsion scalar $T$. We use linear perturbations to obtain the equation of matter density perturbations on sub-Hubble scales. The gravitational coupling is modified in scalar modes with respect to the one of General Relativity, albeit vector modes decay and do not show any significant effects. We thus extend these results by involving multiple scalar field models. Further, we study conformal transformations in teleparallel gravity and we obtain the coupling as the scalar field is non-minimally coupled to both torsion and boundary terms. Finally, we propose the specific model $f(T,ϕ,X)=T + \partial_μϕ \partial^μϕ+ξT ϕ^2$. To check its goodness, we employ the observational Hubble data, constraining the coupling constant, $ξ$, through a Monte Carlo technique based on the Metropolis-Hastings algorithm. Hence, fixing $ξ$ to its best-fit value got from our numerical analysis, we calculate the growth rate of matter perturbations and we compare our outcomes with the latest measurements and the predictions of the $Λ$CDM model.

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arXiv2017

Cosmic acceleration from a single fluid description

We here propose a new class of barotropic factor for matter, motivated by properties of isotropic deformations of crystalline solids. Our approach is dubbed Anton-Schmidt's equation of state and provides a non-vanishing, albeit small, pressure term for matter. The corresponding pressure is thus proportional to the logarithm of universe's volume, i.e. to the density itself since $V\propto ρ^{-1}$. In the context of solid state physics, we demonstrate that by only invoking standard matter with such a property, we are able to frame the universe speed up in a suitable way, without invoking a dark energy term by hand. Our model extends a recent class of dark energy paradigms named \emph{logotropic} dark fluids and depends upon two free parameters, namely $n$ and $B$. Within the Debye approximation, we find that $n$ and $B$ are related to the Grüneisen parameter and the bulk modulus of crystals. We thus show the main differences between our model and the logotropic scenario, and we highlight the most relevant properties of our new equation of state on the background cosmology. Discussions on both kinematics and dynamics of our new model have been presented. We demonstrate that the $Λ$CDM model is inside our approach, as limiting case. Comparisons with CPL parametrization have been also reported in the text. Finally, a Monte Carlo analysis on the most recent low-redshift cosmological data allowed us to place constraints on $n$ and $B$. In particular, we found $n=-0.147^{+0.113}_{-0.107}$ and $B=3.54 \times 10^{-3}$.

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arXiv2017

Cosmographic analysis with Chebyshev polynomials

The limits of standard cosmography are here revised addressing the problem of error propagation during statistical analyses. To do so, we propose the use of Chebyshev polynomials to parameterize cosmic distances. In particular, we demonstrate that building up rational Chebyshev polynomials significantly reduces error propagations with respect to standard Taylor series. This technique provides unbiased estimations of the cosmographic parameters and performs significatively better than previous numerical approximations. To figure this out, we compare rational Chebyshev polynomials with Padé series. In addition, we theoretically evaluate the convergence radius of (1,1) Chebyshev rational polynomial and we compare it with the convergence radii of Taylor and Padé approximations. We thus focus on regions in which convergence of Chebyshev rational functions is better than standard approaches. With this recipe, as high-redshift data are employed, rational Chebyshev polynomials remain highly stable and enable one to derive highly accurate analytical approximations of Hubble's rate in terms of the cosmographic series. Finally, we check our theoretical predictions by setting bounds on cosmographic parameters through Monte Carlo integration techniques, based on the Metropolis-Hastings algorithm. We apply our technique to high-redshift cosmic data, using the JLA supernovae sample and the most recent versions of Hubble parameter and baryon acoustic oscillation measurements. We find that cosmography with Taylor series fails to be predictive with the aforementioned data sets, while turns out to be much more stable using the Chebyshev approach.

astro-ph.CO↗

arXiv2017

Toward unbiased estimations of the statefinder parameters

With the use of simulated supernova catalogs, we show that the statefinder parameters turn out to be poorly and biased estimated by standard cosmography. To this end, we compute their standard deviations and several bias statistics on cosmologies near the concordance model, demonstrating that these are very large, making standard cosmography unsuitable for future and wider compilations of data. To overcome this issue, we propose a new method that consists in introducing the series of the Hubble function into the luminosity distance, instead of considering the usual direct Taylor expansions of the luminosity distance. Moreover, in order to speed up the numerical computations, we estimate the coefficients of our expansions in a hierarchical manner, in which the order of the expansion depends on the redshift of every single piece of data. In addition, we propose two hybrids methods that incorporates standard cosmography at low redshifts. The methods presented here perform better than the standard approach of cosmography both in the errors and bias of the estimated statefinders. We further propose a one-parameter diagnostic to reject non-viable methods in cosmography.

astro-ph.CO↗

arXiv2017

A thermodynamic approach to holographic dark energy

We propose a method to relate the holographic minimal information density to the de Broglie's wavelength at a given universe temperature $T$. To figure this out, we assume that the thermal length of massive and massless constituents represents the cut-off scale of the holographic principle. To perform our analysis, we suppose two plausible universe volumes, i.e. the adiabatic and the horizon volumes, i.e. $V \propto a^3$ and $V \propto H^{-3}$ respectively, assuming zero spatial curvature. With these choices in mind, we evaluate the thermal lengths for massive and massless particles and we thus find two cosmological models associated to late and early cosmological epochs. We demonstrate that both models depend upon a free term $β$ which enters the temperature parametrization in terms of the redshift $z$. For the two treatments, we show evolving dark energy terms which can be compared with the $ω$CDM quintessence paradigm when the barotropic factor takes the formal values: $ω_0=-\frac{1}{3}(2+β)$ and $ω_0=-\frac{1}{3}(1+2β)$ respectively for late and early eras. From our analyses, we candidate the two models as viable alternatives to dark energy determined from thermodynamics in the field of the holographic principle.

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