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S. Capozziello

Publications and source records attributed to S. Capozziello.

At least 127 records · Page 7Linked to original sources
arXiv2009

Classifying and avoiding singularities in the alternative gravity dark energy models

The future finite-time singularities emerging in alternative gravity dark energy models are classified and studied in Jordan and Einstein frames. It is shown that such singularity may occur even in flat spacetime for the specific choice of the effective potential. The conditions for the avoidance of finite-time singularities are presented and discussed. The problem is reduced to the study of a scalar field evolving on an effective potential by using the conformal transformations. Some viable modified gravity models are analyzed in detail and the way to cure singularity is considered by introducing the higher-order curvature corrections. These results maybe relevant for the resolution of the conjectured problem in the relativistic star formation in such modified gravity where finite-time singularity is also manifested.

hep-th↗
arXiv2009

Testing Dark Energy models vs $Λ$CDM Cosmology by Supernovae and Gamma Ray Bursts

A new method to constrain the cosmological equation of state is proposed by using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The Chevallier-Polarski-Linder parameterization is adopted for the equation of state in order to find out a realistic approach to achieve the deceleration/acceleration transition phase of dark energy models. As results, we find that GRBs, calibrated by SNeIa, could be, at least, good distance indicators capable of discriminating cosmological models with respect to $Λ$CDM at high redshift. Besides, GRBs+SNeIa combined redshift-distance diagram puts better in evidence the change of slope around redshift $z\sim 0.5$ which is usually addressed as the "signature" of today observed acceleration. This feature could be interpreted, in more standard way, by the red sequence in galaxy clusters.

astro-ph.CO↗
arXiv2009

Gravitational waves production from stellar encounters around massive black holes

The emission of gravitational waves from a system of massive objects interacting on elliptical, hyperbolic and parabolic orbits is studied in the quadrupole approximation. Analytical expressions are then derived for the gravitational wave luminosity, the total energy output and gravitational radiation amplitude. A crude estimate of the expected number of events towards peculiar targets (i.e. globular clusters) is also given. In particular, the rate of events per year is obtained for the dense stellar cluster at the Galactic Center.

gr-qc↗
arXiv2009

Cosmography by GRBs : Gamma Ray Bursts as possible distance indicators

A new method to constrain the cosmological equation of state is proposed by using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The Chevallier-Polarski-Linder parameterization is adopted for the equation of state in order to find out a realistic approach to achieve the deceleration/acceleration transition phase of dark energy models. As results, we find that GRBs, calibrated by SNeIa, could be, at least, good distance indicators capable of discriminating cosmological models with respect to $Λ$CDM at high redshift.

astro-ph.CO↗
arXiv2009

Can magnetic monopoles and massive photons coexist in the framework of the same classical theory?

It is well known that one cannot construct a self-consistent quantum field theory describing the non-relativistic electromagnetic interaction mediated by massive photons between a point-like electric charge and a magnetic monopole. We show that, indeed, this inconsistency arises in the classical theory itself. No semi-classic approximation or limiting procedure for the Dirac constant approaching to zero is used. As a result, the string attached to the monopole emerges as visible also if finite-range electromagnetic interactions are considered in classical framework.

hep-th↗
arXiv2009

Abelian Magnetic Monopoles and Topologically Massive Vector Bosons in Scalar-Tensor Gravity with Torsion Potential

A Lagrangian formulation describing the electromagnetic interaction - mediated by topologically massive vector bosons - between charged, spin-(1/2) fermions with an abelian magnetic monopole in a curved spacetime with non-minimal coupling and torsion potential is presented. The covariant field equations are obtained. The issue of coexistence of massive photons and magnetic monopoles is addressed in the present framework. It is found that despite the topological nature of photon mass generation in curved spacetime with isotropic dilaton field, the classical field theory describing the nonrelativistic electromagnetic interaction between a point-like electric charge and magnetic monopole is inconsistent.

gr-qc↗
arXiv2009

A scaling law for the cosmological constant from a stochastic model for cosmic structures

A set of scaling laws, based on the stochastic motions of the granular components of astronomical systems, is applied to a cosmological model with a positive cosmological constant. It follows that the mass of the dominant particle in the observable universe must be proportional to the sixth root of the cosmological constant and of the order of the nucleon mass, which is consistent with the Zeldovich scaling law. The approach is a natural way to solve the cosmic coincidence problem. On the other hand, the observed value of the cosmological constant emerges as the result of a scaling law induced by the stochastic mechanism which gives rise to the gravitationally bound systems.

gr-qc↗
arXiv2009

A tomographic description for classical and quantum cosmological perturbations

Classical and quantum perturbations can be described in terms of marginal distribution functions in the framework of tomographic cosmology. In particular, the so called Radon transformation and the mode-parametric quantum oscillator description can give rise to links between quantum and classical regimes. The approach results a natural scheme to discuss the transition from the quantum to the classical perturbations and then it could be a workable scheme to connect primordial fluctuations with the today observed large scale structure.

gr-qc↗
arXiv2009

The Cauchy problem for metric-affine f(R)-gravity in presence of perfect-fluid matter

The Cauchy problem for metric-affine f(R)-gravity `a la Palatini and with torsion, in presence of perfect fluid matter acting as source, is discussed following the well-known Bruhat prescriptions for General Relativity. The problem results well-formulated and well-posed when the perfect-fluid form of the stress-energy tensor is preserved under conformal transformations. The key role of conservation laws in Jordan and in Einstein frame is also discussed.

gr-qc↗
arXiv2009

Position and frequency shifts induced by massive modes of the gravitational wave background in alternative gravity

Alternative theories of gravity predict the presence of massive scalar, vector, and tensor gravitational wave modes in addition to the standard massless spin~2 graviton of general relativity. The deflection and frequency shift effects on light from distant sources propagating through a stochastic background of gravitational waves, containing such modes, differ from their counterparts in general relativity. Such effects are considered as a possible signature for alternative gravity in attempts to detect deviations from Einstein's gravity by astrophysical means.

gr-qc↗
arXiv2009

Fractal large-scale structure from a stochastic scaling law model

A stochastic model relating the parameters of astrophysical structures to the parameters of their granular components is applied to the formation of hierarchical, large-scale structures from galaxies assumed as point-like objects. If the density profile of galaxies on a given scale is described by a power law then the stochastic model leads naturally to a mass function that is proportional to the square of the distance from an occupied point, which corresponds to a two-point correlation function that is inversely proportional to the distance. This result is consistent with observations indicating that galaxies are, on the largest scales, characterized by a fractal distribution with a dimension of order 2 and well-fit with transition to homogeneity at cosmological scales.

astro-ph.CO↗
arXiv2009

The Newtonian limit of metric gravity theories with quadratic Lagrangians

The Newtonian limit of fourth-order gravity is worked out discussing its viability with respect to the standard results of General Relativity. We investigate the limit in the metric approach which, with respect to the Palatini formulation, has been much less studied in the recent literature, due to the higher-order of the field equations. In addition, we refrain from exploiting the formal equivalence of higher-order theories considering the analogy with specific scalar-tensor theories, i.e. we work in the so-called Jordan frame in order to avoid possible misleading interpretations of the results. Explicit solutions are provided for several different types of Lagrangians containing powers of the Ricci scalar as well as combinations of the other curvature invariants. In particular, we develop the Green function method for fourth-order theories in order to find out solutions. Finally, the consistency of the results with respect to General Relativity is discussed.

gr-qc↗
arXiv2009

Detecting the Cosmological Stochastic Background of Gravitational Waves with FastICA

We show that the stochastic background of gravitational waves, produced in the early cosmological epochs, strictly depends on the assumed theory of gravity. In particular, the specific form of the function f(R), where R is the Ricci scalar, is related to the evolution and the production mechanism of gravitational waves. Using a neural network algorithm which only requires non-Gaussian nature and independence of the input signals we conclude that, in order to detect a CSB signal, the interferometric sensitivity of detector like VIRGO will be improved.

gr-qc↗
arXiv2009

Stochastic background of gravitational waves "tuned" by $f(R)$ gravity

The cosmological background of gravitational waves can be tuned by Extended Theories of Gravity. In particular, it can be shown that assuming a generic function $f(R)$ of the Ricci scalar $R$ gives a parametric approach to control the evolution and the production mechanism of gravitational waves in the early Universe.

gr-qc↗
arXiv2009

A general solution in the Newtonian limit of f(R)- gravity

We show that any analytic $f(R)$-gravity model, in the metric approach, presents a weak field limit where the standard Newtonian potential is corrected by a Yukawa-like term. This general result has never been pointed out but often derived for some particular theories. This means that only $f(R)=R$ allows to recover the standard Newton potential while this is not the case for other relativistic theories of gravity. Some considerations on the physical consequences of such a general solution are addressed.

gr-qc↗
arXiv2008

Relativistic orbits with gravitomagnetic corrections

Corrections to the relativistic orbits are studied considering higher order approximations induced by gravitomagnetic effects. We discuss in details how such corrections come out taking into account magnetic components in the weak field limit of gravitational field and then the theory of orbits is developed starting from the Newtonian one, the lowest order in the approximation. Finally, the orbital structure and the stability conditions are discussed giving numerical examples. Beside the standard periastron corrections of General Relativity, a new nutation effect is due to the c^{-3} corrections. The transition to a chaotic behavior strictly depends on the initial conditions. The orbital phase space portrait is discussed.

gr-qc↗