arXiv subjects

Salvatore Capozziello

Publications and source records attributed to Salvatore Capozziello.

403 records · Page 23Linked to original sources

arXiv1996

Defocusing gravitational microlensing

We introduce the notion of defocusing gravitational lens considering a MACHO located behind a light source with respect to an observer. The consequence of defocusing effect is a temporal variability of star luminosity which produces a gap instead of a peak as tell--tale signature in the light curve. General theory of (de)focusing rays (geodesics) in a gravitational field is presented. Furthermore, we give estimations of the mass of the lens and the optical depth connected to such a phenomenon.

astro-ph↗

arXiv1996

Luminosity variation in (de)focusing microlensing

The luminosity variation of a stellar source due to the gravitational microlensing effect can be considered also if the light rays are defocused (instead of focused) toward the observer. In this case, we should detect a gap instead of a peak in the light curve of the source. Actually, we describe how the phenomenon depends on the relative position of source and lens with respect to the observer: if the lens is between, we have focusing, if the lens is behind, we have defocusing. It is shown that the number of events with predicted gaps is equal to the number of events with peaks in the light curves.

astro-ph↗

arXiv1996

Gravitational Waveguides in Cosmology

We discuss the possibility that, besides the usual gravitational lensing, there may exist a sort of gravitational waveguiding in cosmology which could explain some anomalous phenomena which cannot be understood by the current gravitational lensing models as the existence of "brothers" objects having different brilliancy but similar spectra and redshifts posed on the sky with large angular distance. Furthermore, such a phenomena could explain the huge luminosities coming from quasars using the cosmological structures as selfoc-type or planar waveguide. We describe the gravitational waveguide theory and then we discuss possible realizations in cosmology.

astro-ph↗

arXiv1996

A time--dependent Cosmological Constant

We extend the Wald cosmic no-hair theorem to a general class of scalar-tensor nonminimally coupled theories of gravity where ordinary matter is also present in the form of a perfect fluid. We give a set of conditions for obtaining a de Sitter expansion independently of any initial conditions, generalizing the treatment of such a problem as given by Wald. Finally we apply the results to some specific models.

astro-ph↗

arXiv1996

Noether symmetries in Bianchi universes

We use our Nöther Symmetry Approach to study the Einstein equations minimally coupled with a scalar field, in the case of Bianchi universes of class A and B. Possible cases, when such symmetries exist, are found and two examples of exact integration of the equations of motion are given in the cases of Bianchi AI and BV.

gr-qc↗

arXiv1996

A Time-dependent Cosmological Constant Phenomenology

We construct a cosmological toy model in which a step-function ``cosmological constant'' is taken into consideration beside ordinary matter. We assume that $Λ$ takes two values depending on the epoch, and matter goes from a radiation dominated era to a dust dominated era. The model is exactly solvable and it can be compared with recent observations.

astro-ph↗

arXiv1996

Asymptotic freedom from induced gravity cosmology

We give conditions to obtain cosmological asymptotic freedom in scalar-tensor theories of gravity. We show that this feature can be achieved in FRW flat spacetimes since we obtain singularity free solutions where the effective gravitational constant $G_{eff}\rightarrow 0$ for $t\rightarrow -\infty$ and, for some of them, $G_{eff}\rightarrow G_{N}$ for $t\rightarrow\infty$, where $G_{N}$ is the Newton constant.

gr-qc↗