Compl University Of Monte gelo

Napoli, Italy

Compl University Of Monte gelo

Napoli, Italy
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Tsamparlis M.,National and Kapodistrian University of Athens | Paliathanasis A.,National and Kapodistrian University of Athens | Basilakos S.,Academy of Athens | Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo
General Relativity and Gravitation | Year: 2013

Conformally related metrics and Lagrangians are considered in the context of scalar-tensor gravity cosmology. After the discussion of the problem, we pose a lemma in which we show that the field equations of two conformally related Lagrangians are also conformally related if and only if the corresponding Hamiltonian vanishes. Then we prove that to every non-minimally coupled scalar field, we may associate a unique minimally coupled scalar field in a conformally related space with an appropriate potential. The latter result implies that the field equations of a non-minimally coupled scalar field are the same at the conformal level with the field equations of the minimally coupled scalar field. This fact is relevant in order to select physical variables among conformally equivalent systems. Finally, we find that the above propositions can be extended to a general Riemannian space of n-dimensions. © 2013 Springer Science+Business Media New York.


Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | Lobo F.S.N.,University of Lisbon | Mimoso J.P.,University of Lisbon
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

We consider generalized energy conditions in modified theories of gravity by taking into account the further degrees of freedom related to scalar fields and curvature invariants. The latter are usually recast as generalized geometrical fluids that have different meanings with respect to the standard matter fluids generally adopted as sources of the field equations. More specifically, in modified gravity the curvature terms are grouped in a tensor Hab and a coupling g(Ψi) that can be reorganized in effective Einstein field equations, as corrections to the energy-momentum tensor of matter. The formal validity of such inequalities does not assure some basic requirements such as the attractive nature of gravity, so that the energy conditions have to be considered in a wider sense. © 2014 Elsevier B.V.


Stabile A.,University of Sannio | Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We investigate the possibility of explaining theoretically the galaxy rotation curves by a gravitational potential in total absence of dark matter. To this aim an analytic fourth-order theory of gravity, nonminimally coupled with a massive scalar field, is considered. Specifically, the interaction term is given by an analytic function f(R,φ), where R is the Ricci scalar and φ is the scalar field. The gravitational potential is generated by a pointlike source and compared with the so-called Sanders's potential that can be exactly reproduced in this case. This result means that the problem of dark matter in spiral galaxies could be fully addressed by revising general relativity at galactic scales and requiring further gravitational degrees of freedom instead of new material components that have not been found up to now. © 2013 American Physical Society.


Astashenok A.V.,Immanuel Kant Baltic Federal University | Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | Capozziello S.,National Institute of Nuclear Physics, Italy | And 2 more authors.
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2015

Quark star models with realistic equation of state in nonperturbative f(R) gravity are considered. The mass-radius relation for f(R)=R+αR2 model is obtained. Considering scalar curvature R as an independent function, one can find out, for each value of central density, the unique value of central curvature for which one has solutions with the required asymptotic R→0 for r→∞. In other words, one needs a fine-tuning for R to achieve quark stars in f(R) gravity. We consider also the analogue description in corresponding scalar-tensor gravity. The fine-tuning on R is equivalent to the fine-tuning on the scalar field ϕ in this description. For distant observers, the gravitational mass of the star increases with increasing α (α>0) but the interpretation of this fact depends on frame where we work. Considering directly f(R) gravity, one can say that increasing of mass occurs by the "gravitational sphere" outside the star with some "effective mass". On the other hand, in conformal scalar-tensor theory, we also have a dilaton sphere (or "disphere") outside the star but its contribution to gravitational mass for distant observer is negligible. We show that it is possible to discriminate modified theories of gravity from General Relativity due to the gravitational redshift of the thermal spectrum emerging from the surface of the star. © 2015 The Authors.


Laurentis M.D.,University of Naples Federico II | Laurentis M.D.,Compl University Of Monte gelo | Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo
Astroparticle Physics | Year: 2011

The debate concerning the viability of f(R)-gravity as a natural extension of General Relativity could be realistically addressed by using results coming from binary pulsars like PSR 1913 + 16. To this end, we develop a quadrupolar approach to the gravitational radiation for a class of analytic f(R)-models. We show that experimental results are compatible with a consistent range of f(R)-models. This means that f(R)-gravity is not ruled out by the observations and gravitational radiation (in strong field regime) could be a test-bed for such theories. © 2011 Published by Elsevier B.V.


Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | Makarenko A.N.,Tomsk State Pedagogical University | Odintsov S.D.,Catalan Institution for Research and Advanced Studies | Odintsov S.D.,Tomsk State Pedagogical University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

A string-inspired effective theory of gravity, containing Gauss-Bonnet invariant interacting with a scalar field, is considered in view of obtaining cosmological dark energy solutions. A Lagrange multiplier is inserted into the action in order to achieve the cosmological reconstruction by selecting suitable forms of couplings and potentials. Several cosmological exact solutions (including dark energy of quintessence, phantom, or Little Rip type) are derived in the presence and in the absence of the Lagrange multiplier showing the difference in the two dynamical approaches. In the models that we consider, the Lagrange multiplier behaves as a sort of dust fluid that realizes the transitions between matter-dominated and dark energy epochs. The relation between Lagrange multipliers and Noether symmetries is discussed. © 2013 American Physical Society.


De Laurentis M.,Tomsk State Pedagogical University | Paolella M.,University of Naples Federico II | Paolella M.,Compl University Of Monte gelo | Capozziello S.,University of Naples Federico II | And 2 more authors.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

Cosmological inflation is discussed in the framework of F(R,G) gravity where F is a generic function of the curvature scalar R and the Gauss-Bonnet topological invariant G. The main feature that emerges in this analysis is the fact that this kind of theory can exhaust all the curvature budget related to curvature invariants without considering derivatives of R, Rμν, Rσμνλ, etc., in the action. Cosmological dynamics results driven by two effective masses (lengths) are related to the R scalaron and the G scalaron working respectively at early and very early epochs of cosmic evolution. In this sense, a double inflationary scenario naturally emerges. © 2015 American Physical Society.


Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | De Laurentis M.,University of Naples Federico II | De Laurentis M.,Compl University Of Monte gelo
Annalen der Physik | Year: 2012

An alternative view to the dark matter puzzle is represented by Extended Theories of Gravity. The approach consists in addressing issues like dark components from the point of view of gravitational field instead of requiring new material ingredients that, up to now, have not been detected at fundamental level. In this review paper, by extending the Hilbert-Einstein action of gravitational field to more general actions (e.g. f (R) gravity), it is shown that several gravitating structures like stars, spiral galaxies, elliptical galaxies and clusters of galaxies can be selfconsistently described without asking for dark matter. It is also shown that standard General Relativity tests and Equivalence Principle constraints can be evaded at Solar System scales. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | Martin-Moruno P.,Victoria University of Wellington
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013

In this Letter, we consider cosmological solutions of bimetric theory without assuming that only one metric is coupled to gravity. We conclude that any cosmology can be described by fixing the matter content of the space that we are not inhabiting. On the other hand, we show that some conclusions can still be extracted independently of the matter content filling both spaces. In particular, we can conclude the occurrence of some extremality events in one universe if we know that they take place in the other space. © 2013 Elsevier B.V.


Capozziello S.,University of Naples Federico II | Capozziello S.,Compl University Of Monte gelo | Roshan M.,Ferdowsi University of Mashhad
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013

We present a new approach to find exact solutions for cosmological models. By requiring the existence of a symmetry transformation vector for the equations of motion of the given cosmological model (without using either Lagrangian or Hamiltonian), one can find corresponding Hojman conserved quantities. With the help of these conserved quantities, the analysis of the cosmological model can be simplified. In the case of quintessence scalar-tensor models, we show that the Hojman conserved quantities exist for a wide range of V(φ)-potentials and allow to find exact solutions for the cosmic scale factor and the scalar field. Finally, we investigate the general cosmological behavior of solutions by adopting a phase-space view. © 2013 Elsevier B.V.

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