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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. Source


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. Source


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. Source


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. Source


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. Source

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