Viable f(R) gravity and future cosmological evolution

One of the so-called viable modified gravities is analyzed. This kind of gravity theories are characterized by a well behavior at local scales, where General Relativity is recovered, while the modified terms become important at the cosmological level, where the late-time accelerating era is reproduced, and even the inflationary phase. In the present work, the future cosmological evolution for one of these models is studied. A transition to the phantom phase is observed. Furthermore, the scalar-tensor equivalence of f(R) gravity is also considered, which provides important information concerning this kind of models.Comment: 4 pages. To appear in the proceedings of the Multiverse and Fundamental Cosmology Conference (Multicosmofun 2012), Szczecin, Polan

Reconstructing cosmological solutions in F(R) gravity. Towards a unified model of the Universe evolution

It is accepted by the majority of scientific community that the Universe is currently in an accelerated epoch. In order to explain this shock of late 90's, a lot of dark energy candidates have been proposed. We study in the context of f (R) gravity, how some modifications on General Relativity could reproduce such behavior of the cosmic evolution. It is showed that in general, an f (R) theory can be reconstructed from a non-minimal scalar-tensor theory, where a desired cosmological solution can be achieved. Some viable models are studied as well as its cosmological evolution, and the possibility that inflationary epoch is also a result of these fine modifications.Comment: 4 pages. Contribution to the Proceedings of the Spanish Relativity Meeting (ERE) 2009, Bilbao, Spai

Cosmological solutions in F(R) Horava-Lifshitz gravity

At the present work, it is studied the extension of F (R) gravities to the new recently proposed theory of gravity, the so-called Horava-Lifshitz gravity, which provides a way to make the theory power counting renormalizable by breaking Lorentz invariance. It is showed that dark energy can be well explained in the frame of this extension, just in terms of gravity. It is also explored the possibility to unify inflation and late-time acceleration under the same mechanism, providing a natural explanation the accelerated expansion.Comment: 4 pages. Contribution to the Proceedings of the Spanish Relativity Meeting (ERE) 2010, Granada, Spai

Conformal frames and the validity of Birkhoff's theorem

Birkhoff's theorem is one of the most important statements of Einstein's general relativity, which generally can not be extended to modified theories of gravity. Here we study the validity of the theorem in scalar-tensor theories using a perturbative approach, and compare the results in the so-called Einstein and Jordan frames. The implications of the results question the physical equivalence between both frames, at least in perturbations.Comment: 4 pages. Contribution to the Proceedings of Spanish Relativity Meeting ERE2011, Madri