Physics

We explore the predictions of a class of dark energy models, quinstant dark energy, concerning the structure formation in the Universe, both in the linear and non-linear regimes. Quinstant dark energy is considered to be formed by quintessence and a negative cosmological constant. We conclude that these models give good predictions for structure formation in the linear regime, but fail to do so in the non-linear one, for redshifts larger than one.

In order to solve the problem of eternal acceleration, a model has been recently proposed including both a negative cosmological constant Λ and a scalar field evolving under the action of an exponential potential. We further explore this model by contrasting it against the Hubble diagram of Type Ia supernovae, the gas mass fraction in galaxy clusters and the acoustic peak and shift parameters. It turns out that the model is able to fit quite well this large dataset so that we conclude that a negative Λ is indeed allowed and could represent a viable mechanism to halt eternal acceleration. In order to avoid problems with theoretical motivations for both a negative Λ term and the scalar field, we reconstruct the gravity Lagrangian f (R) of a fourth order theory of gravity predicting the same dynamics (scale factor and Hubble parameter) as the starting model. We thus end up with a f (R) theory able to both fit the data and solve the problem of eternal acceleration without the need of unusual negative Λ and ad hoc scalar fields.

In the previous paper was investigated the phase space of quintom cosmologies for a class of exponential potentials. This study suggests that the past asymptotic dynamics of such a model can be approximated by the dynamics near a hyperbola of critical points. In this paper we obtain a normal form expansion near a fixed point located on this equilibrium set. We get computationally treatable system up to fourth order. From the structure of the unstable manifold (up to fourth order) we see that that the past asymptotic behavior of this model is given by a massless scalar field cosmology for an open set of orbits (matching the numerical results given in ). We complement the results discussed there by including the analysis at infinity. Although there exists unbounded orbits towards the past, by examining the orbits at infinity, we get that the sources satisfy the evolution ratesφ 2 /V ∼ 2m 2 n 2 −m 2 ,φ/φ ∼ −m/n, with H/φ approaching zero. PACS numbers: 47.10.Fg, 05.45.-a, 02.30.Mv, 98.80.-k, 95.36.+x, 98.80.Jk

We study a non-Riemannian modification of 5-dimensional Kaluza–Klein theory. In our proposal the Riemannian structure of the five-dimensional manifold is replaced by a Weyl-integrable one. In this context a 4-dimensional Poincaré invariant solution is studied. Spacetime structures with thick smooth branes separated in the extra dimension arise. We focus our attention, mainly, in a case where the massless graviton is

In this work we investigate the evolution of matter density perturbations for two different quintessence models. One of them is based on the Einstein theory of gravity, while the other is based on the Brans-Dicke scalar tensor theory. We show that it is possible to constrain the parameter space of the models using the determinations for the growth rate of

0 <γ< 2 of the parameter space. In the present paper we give conclusive arguments pointing at physical equivalence among conformally related metrics, so this result acquires a new dimension. Based on the argument that any consistent effective theory of spacetime must ...

In this talk we present a model of the universe in which dark energy is modelled explicitely with both a dynamical quintessence field and a cosmological constant. Our results confirm the possibility of a collapsing universe (for a given region of the parameter space), which is advantageous for an adequate formulation of both perturbative quantum field and string theories. We have also reproduced the measurements of modulus distance from supernovae with good accuracy.

0 <γ< 2 of the parameter space. In the present paper we give conclusive arguments pointing at physical equivalence among conformally related metrics, so this result acquires a new dimension. Based on the argument that any consistent effective theory of spacetime must ...