José P. Mimoso

Energy conditions in modified Gauss-Bonnet gravity

In considering alternative higher-order gravity theories, one is liable to be motivated in pursuing models consistent and inspired by several candidates of a fundamental theory of quantum gravity. Indeed, motivations from string/M theory predict that scalar field couplings with the Gauss-Bonnet invariant, G, are important in the appearance of nonsingular early time cosmologies. In this work, we discuss the viability of an interesting alternative gravitational theory, namely, modified Gauss-Bonnet gravity or f(G) gravity. We consider specific realistic forms of f(G) analyzed in the literature that account for the late-time cosmic acceleration and that have been found to cure the finite-time future singularities present in the dark energy models. We present the general inequalities imposed by the energy conditions and use the recent estimated values of the Hubble, deceleration, jerk and snap parameters to examine the viability of the above-mentioned forms of f(G) imposed by the weak energy condition.

Asymptotic behavior of the warm inflation scenario with viscous pressure

We analyze the dynamics of models of warm inflation with general dissipative effects. We consider phenomenological terms both for the inflaton decay rate and for viscous effects within matter. We provide a classification of the asymptotic behavior of these models and show that the existence of a late-time scaling regime depends not only on an asymptotic behavior of the scalar field potential, but also on an appropriate asymptotic behavior of the inflaton decay rate. There are scaling solutions whenever the latter evolves to become proportional to the Hubble rate of expansion regardless of the steepness of the scalar field exponential potential. We show from thermodynamic arguments that the scaling regime is associated to a power-law dependence of the matter-radiation temperature on the scale factor, which allows a mild variation of the temperature of the matter/radiation fluid. We also show that the late time contribution of the dissipative terms alleviates the depletion of matter, and increases the duration of inflation.

Massless fields in scalar - tensor cosmologies

We derive exact Friedmann--Robertson--Walker cosmological solutions in general scalar--tensor gravity theories, including Brans--Dicke gravity, for stiff matter or radiation. These correspond to the long or short wavelength modes respectively of massless scalar fields. If present, the long wavelength modes of such fields would be expected to dominate the energy density of the universe at early times and thus these models provide an insight into the classical behaviour of these scalar--tensor cosmologies near an initial singularity, or bounce. The particularly simple exact solutions also provide a useful example of the possible evolution of the Brans--Dicke (or dilaton) field,

Generalized energy conditions in Extended Theories of Gravity

\phi

Local conditions separating expansion from collapse in spherically symmetric models with anisotropic pressures

, and the Brans--Dicke parameter,

Anisotropic scalar-tensor cosmologies

\omega(\phi)

Energy conditions in modified gravity

, at late times in spatially curved as well as flat universes. We also discuss the corresponding solutions in the conformally related Einstein metric.

On the potentials yielding cosmological scaling solutions

In this work, we consider the further degrees of freedom related to curvature invariants and scalar fields in Extended Theories of Gravity (ETG). These new degrees of freedom can be recast as effective fluids that differ in nature with respect to the standard matter fluids generally adopted as sources of the field equations. It is thus somewhat misleading to apply the standard general relativistic energy conditions to this effective energy-momentum tensor, as the latter contains the matter content and a geometrical quantity, which arises from the specific ETG considered. Here, we explore this subtlety, extending on previous work, in particular, to cases with the contracted Bianchi identities with diffeomorphism invariance and to cases with generalized explicit curvature-matter couplings, which imply the non-conservation of the energy-momentum tensor. Furthermore, we apply the analysis to specific ETGs, such as scalar-tensor gravity and f(R) gravity. Thus, in the context of ETGs, interesting results appear such as matter that may exhibit unusual thermodynamical features, for instance, gravity that retains its attractive character in the presence of large negative pressures; or alternatively, we verify that repulsive gravity may occur for standard matter.

Gravitational induced particle production through a nonminimal curvature–matter coupling

The authors wish to thank the anonymous referee for many enlightening comments and suggestions and Jose Fernando Pascual-Sanchez for helpful discussions. M. Le D. also wishes to thank Michele Fontanini, Daniel Guariento and Elcio Abdalla for helpful discussions. The work of M. Le D. has been supported by CSIC (JAEDoc072), CICYT (FPA2006-05807) in Spain and FAPESP (2011/24089-5) in Brazil. F.C.M. thanks CMAT, Univ. Minho, for support through FEDER Funds COMPETE and FCT Projects No. Est-C/MAT/UI0013/2011, No. PTDC/MAT/108921/2008 and No. CERN/FP/123609/2011. M. Le D. and J. P. M. acknowledge the CAAULs Project No. PEst-OE/FIS/UI2751/2011. J. P. M. also wishes to thank FCT for Grants No. CERN/FP/123615/2011 and No. CERN/FP/123618/2011.

f(G) modified gravity and the energy conditions

We examine homogeneous but anisotropic cosmologies in scalar-tensor gravity theories, including Brans-Dicke gravity. We present a method for deriving solutions for any isotropic perfect fluid with a barotropic equation of state (p\ensuremath{\propto}\ensuremath{\rho}) in a spatially flat (Bianchi type I) cosmology. These models approach an isotropic, general relativistic solution as the expansion becomes dominated by the barotropic fluid. All models that approach general relativity isotropize except for the case of a maximally stiff fluid. For stiff fluid or radiation or in vacuum we are able to give solutions for arbitrary scalar-tensor theories in a number of anisotropic Bianchi and Kantowski-Sachs metrics. We show how this approach can also be used to derive solutions from the low-energy string effective action. We discuss the nature, and possibly avoidance of, the initial singularity where both shear and non-Einstein behavior is important.