MG12 - Talk detail
 

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Abstract

Defrosting the big freeze quantum mechanically

Classical models for dark energy can exhibit a variety of singularities, many of which occur for scale factors much bigger than the Planck length. We address here the issue whether some of these singularities, the big freeze and the big démarrage, can be avoided in quantum cosmology. We use the framework of quantum geometrodynamics. We restrict our attention to a class of models whose matter content can be described by a generalized Chaplygin gas and be represented by a scalar field with an appropriate potential. Employing the DeWitt criterium that the wave function be zero at the classical singularity, we show that a class of solutions to the Wheeler-DeWitt equation fulfilling this condition can be found. We discuss the reasons for the remaining ambiguity in fixing the solution.

Self-accelerating the normal DGP branch

We propose a generalised induced gravity brane-world model where the brane action contains an arbitrary f(R) term, R being the scalar curvature of the brane. We show that the effect of the f(R)term on the dynamics of a homogeneous and isotropic brane is twofold: (i) an evolving induced gravity parameter and (ii) a shift on the energy density of the brane. This new shift term, which is absent on the Dvali, Gabadadze and Porrati (DGP) model, plays a crucial role to self-accelerate the generalised normal DGP branch of our model. We analyse as well the stability of de Sitter self-accelerating solutions under homogeneous perturbations and compare our results with the standard 4-dimensional one. Finally, we obtain power law solutions which either correspond to conventional acceleration or super-acceleration of the brane. In the latter case, no phantom matter is invoked on the brane nor in the bulk.

Gravitons production in a new type of Generalized Chaplygin gas

We propose a new early scenario for the Universe where there is a smooth transition between an early de Sitter-like phase which is followed by a radiation dominated era. In this model, the matter content is modelled either by a scalar field or a new modified generalised Chaplygin gas. Such a description is similar in spirit to that introduced by Kamenshchik et al. to unify the dark sectors of the universe. We study the scalar and tensorial perturbations - density perturbations and gravitons, respectively - generated by quantum fluctuations of the scalar field. In particular, we calculate the gravitational wave power spectrum, as would be measured today, following the method of the Bogoliubov coefficients. We show that the high frequencies range of the spectrum depends strongly on one of the parameter of our new model.

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