No abstract

Projecting the closed form expression of the de Sitter scalar field operator onto the Minkowskian positive frequency massless modes, we compute the corresponding Bogolubov coefficient which is associated to the (massless) quasiparticle creation during the stationary quasi-de Sitter stage of the Universe. Thereafter, we derive the expression of the thermalized energy density which reveals an interesting mixture of de Sitter false vacuum and dark-radiation, exotic dust and black body radiation. Setting the temperature to the value of the Hawking one for the de Sitter spacetime, we finally analyze the (straightforward) back-reaction of the newly created "matter" on the scale function. It basically points out three stages of highly continuous evolution represented by an initially short radiationlike era, a somewhat long-lasting connecting phase made of coherent massless oscillations, in its beginnings, ended up by the dark-radiation (i.e. curvature-like term) contribution and, finally, a much slower exponential expansion than the initial de Sitter one.

In the spacetime described by a metric with g44 = - cosh 2(αz), we are pointing out some unusual (pathological) features as for example a genuine temporal imprisonment of null geodesics. For the massless bosons evolving in such an Universe, we solve the Gordon equation and derive the characteristic function and the main thermodynamic quantities.