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- Gravitational waves in hybrid quintessential inflationary modelsPublication . Sá, Paulo; Henriques, Alfredo B.; Aldaya, V; Barcelo, C; Jaramillo, JL
Expand The generation of primordial gravitational waves is investigated within the hybrid quintessential inflationary model. Using the method of continuous Bogoliubov coefficients, we calculate the full gravitational-wave energy spectrum. The post-inflationary kination period, characteristic of quintessential inflationary models, leaves a clear signature on the spectrum, namely, a sharp rise of the gravitational-wave spectral energy density Omega(GW) at high frequencies. For appropriate values of the parameters of the model, Omega(GW) can be as high as 10(-12) in the MHz-GHz range of frequencies.Expand - Late-time evolution of the Universe within a two-scalar-field cosmological modelPublication . Sá, Paulo
Expand We investigate the late-time evolution of the Universe within a cosmological model in which dark matter and dark energy are identified with two interacting scalar fields. Using methods of qualitative analysis of dynamical systems, we identify all cosmological solutions of this model. We show that viable solutions -- in the sense that they correspond to a cosmic evolution in which a long enough matter-dominated era is followed by a current era of accelerated expansion -- can be found in several regions of the parameter space. These solutions can be divided into two categories, namely, solutions that evolve to a state of everlasting accelerated expansion, in which the energy density of the dark-matter field rapidly approaches zero and the evolution becomes entirely dominated by the dark-energy field, and solutions in which the stage of accelerated expansion is temporary and the ratio between the energy densities of dark energy and dark matter tends, asymptotically, to a constant nonzero value.Expand - Coupled quintessence with a generalized interaction termPublication . Potting, Robertus; Sá, Paulo
Expand We investigate a cosmological model in which dark energy, represented by a quintessential scalar field, is directly coupled to a dark-matter perfect fluid. We are interested in solutions of cosmological relevance, namely those for which a dark-matter-dominated era long enough to allow for structure formation is followed by an era of accelerated expansion driven by dark energy. For the coupling between these two dark components of the Universe, we choose forms that generalize the one most commonly used in the literature. Resorting to powerful methods of qualitative analysis of dynamical systems, we show that, for certain generalized forms of the coupling, final states of our coupled quintessential model correspond to solutions in which the evolution of the Universe is completely dominated by dark energy. In this case, there are no scaling solutions. Interestingly, however, for certain values of a relevant parameter, during the approach to the final state of evolution, the cosmological parameters change so slowly that, for all practical purposes, the solution appears to be stuck in a state corresponding to a scaling solution.Expand - Unified description of dark energy and dark matter within the generalized hybrid metric-palatini theory of gravityPublication . Sá, Paulo
Expand The generalized hybrid metric-Palatini theory of gravity admits a scalar-tensor representation in terms of two interacting scalar fields. We show that, upon an appropriate choice of the interaction potential, one of the scalar fields behaves like dark energy, inducing a late-time accelerated expansion of the universe, while the other scalar field behaves like pressureless dark matter that, together with ordinary baryonic matter, dominates the intermediate phases of cosmic evolution. This unified description of dark energy and dark matter gives rise to viable cosmological solutions, which reproduce the main features of the evolution of the universe.Expand - Multi-field cold and warm inflation and the de Sitter swampland conjecturesPublication . Bertolami, Orfeu; Sá, Paulo
Expand We discuss under which conditions multi-field cold and warm inflationary models with canonical kinetic energy terms are compatible with the swampland conjectures about the emergence of de Sitter solutions in string theory. We find that under quite general con-ditions the slow-roll conditions for multi-field cold inflation are at odds with the swampland conjectures for an arbitrary number of scalar fields driving inflation. However, slow-roll con-ditions can be reconciled with the swampland conjectures in the strong dissipative regime of warm inflation.Expand - Triple unification of inflation, dark energy, and dark matter in two-scalar-field cosmologyPublication . Sá, Paulo
Expand A unified description of inflation, dark energy, and dark matter is presented within a two-scalar-field cosmological model. Inflation, assumed to be of the warm type, is driven by one of the scalar fields, which, shortly after the end of the inflationary period, decouples from radiation and begins to oscillate rapidly around the minimum of its potential, thus behaving like cold dark matter; the second scalar field emerges, at recent times, as the dominant component of the Universe, giving rise to a second era of accelerated expansion. For certain values of the parameters of the model, the cosmological solutions arising in this triple unification of inflation, dark energy, and dark matter are viable, reproducing the main features of the evolution of the Universe.Expand