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- Gravitational waves in hybrid quintessential inflationary modelsPublication . Sá, Paulo; Henriques, Alfredo B.; Aldaya, V; Barcelo, C; Jaramillo, JLThe 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.
- Late-time evolution of the Universe within a two-scalar-field cosmological modelPublication . Sá, PauloWe 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.
- Coupled quintessence with a generalized interaction termPublication . Potting, Robertus; Sá, PauloWe 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.
- Unified description of dark energy and dark matter within the generalized hybrid metric-palatini theory of gravityPublication . Sá, PauloThe 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.
- Triple unification of inflation, dark energy, and dark matter in two-scalar-field cosmologyPublication . Sá, PauloA 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.
- Coupled quintessence inspired by warm inflationPublication . Sá, PauloWe investigate a coupled quintessence cosmological model in which a dark-energy scalar field with an exponential potential interacts directly with a dark-matter fluid through a dissipa tive term inspired by warm inflation. The evolution equations of this model give rise to a three dimensional dynamical system for which a thorough qualitative analysis is performed for all values of the relevant parameters. We find that the model is able to replicate the observed sequence of late-time cosmological eras, namely, a long enough matter-dominated era followed by a present era of accelerated expansion. In situations where there is a significant transfer of energy from dark energy to dark matter, temporary scaling-type solutions may arise, but, asymptotically, all solutions are dominated by dark energy.
- Multi-field cold and warm inflation and the de sitter swampland conjecturesPublication . Bertolami, Orfeu; Sá, PauloWe 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.
- Gravitational waves from two scalar fields unifying the dark sector with inflationPublication . Luongo, Orlando; Mengoni, Tommaso; Sá, PauloWe investigate the gravitational-wave background predicted by a two-scalar-field cosmological model that aims to unify primordial inflation with the dark sector, namely late-time dark energy and dark matter, in a single and self-consistent theoretical framework. The model is constructed from an action inspired by several extensions of general relativity and string-inspired scenarios and features a non-minimal interaction between the two scalar fields, while both remain minimally coupled to gravity. In this context, we derive the gravitational-wave energy spectrum over wavelengths ranging from today’s Hubble horizon to those at the end of inflation. We employ the continuous Bogoliubov coefficient formalism, originally introduced to describe particle creation in an expanding Universe, in analogy to the well-established mechanism of gravitational particle production and, in particular, generalized to gravitons. Using this method, which enables an accurate description of graviton creation across all cosmological epochs, we find that inflation provides the dominant gravitational-wave contribution, while subdominant features arise at the inflation-radiation, radiation-matter, and matter-dark energy transitions, i.e., epochs naturally encoded inside our scalar field picture. The resulting energy density spectrum is thus compared with the sensitivity curves of the planned nextgeneration ground- and space-based gravitational-wave observatories. The comparison identifies frequency bands where the predicted signal could be probed, providing those windows associated with potentially detectable signals, bounded by our analyses. Consequences of our recipe are thus compared with numerical outcomes and the corresponding physical properties discussed in detail.
- Phantom cosmology with arbitrary potential: New accelerating scaling attractorsPublication . Halde, Sudip; Pan, Supriya; Sá, Paulo; Saha, TapanIn this article, we investigate the existence of accelerating scaling solutions in coupled phantom cosmology without assuming any specific potential for the phantom scalar field. The coupling between phantom dark energy and dark matter is motivated by the warm inflationary paradigm, with the dissipation coefficient assumed to be either constant or variable. The evolution equations are written in the form of autonomous systems, whose stability is studied using methods of qualitative analysis of dynamical systems. For this analysis, the only requirement imposed on the otherwise arbitrary phantom potential is that a particular dynamical variable, defined in terms of the potential and its derivative, must be invertible. For such a generic potential, we show that accelerated scaling solutions do exist, for both constant and variable dissipation coefficients. Although there is a limitation to these scaling solutions – specifically, the current stage of accelerated expansion is not preceded by a long enough matter-dominated era – our results show that the existence of a direct coupling between phantom dark energy and dark matter yields great potential for addressing the cosmic coincidence problem.
- The de sitter swampland conjectures in the context of chaplygin-inspired inflationPublication . Bertolami, Orfeu; Potting, Robertus; Sá, PauloIn this work, we discuss the de Sitter swampland conjectures in the context of the generalized Chaplygin-inspired inflationary model. We demonstrate that these conjectures can be satisfied, but only in the region of the parameter space far away from the General Relativity limit. The cosmic microwave background data had already been found to restrict the allowed inflationary potentials of this model. Our results impose a further limitation on the possible potentials.