We provide a detailed description for power-law scaling Friedmann-Robertson-Walker cosmological scenarios dominated by two interacting perfect fluid components during the expansion. As a consequence of the mutual interaction between the two fluids, neither component is conserved separately and the energy densities are proportional to 1/t 2 . It is shown that in flat FRW cosmological models there can exist interacting superpositions of two perfect fluids (each of them having a positive energy density) which accelerate the expansion of the universe. In this family there also exist flat power law cosmological scenarios where one of the fluids may have a "cosmological constant" or "vacuum energy" equation of state (p = −ρ) interacting with the other component; this scenario exactly mimics the behavior of the standard flat Friedmann solution for a single fluid with a barotropic equation of state. These possibilities of combining interacting perfect fluids do not exist for the non-interacting mixtures of two perfect cosmic fluids, where the general solution for the scale factor is not described by power-law expressions and has a more complicated behavior. In this study is considered also the associated single fluid model interpretation for the interaction between two fluids.
We investigate an interacting two-fluid model in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) Universe, when the energy transfer between these two dark components is produced by a factorisable nonlinear sign-changeable interaction depending linearly on the energy density and quadratically on the deceleration parameter. We solve the source equation and obtain the effective energy densities of the dark sector and their components. We show that the effective equation of state of the dark sector includes some of the several kind of Chaplygin gas equations of state as well as a generalization of the polytropic equation of state. We use bayesian statistics methods to constrain free parameters in the models during its most recent evolution considering supernovae type Ia and measurements of the Hubble expansion rate. The resulting constraints provide new information on sign-changeable interactions, its equivalences and compatibility with previous models and novel late time universe
In this article we investigate and develop specific aspects of Friedmann-Robertson-Walker (FRW) scalar field cosmologies related to the interpretation that canonical and phantom scalar field sources may be interpreted as cosmological configurations with a mixture of two interacting barotropic perfect fluids: a matter component ρ1(t) with a stiff equation of state (p1 = ρ1), and an "effective vacuum energy" ρ2(t) with a cosmological constant equation of state (p2 = −ρ2). An important characteristic of this alternative equivalent formulation in the framework of interacting cosmologies is that it gives, by choosing a suitable form of the interacting term Q, an approach for obtaining exact and numerical solutions. The choice of Q merely determines a specific scalar field with its potential, thus allowing to generate closed, open and flat FRW scalar field cosmologies.
It is well known that Kasner-type cosmologies provide a useful framework for analyzing the threedimensional anisotropic expansion because of the simplification of the anisotropic dynamics. In this paper relativistic multi-fluid Kasner-type scenarios are studied. We first consider the general case of a superposition of two ideal cosmic fluids, as well as the particular cases of non-interacting and interacting ones, by introducing a phenomenological coupling function q(t). For two-fluid cosmological scenarios there exist only cosmological scaling solutions, while for three-fluid configurations there exist not only cosmological scaling ones, but also more general solutions. In the case of triply interacting cosmic fluids we can have energy transfer from two fluids to a third one, or energy transfer from one cosmic fluid to the other two. It is shown that by requiring the positivity of energy densities there always is a matter component which violates the dominant energy condition in this kind of anisotropic cosmological scenarios.
Abstract:Previously it was shown that there exists a class of viscous cosmological models which violate the dominant energy condition for a limited amount of time after which they are smoothly connected to the ordinary radiation era (which preserves the dominant energy conditions). This violation of the dominant energy condition at an early cosmological epoch may influence the slopes of energy spectra of relic gravitons that might be of experimental relevance. However, the bulk viscosity coefficient of these cosmologies became negative during the ordinary radiation era, and then the entropy of the sources driving the geometry decreases with time.We show that in the presence of viscous sources with a linear barotropic equation of state p = γρ we get viscous cosmological models with positive bulk viscous stress during all their evolution, and hence the matter entropy increases with the expansion time. In other words, in the framework of viscous cosmologies, there exist isotropic models compatible with the standard second law of thermodynamics which also may influence the slopes of energy spectra of relic gravitons. Our universe can be viewed as containing a sea of stochastically distributed gravitational waves of primordial origin. Among all observational cosmological evidences (present and future), primordial gravitational waves should have a sufficiently enlightened character in order to better understand the very early universe. The gravitational waves of cosmological origin are nothing but squeezed states of many gravitons produced from the vacuum fluctuations of the background metric. A qualitative analysis can be performed in the context of different physical frameworks, since all models for the very early universe predict the formation of stochastic gravitational wave backgrounds. As examples we can mention inflationary quintessential models [1], inflationary models in Brans-Dicke theory of gravity [2], cosmological models in the Brane-world scenario [3], and superstring theories [4]. The shape of the stochastic graviton background spectrum is affected by the variations of the background dynamics.In this context, Giovannini [5] has considered the interesting possibility of constructing flat FriedmannRobertson-Walker (FRW) cosmologies endowed with a bulk viscous stress which induces a violation of the dominant energy condition (DEC) for a limited amount of time at an early cosmological epoch. This kind of cos- * mcataldo@ubiobio.cl † patriciomella@udec.cl mological models may be connected to some of the recent remarks of Grishchuk [6] concerning the detectability of stochastic gravitational wave background by forthcoming interferometric detectors, such as LIGO, VIRGO, GEO600, LISA [7]. Effectively, bulk viscous dissipative processes may influence the slopes of the energy spectra of relic gravitons (generated at the time of violation of the DEC) producing an increasing with frequency in a calculable way. These slopes are crucially related to the sign of the ρ + p, where ρ and p are, respectively, the energy...
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