118523877
The slow-roll condition and the amplitude of the primordial spectrum of cosmic fluctuations: contrasts and similarities of the standard account and the 'collapse scheme'
The inflationary paradigm enjoys a very wide acceptance in the cosmological community, due largely to the fact that it is said to 'naturally account' for a nearly scale-independent power primordial spectrum of fluctuations which is in very good agreement with the observations. The expected overall scale of the fluctuations in most models turns out to be too large because it is inversely proportional to the slow-roll parameter, which is expected to be very small. This fact requires the fine tuning of the inflaton potential. In series of recent works it has been argued that the success of the inflationary picture is not fully justified in terms of the rules of quantum theory as applied to the cosmological setting and that an extra element, something akin to a self-induced collapse of the wavefunction, is required. There, it was suggested that the incorporation of such a collapse in the treatment might avoid the need for fine tuning of the potential that afflicts most inflationary models. In this paper we will discuss in detail the manner in which one obtains the estimation of the magnitude of the perturbations in the new scheme and that of the standard accounts, comparing one of the most popular among the latter and that corresponding to the new proposal. We will see that the proposal includes a collapse scheme that bypasses the problem, but we will see that the price seems to be a teleological, and thus unphysical, fine tuning of the characteristics of collapse.
54073503
Quantum origin of the primordial fluctuation spectrum and its statistics
The usual account for the origin of cosmic structure during inflation is not fully satisfactory, as it lacks a physical mechanism capable of generating the inhomogeneity and anisotropy of our Universe, from an exactly homogeneous and isotropic initial state associated with the early inflationary regime. The proposal in [A. Perez, H. Sahlmann, and D. Sudarsky, Classical Quantum Gravity, 23, 2317, (2006)] considers the spontaneous dynamical collapse of the wave function, as a possible answer to that problem. In this work, we review briefly the difficulties facing the standard approach, as well as the answers provided by the above proposal and explore their relevance to the investigations concerning the characterization of the primordial spectrum and other statistical aspects of the cosmic microwave background and large-scale matter distribution. We will see that the new approach leads to novel ways of considering some of the relevant questions, and, in particular, to distinct characterizations of the non-Gaussianities that might have left imprints on the available data.
119111840
Phase-space analysis of teleparallel dark energy
We perform a detailed dynamical analysis of the teleparallel dark energy scenario, which is based on the teleparallel equivalent of General Relativity, in which one adds a canonical scalar field, allowing also for a nonminimal coupling with gravity. We find that the universe can result in the quintessence-like, dark-energy-dominated solution, or to the stiff dark-energy late-time attractor, similarly to standard quintessence. However, teleparallel dark energy possesses an additional late-time solution, in which dark energy behaves like a cosmological constant, independently of the specific values of the model parameters. Finally, during the evolution the dark energy equation-of-state parameter can be either above or below -1, offering a good description for its observed dynamical behavior and its stabilization close to the cosmological-constant value.
119120062
Phase space analysis of quintessence fields trapped in a Randall-Sundrum Braneworld: a refined study
In this paper, we investigate, from the perspective of dynamical systems, the evolution of a scalar field with an arbitrary potential trapped in a Randall?Sundrum's braneworld of type II. We consider an homogeneous and isotropic Friedmann?Robertson?Walker brane filled also with a perfect fluid. Center manifold theory is employed to obtain sufficient conditions for the asymptotic stability of the de Sitter solution. We obtain conditions on the potential for the stability of scaling solutions as well for the stability of the scalar-field-dominated solution. We prove the fact that there are not late-time attractors with 5D-modifications (they are saddle like). This fact correlates with a transient primordial inflation. In the particular case of a scalar field with the potential V = V0e??? + ?, we prove that for ? 0, the de Sitter solution is unstable (of saddle type).
119460309
Quintom cosmologies admitting either tracking or phantom attractors
Abstract In this Letter we investigate the evolution of a class of cosmologies fueled by quintom dark energy and dark matter. Quintom dark energy is a hybrid of quintessence and phantom which involves the participation of two real scalar fields playing the roles of those two types of dark energy. In that framework we examine, from a dynamical systems perspective, the possibility that those fields are coupled among them by considering an exponential potential with an interesting functional dependence similar but not identical to others studied before. The model we consider represents a counterexample to the typical behavior of quintom models with exponential potentials because it admits either tracking attractors ( w = 0 ), or phantom attractors ( w − 1 ).
119391101
Dynamics of quintessence models of dark energy with exponential coupling to dark matter
We explore quintessence models of dark energy which exhibit non-minimal coupling between the dark matter and dark energy components of the cosmic fluid. The kind of coupling chosen is inspired by scalar–tensor theories of gravity. We impose a suitable dynamics of the expansion allowing us to derive exact Friedmann–Robertson–Walker solutions once the coupling function is given as input. Self-interaction potentials of single and double exponential types emerge as a result of our choice of the coupling function. The stability and existence of the solutions are discussed in some detail. Although, in general, models with appropriate interaction between the components of the cosmic mixture are useful for handling the coincidence problem, in the present study this problem cannot be avoided due to the choice of solution generating ansatz.