arXiv subjects

Carlos R. Fadragas

Publications and source records attributed to Carlos R. Fadragas.

13 recordsLinked to original sources

arXiv2014

Cosmological dynamical systems

In this book are studied, from the perspective of the dynamical systems, several Universe models. In chapter 1 we give a bird's eye view on cosmology and cosmological problems. Chapter 2 is devoted to a brief review on some results and useful tools from the qualitative theory of dynamical systems. They provide the theoretical basis for the qualitative study of concrete cosmological models. Chapters 1 and 2 are a review of well-known results. Chapters 3, 4, 5 and 6 are devoted to our main results. In these chapters are extended and settled in a substantially different, more strict mathematical language, several results obtained by one of us in arXiv:0812.1013 [gr-qc]; arXiv:1009.0689 [gr-qc]; arXiv:0904.1577[gr-qc]; and arXiv:0909.3571 [hep-th]. In chapter 6, we provide a different approach to the subject discussed in astro-ph/0503478. Additionally, we perform a Poincaré compactification process allowing to construct a global phase space containing all the cosmological information in both finite and infinite regions for all the models.

gr-qc↗

arXiv2014

Some remarks about non-minimally coupled scalar field models

Several results related to flat Friedmann-Lema\^ıtre-Robertson-Walker models in the conformal (Einstein) frame of scalar-tensor gravity theories are extended. Scalar fields with arbitrary (positive) potentials and arbitrary coupling functions are considered. Mild assumptions under such functions (differentiable class, number of singular points, asymptotes, etc) are introduced in a straightforward manner in order to characterize the asymptotic structure on a phase space. We pay special attention to the possible scaling solutions. Numerical evidence confirming our results is presented.

gr-qc↗

arXiv2014

Dynamical analysis of anisotropic scalar-field cosmologies for a wide range of potentials

We perform a detailed dynamical analysis of anisotropic scalar-field cosmologies, and in particular of the most significant Kantowski-Sachs, Locally Rotationally Symmetric (LRS) Bianchi I and LRS Bianchi III cases. We follow the new and powerful method of $f$-devisers, which allows us to perform the whole analysis for a wide range of potentials. Thus, one can just substitute the specific potential form in the final results and obtain the corresponding behavior, without the need of new calculations. We find a very rich behavior, and amongst others the universe can result in isotropized solutions with observables in agreement with observations, such as de Sitter, quintessence-like, or stiff-dark energy solutions. In particular, all expanding, accelerating, stable attractors are isotropic. Additionally, we prove that as long as matter obeys the null energy condition, bounce behavior is impossible. Finally, applying the general results to the well-studied exponential and power-law potentials, we find that some of the general stable solutions disappear. This feature may be an indication that such simple potentials might restrict the dynamics in scalar-field cosmology, opening the way to the introduction of more complicated ones.

gr-qc↗

arXiv2013

Asymptotic behavior of a scalar field with an arbitrary potential trapped on a Randall-Sundrum's braneworld: the effect of a negative dark radiation term on a Bianchi I brane

In this work we present a phase space analysis of a quintessence field and a perfect fluid trapped in a Randall-Sundrum's Braneworld of type 2. We consider a homogeneous but anisotropic Bianchi I brane geometry. Moreover, we consider the effect of the projection of the five dimensional Weyl tensor onto the three-brane in the form of a negative Dark Radiation term. For the treatment of the potential we use the "Method of $f$-devisers" that allows investigating arbitrary potentials in a phase space. We present general conditions on the potential in order to obtain the stability of standard 4D and non-standard 5D de Sitter solutions, and we provide the stability conditions for both scalar field-matter scaling solutions, scalar field-dark radiation solutions and scalar field-dominated solutions. We find that the shear-dominated solutions are unstable (particularly, contracting shear-dominated solutions are of saddle type). As a main difference with our previous work, the traditionally ever-expanding models could potentially re-collapse due to the negativity of the dark radiation. Additionally, our system admits a large class of static solutions that are of saddle type. These kinds of solutions are important at intermediate stages in the evolution of the universe, since they allow the transition from contracting to expanding models and viceversa. New features of our scenario are the existence of a bounce and a turnaround, which lead to cyclic behavior, that are not allowed in Bianchi I branes with positive dark radiation term. Finally, as specific examples we consider the potentials $V\propto\sinh^{-α}(βϕ)$ and $V\propto\left[\cosh\left(ξϕ\right)-1\right]$ which have simple $f$-devisers.

gr-qc↗

arXiv2013

Thawing models in the presence of a generalized Chaplygin gas

In this paper we consider a cosmological model whose main components are a scalar field and a generalized Chaplygin gas. We obtain an exact solution for a flat arbitrary potential. This solution have the right dust limit when the Chaplygin parameter $A\rightarrow 0$. We use the dynamical systems approach in order to describe the cosmological evolution of the mixture for an exponential self-interacting scalar field potential. We study the scalar field with an arbitrary self-interacting potential using the "Method of $f$-devisers." Our results are illustrated for the special case of a coshlike potential. We find that usual scalar-field-dominated and scaling solutions cannot be late-time attractors in the presence of the Chaplygin gas (with $α>0$). We recover the standard results at the dust limit ($A\rightarrow 0$). In particular, for the exponential potential, the late-time attractor is a pure generalized Chaplygin solution mimicking an effective cosmological constant. In the case of arbitrary potentials, the late-time attractors are de Sitter solutions in the form of a cosmological constant, a pure generalized Chaplygin solution or a continuum of solutions, when the scalar field and the Chaplygin gas densities are of the same orders of magnitude. The different situations depend on the parameter choices.

astro-ph.CO↗

arXiv2012

Phase space analysis of quintessence fields trapped in a Randall-Sundrum Braneworld: anisotropic Bianchi I brane with a Positive Dark Radiation term

In this paper we investigate, from the dynamical systems perspective, the evolution of an scalar field with arbitrary potential trapped in a Randall-Sundrum's Braneworld of type 2. We consider an homogeneous but anisotropic Bianchi I (BI) brane filled also with a perfect fluid. We also consider the effect of the projection of the five-dimensional Weyl tensor onto the three-brane in the form of a positive Dark Radiation term. Using the center manifold theory we obtain sufficient conditions for the asymptotic stability of de Sitter solution with standard 4D behavior. We also prove that there are not late time de Sitter attractors with 5D-modifications since they are always saddle-like. This fact correlates with a transient primordial inflation. We present here sufficient conditions on the potential for the stability of the scalar field-matter scaling solution, the scalar field-dominated solution, and the scalar field-dark radiation scaling solution. We illustrate our analytical findings using a simple $f$-deviser as a toy model. All these results are generalizations of our previous results obtained for FRW branes.

gr-qc↗

arXiv2012

Phase space analysis of quintessence fields trapped in a Randall-Sundrum braneworld: a refined study

In this paper we investigate, from the dynamical systems perspective, the evolution of an scalar field with arbitrary potential trapped in a Randall-Sundrum's Braneworld of type II. We consider an homogeneous and isotropic Friedmann-Robertson-Walker (FRW) brane filled also with a perfect fluid. Center Manifold Theory is employed to obtain sufficient conditions for the asymptotic stability of 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 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 potential $V=V_{0}e^{-χϕ}+Λ$ we prove that for $χ<0$ the de Sitter solution is asymptotically stable. However, for $χ>0$ the de Sitter solution is unstable (of saddle type).

gr-qc↗

arXiv2010

Phase-space of flat Friedmann-Robertson-Walker models with both a scalar field coupled to matter and radiation

We investigate the phase-space of a flat FRW universe including both a scalar field, $ϕ,$ coupled to matter, and radiation. The model is inspired in scalar-tensor theories of gravity, and thus, related with $F(R)$ theories through conformal transformation. The aim of the chapter is to extent several results to the more realistic situation when radiation is included in the cosmic budget particularly for studying the early time dynamics. Under mild conditions on the potential we prove that the equilibrium points corresponding to the non-negative local minima for $V(ϕ)$ are asymptotically stable. Normal forms are employed to obtain approximated solutions associated to the inflection points and the strict degenerate local minimum of the potential. We prove for arbitrary potentials and arbitrary coupling functions $χ(ϕ),$ of appropriate differentiable class, that the scalar field almost always diverges into the past. It is designed a dynamical system adequate to studying the stability of the critical points in the limit $|ϕ|\to\infty.$ We obtain there: radiation-dominated cosmological solutions; power-law scalar-field dominated inflationary cosmological solutions; matter-kinetic-potential scaling solutions and radiation-kinetic-potential scaling solutions. Using the mathematical apparatus developed here, we investigate the important examples of higher order gravity theories $F(R) = R + αR^2$ (quadratic gravity) and $F(R) =R^n.$ We illustrated both analytically and numerically our principal results. In the case of quadratic gravity we prove, by an explicit computation of the center manifold, that the equilibrium point corresponding to de Sitter solution is locally asymptotically unstable (saddle point).

gr-qc↗

arXiv2003

Modelling the propagation of an ultrasonic ray through a heterogeneous medium

The estimation of mechanical properties of concrete is one of the most important applications of the ultrasonic method. This technique often involves the measuring of the ultrasonic pulse propagation velocity. However, there exist some factors influencing the effective ultrasonic pulse propagation velocity in concrete. This paper is devoted to a discussion of the behavior of the effective ultrasonic pulse propagation velocity as a function of the free water contained in concrete. The travelling of an ultrasonic ray through a heterogeneous medium composed by cement and aggregates(solid phase), water(liquid phase) and air(gaseous phase) was empirically modelled. A relationship between the transition time of the ultrasonic ray in the testing piece and the free water quantity was considered. We assumed an empirical intermediate relationship between the ultrasonic path length in porous space in concrete filled with water and the free water quantity in the testing piece. From experimental data points we evaluated the parameters of the model and obtained a fitting function for describing the dependence between the effective ultrasonic pulse propagation velocity and the free water quantity in concrete. We also analyzed the error propagation through the algorithm used. It allowed to decide under which conditions the linear approach between the effective ultrasonic pulse propagation velocity and the free water quantity in concrete can be applied.

cond-mat↗

arXiv2003

Using the Magnitude-Squared Coherence for Determining Order-Chaos Transition in a System Governed by Logistic Equation Dynamics

This paper is devoted to show the results obtained by using the magnitude-squared coherence for determining order-chaos transition in a system described by the logistic equation dynamics. For determining the power spectral density of a chaotic finite-duration discrete-time sequence the Welch average periodogram method was used. This method has the advantage that can be applied to any stationary signal by using the discrete Fourier transform (DFT) representation of a discrete-time series which allows an effective computation via fast Fourier transform (FFT) algorithm, and that can be applied to a discrete-time series shorter than that required by nonlinear dynamical analysis methods. The estimate of the Inverse Average Magnitude-Squared Coherence Index (IAMSCI) for each discrete-time series in the set obtained from the logistic mapping was calculated. The control parameter, {\bf r}, ranges in the interval [2.8,4] for producing the discrete-time series set. When the condition $r \geq 3.57$ is satisfied, each discrete-time series exhibited a positive value for IAMSCI estimate, indicating a high level of coherence loss of the signal and corresponding to a chaotic behavior of the dynamical system. Its effectiveness was demonstrated by comparing the results with those obtained by calculating the largest Lyapounov exponent of the time series set obtained from the logistic equation.

nlin.CD↗

arXiv2003

Applying the Inverse Average Magnitude Squared Coherence Index for Determining Order-Chaos Transition in a System Governed by Hénon Mapping Dynamics

The quantitative determination of the order-chaos transition in a nonlinear dynamical system described by Hénon mapping defined as $x[n+1]=1.0-A*x[n]^2+B*y[n], y[n+1]=B*x[n]$, where $B=0.3$, and $A$ is an adjustable control parameter, was made. This was achieved by applying the Inverse Average Magnitude-Squared Coherence Index (IAMSCI). This method is based on the Welch average periodogram technique and it has the advantage respect to nonlinear dynamical methods that it may be applied to any stationary signal by using discrete Fourier transform (DFT) representation which allows to operate on a short discrete-time series. Its effectiveness was demonstrated by comparing the results obtained by applying IAMSCI method with those obtained by calculating the largest Lyapounov exponent (LLE), both applied to the same discrete-time series set derived from the Hénon mapping.

nlin.CD↗

arXiv2002

Applying the DFT representation for describing the order-chaos transition in Hénon mapping dynamics

The DFT representation is applied, to a ordered-chaotic finite-duration sequences set generated by the Hénon mapping, for describing the order-chaos transition. This representation has the advantage that it may be applied to a relatively shorter discret-time series. The bifurcation diagram is produced and the largest Lyapounov exponent is calculated for each time series of the set, showing a good agreement with the results obtained by the DFT representation. The threshold value of the control parameter for the order-chaos transition was determined to be A=1.0532.

nlin.CD↗

arXiv2002

Possibilities of the Discrete Fourier Transform for Determining the Order-Chaos Transition in a Dynamical System

This paper is devoted to a discussion of the Discrete Fourier Transform (DFT) representation of a chaotic finite-duration sequence. This representation has the advantage that is itself a finite-duration sequence corresponding to samples equally spaced in the frequency domain. The Fast Fourier Transform (FFT) algoritm allows us an effective computation, and it can be applied to a relatively short time series. DFT representation requirements were analized and applied for determining the order-chaos transition in a nonlinear system described by the equation $x[n+1]=rx[n](1-x[n])$. Its effectiveness was demonstrated by comparing the results with those obtained by calculating the largest Lyapounov exponent for the time series set, obtained from the logistic equation.

nlin.CD↗