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German Izquierdo

Publications and source records attributed to German Izquierdo.

17 recordsLinked to original sources
arXiv2022

Evolution equations dynamical system of the Lemaître--Tolman--Bondi metric containing coupled dark energy

We consider inhomogeneous spherically symmetric models based on the Lemaître-Tolman-Bondi (LTB) metric, assuming as its source an interactive mixture of ordinary baryonic matter, cold dark matter and dark energy with a coupling term proportional to the addition of energy densities of both dark fluids. We reduce Einstein's field equations to a first order 7-dimensional autonomous dynamical system of evolution equations and algebraic constraints. We study in detail the evolution of the energy density and spatial curvature profiles along the phase space by means of two subspace projections: a three-dimensional projection associated with the solutions of the Friedman-Lema\^ıtre-Robertson-Walker metric (invariant subspace) and a four-dimensional projection describing the evolution of the inhomogeneous fluctuations. We also classify and study the critical points of the system in comparison with previous work on similar sources, as well as solving numerically the equations for initial energy density and curvature profiles that lead to a spherical bounce whose collapsing time we estimate appropriately.

gr-qc↗
arXiv2022

Primordial gravitational waves spectrum in the interacting Bose-Einstein gas model

We study the evolution and power spectrum of primordial gravitational waves in the interactive Bose-Einstein gas model for dark energy, relevant, as it addresses the coincidence problem. The model is applied in the radiation, matter and dark-energy domination stages. The model introduces a scale factor associated to the radiation-matter transition which influences the gravitational spectrum. We focus on the impact of the free parameters on both the gravitational waves amplitude and its power-spectrum slope. For sets of parameters fitting Hubble's law, we show that the model's parameter for today's dark-matter energy density has a noticeable impact on such waves, while the others produce an indistinguishable effect. The feasibility of detecting such waves under present and future measurements is discussed.

gr-qc↗
arXiv2019

Observational constraints on the free parameters of an interacting Bose-Einstein gas as a dark-energy model

Dark energy is modelled by a Bose-Einstein gas of particles with an attractive interaction. It is coupled to cold dark matter, within a flat universe, for the late-expansion description, producing variations in particle-number densities. The model's parameters, and physical association, are: $Ω_{G0}$, $Ω_{m0}$, the dark-energy rest-mass energy density and the dark-matter term scaling as a mass term, respectively; $Ω_{i0}$, the self-interaction intensity; $x$, the energy exchange rate. Energy conservation relates such parameters. The Hubble equation omits $Ω_{G0}$, but also contains $h$, the present-day expansion rate of the flat Friedman--Lemâitre--Robertson--Walker metric, and $Ω_{b0}$, the baryon energy density, used as a prior. This results in the four effective chosen parameters $Ω_{b0}$, $h$, $Ω_{m0}$, $Ω_{i0}$, fit with the Hubble expansion rate $H(z)$, and data from its value today, near distance, and supernovas. We derive wide $1σ$ and $2σ$ likelihood regions compatible with definite positive total CDM and IBEG mass terms. Additionally, the best-fit value of parameter $x$ relieves the coincidence problem, and a second potential coincidence problem related to the choice of $Ω_{G0}$.

astro-ph.CO↗
arXiv2017

Dynamics of a spherically symmetric inhomogeneous coupled dark energy model with coupling term proportional to non relatvistic matter

Quasi--local scalar variables approach is applied to a spherically symmetric inhomogeneous Lema\^ıtre--Tolman--Bondi metric containing a mixture of non-relativistic cold dark matter and coupled dark energy with constant equation of state. The quasi--local coupling term considered is proportional to the quasi--local cold dark matter energy density and a quasi--local Hubble factor-like scalar via a coupling constant $α$. The autonomous numerical system obtained from the evolution equations is classified for different choices of the free parameters: the adiabatic constant of the dark energy $w$ and $α$. The presence of a past attractor in a non-physical region of the energy densities phase-space of the system makes the coupling term non physical when the energy flows from the matter to the dark energy in order to avoid negative values of the dark energy density in the past. On the other hand, if the energy flux goes from dark energy to dark matter, the past attractor lays in a physical region. The system is also numerically solved for some interesting initial profiles leading to different configurations: an ever expanding mixture, a scenario where the dark energy is completely consumed by the non-relativistic matter by means of the coupling term, a scenario where the dark energy disappears in the inner layers while the outer layers expand as a mixture of both sources, and, finally, a structure formation toy model scenario, where the inner shells containing the mixture collapse while the outer shells expand.

gr-qc↗
arXiv2014

Coincidence problem within dark energy as a coupled self-interacting Bose-Einstein gas

A late accelerated expansion of the Universe is obtained from non-relativistic particles with a short-range attractive interaction, and low enough temperature to produce a Bose-Einstein condensate; by considering coupled dark-energy particles, energy is interchanged with dark matter, allowing it to describe recent acceleration by strengthening its effect. We show that for a sizable range of parameters, dark energy and dark matter evolve with similar energy densities, solving the coincidence problem, and in agreement with the luminosity distance vs redshift, derived from supernova data.

astro-ph.CO↗
arXiv2014

Late evolution of relic gravitational waves in coupled dark energy models

In this paper we study the late evolution of Relic Gravitational Waves in coupled dark energy models, where dark energy interacts with cold dark matter. Relic Gravitational Waves are second tensorial order perturbations of the Lemaitre-Friedman-Robertson-Walker metric and experiment an evolution ruled by the scale factor of the metric. We and that the amplitude of Relic Gravitational Waves is smaller in coupled dark energy models than in models with non interacting dark energy . We also find that the amplitude of the waves predicted by the models with coupling term proportional to the dark energy density is smaller than those of the models with coupling term proportional to dark matter density.

astro-ph.CO↗
arXiv2011

A dynamical systems study of the inhomogeneous Lambda-CDM model

We consider spherically symmetric inhomogeneous dust models with a positive cosmological constant, $Λ$, given by the Lemaitre-Tolman-Bondi metric. These configurations provide a simple but useful generalization of the Lambda-CDM model describing cold dark matter (CDM) and a Lambda term, which seems to fit current cosmological observations. The dynamics of these models can be fully described by scalar evolution equations that can be given in the form of a proper dynamical system associated with a 4-dimensional phase space whose critical points and invariant subspaces are examined and classified. The phase space evolution of various configurations is studied in detail by means of two 2-dimensional subspaces: a projection into the invariant homogeneous subspace associated with Lambda-CDM solutions with FLRW metric, and a projection into a subspace generated by suitably defined fluctuations that convey the effects of inhomogeneity. We look at cases with perpetual expansion, bouncing and loitering behavior, as well as configurations with "mixed" kinematic patters, such as a collapsing region in an expanding background. In all cases, phase space trajectories emerge from and converge to stable past and future attractors in a qualitatively analogous way as in the case of the FLRW limit. However, we can identify in both projections of the phase space various qualitative features absent in the FLRW limit that can be useful in the construction of toy models of astrophysical and cosmological inhomogeneities.

gr-qc↗
arXiv2010

Limits on the parameters of the equation of state for interacting dark energy

Under the assumption that cold dark matter and dark energy interact with each other through a small coupling term, $Q$, we constrain the parameter space of the equation of state $w$ of those dark energy fields whose variation of the field since last scattering do not exceed Planck's mass. We use three parameterizations of $w$ and two different expressions for $Q$. Our work extends previous ones.

astro-ph.CO↗
arXiv2010

Accelerated expansion of a universe containing a self-interacting Bose-Einstein gas

Acceleration of the universe is obtained from a model of non-relativistic particles with a short-range attractive interaction, at low enough temperature to produce a Bose-Einstein condensate. Conditions are derived for negative-pressure behavior. In particular, we show that a phantom-accelerated regime at the beginning of the universe solves the horizon problem, consistently with nucleosynthesis.

astro-ph↗
arXiv2006

The Generalized Second Law in Dark Energy Dominated Universes

The generalized second law of gravitational thermodynamics is examined in scenarios where the dark energy dominates the cosmic expansion. For quintessence and phantom fields this law is fulfilled but it may fail when the dark energy is in the form of a Chaplygin gas. However, if a black hole is allowed in the picture, the law can be violated if the field is of phantom type.

gr-qc↗
arXiv2006

Relic gravitational waves in the expanding Universe

In this thesis, we have reviewed how the expansion of the Universe amplifies the quantum vacuum fluctuations, and how the relic GWs spectrum is related with the scale factor. We have evaluated the spectrum in different scenarios: a model of expanding universe with an era dominated by mini black holes and radiation right after the inflation and a model with an era of accelerated expansion right after the radiation era. Next, we have applied the generalized second law of thermodynamics to the latter model. Finally, we have extended the GSL study to a single stage universe model dominated by dark energy (either phantom or not).

gr-qc↗
arXiv2006

Relic gravitational waves and the cosmic accelerated expansion

The possibility of reconstructing the whole history of the scale factor of the Universe from the power spectrum of relic gravitational waves (RGWs) makes the study of these waves quite interesting. First, we explore the impact of a hypothetical era -right after reheating- dominated by mini black holes and radiation that may lower the spectrum several orders of magnitude. Next, we calculate the power spectrum of the RGWs taking into account the present stage of accelerated expansion and an hypothetical second dust era. Finally, we study the generalized second law of gravitational thermodynamics applied to the present era of accelerated expansion of the Universe.

gr-qc↗
arXiv2005

Dark energy and the generalized second law

We explore the thermodynamics of dark energy taking into account the existence of the observer's event horizon in accelerated universes. Except for the initial stage of Chaplygin gas dominated expansion, the generalized second law of gravitational thermodynamics is fulfilled and the temperature of the phantom fluid results positive. This substantially extends the work of Pollock and Singh [10] on the thermodynamics of super--inflationary expansion.

astro-ph↗
arXiv2005

Relic gravitational waves and the generalized second law

The generalized second law of gravitational thermodynamics is applied to the present era of accelerated expansion of the Universe. In spite of the fact that the entropy of matter and relic gravitational waves inside the event horizon diminish, the mentioned law is fulfilled provided that the expression for the entropy density of the gravitational waves satisfies a certain condition.

gr-qc↗
arXiv2004

Relic gravitational waves and present accelerated expansion

We calculate the current power spectrum of the gravitational waves created at the big bang (and later amplified by the different transitions during the Universe expansion) taking into account the present stage of accelerated expansion. Likewise, we determine the power spectrum in a hypothetical second dust era that would follow the present one if at some future time the dark energy, that supposedly drives the current accelerated expansion, evolved in such a way that it became dynamically equivalent to cold dark matter. The calculated power spectrum as well as the evolution of the density parameter of the waves may serve to discriminate between phases of expansion and may help ascertain the nature of dark energy.

astro-ph↗