Showing papers by "Joan Sola published in 2009"

Hubble expansion and structure formation in time varying vacuum models

Abstract: We investigate the properties of the FLRW flat cosmological models in which the vacuum energy density evolves with time, $\ensuremath{\Lambda}(t)$. Using different versions of the $\ensuremath{\Lambda}(t)$ model, namely, quantum field vacuum, power series vacuum and power law vacuum, we find that the main cosmological functions such as the scale factor of the Universe, the Hubble expansion rate $H$, and the energy densities are defined analytically. Performing a joint likelihood analysis of the recent supernovae type Ia data, the cosmic microwave background shift parameter and the baryonic acoustic oscillations traced by the Sloan Digital Sky Survey galaxies, we put tight constraints on the main cosmological parameters of the $\ensuremath{\Lambda}(t)$ scenarios. Furthermore, we study the linear matter fluctuation field of the above vacuum models. We find that the patterns of the power series vacuum $\ensuremath{\Lambda}={n}_{1}H+{n}_{2}{H}^{2}$ predict stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the other two vacuum cases (quantum field and power law), despite the fact that all the cosmological models share the same equation of state parameter. In the case of the quantum field vacuum $\ensuremath{\Lambda}={n}_{0}+{n}_{2}{H}^{2}$, the corresponding matter fluctuation field resembles that of the traditional $\ensuremath{\Lambda}$ cosmology. The power law vacuum ($\ensuremath{\Lambda}\ensuremath{\propto}{a}^{\ensuremath{-}n}$) mimics the classical quintessence cosmology, the best fit being tilted in the phantom phase. In this framework, we compare the observed growth rate of clustering measured from the optical galaxies with those predicted by the current $\ensuremath{\Lambda}(t)$ models. Performing a Kolmogorov-Smirnov statistical test we show that the cosmological models which contain a constant vacuum ($\ensuremath{\Lambda}\mathrm{CDM}$), quantum field vacuum, and power law vacuum provide growth rates that match well with the observed growth rate. However, this is not the case for the power series vacuum models (in particular, the frequently adduced $\ensuremath{\Lambda}\ensuremath{\propto}H$ model) in which clusters form at significantly earlier times ($z\ensuremath{\ge}4$) with respect to all other models ($z\ensuremath{\sim}2$). Finally, we derived the theoretically predicted dark matter halo mass function and the corresponding distribution of cluster-size halos for all the models studied. Their expected redshift distribution indicates that it will be difficult to distinguish the closely resembling models (constant vacuum, quantum field, and power law vacuum), using realistic future x-ray surveys of cluster abundances. However, cluster surveys based on the Sunayev-Zeldovich detection method give some hope to distinguish the closely resembling models at high redshifts.

Dark energy perturbations and cosmic coincidence

Abstract: While there is plentiful evidence in all fronts of experimental cosmology for the existence of a nonvanishing dark energy (DE) density {rho}{sub D} in the Universe, we are still far away from having a fundamental understanding of its ultimate nature and of its current value, not even of the puzzling fact that {rho}{sub D} is so close to the matter energy density {rho}{sub M} at the present time (i.e. the so-called 'cosmic coincidence' problem). The resolution of some of these cosmic conundrums suggests that the DE must have some (mild) dynamical behavior at the present time. In this paper, we examine some general properties of the simultaneous set of matter and DE perturbations ({delta}{rho}{sub M},{delta}{rho}{sub D}) for a multicomponent DE fluid. Next we put these properties to the test within the context of a nontrivial model of dynamical DE (the {lambda}XCDM model) which has been previously studied in the literature. By requiring that the coupled system of perturbation equations for {delta}{rho}{sub M} and {delta}{rho}{sub D} has a smooth solution throughout the entire cosmological evolution, that the matter power spectrum is consistent with the data on structure formation, and that the 'coincidence ratio' r={rho}{sub D}/{rho}{sub M} stays bounded and not unnaturallymore » high, we are able to determine a well-defined region of the parameter space where the model can solve the cosmic coincidence problem in full compatibility with all known cosmological data.« less

Dark energy perturbations and a possible solution to the coincidence problem

Abstract: We analyze some generic properties of the dark energy (DE) perturbations, in the case of a self-conserved DE fluid. We also apply a simple test (the "F-test") to compare a model to the data on large scale structure (LSS) under the assumption of negligible DE perturbations. We exemplify our discussions by means of the LXCDM model, showing that it provides a viable solution to the cosmological coincidence problem.

Dark energy perturbations and a possible solution to the coincidence problem

Abstract: We analyze some generic properties of the dark energy (DE) perturbations, in the case of a self-conserved DE fluid. We also apply a simple test (the "F-test") to compare a model to the data on large scale structure (LSS) under the assumption of negligible DE perturbations. We exemplify our discussions by means of the LXCDM model, showing that it provides a viable solution to the cosmological coincidence problem.

Matter density fluctuations in the running ΛCDM and ΛXCDM models

Abstract: We have investigated the matter density fluctuations, δM (a ), in the running ΛCDM and ΛXCDM models. The latter was proposed as an interesting solution to the cosmic coincidence problem. It includes an extra dynamical component, the “cosmon” X , which interacts with a running Λ, but not with matter. Adopting a dark energy (DE) “picture”, in which the total DE and matter components are conserved separately, the growth of density fluctuations = δM (a )/a can be written in terms of the effective equation of state. We made use of the measured galaxy fluctuation power spectrum, P GG , and of the linear bias parameter b 2 (z = 0) = P GG /P MM , where P MM ∝ δM 2 is the matter power spectrum of the model under consideration. According to the 2dFGRS survey, b Λ 2 (z 0) = 1 within a 10% accuracy for the ΛCDM model. We adopted this limit to put constraints on the fundamental parameter, ν , of the running ΛCDM model. We found an agreement with previous estimates obtained by a number of different methods and authors. This provided a good test of the procedure, which we used then to determine the physical region of the ΛXCDM parameter space.