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UB-ECM-PF-08/17 Dark energy perturbations and cosmic coincidence

By Javier Gr, Ana Pelinson and Joan Solà

Abstract

Abstract. While there is plentiful evidence in all fronts of experimental cosmology for the existence of a non-vanishing dark energy (DE) density ρD in the Universe, we are still far away from having a fundamental understanding of its ultimate nature or of its current value, not even of the puzzling fact that ρD is so close to the matter energy density ρM 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 (δρM,δρD) for a multicomponent DE fluid. Next we put these properties to the test within the context of a non-trivial model of dynamical DE (the ΛXCDM model) which has been previously studied in the literature. By requiring that the coupled system of perturbation equations for δρM and δρD 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 = ρD/ρM stays bounded and not unnaturally 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. PACS numbers: 95.36.+x, 04.62.+v, 11.10.Hi 1

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