We consider the possibility that the total dark energy (DE) of the Universe is made out of two dynamical components of different nature: a variable cosmological term, Λ, and a dynamical "cosmon", X, possibly interacting with Λ but not with matter -which remains conserved. We call this scenario the ΛXCDM model. One possibility for X would be a scalar field χ, but it is not the only one. The overall equation of state (EOS) of the ΛXCDM model can effectively appear as quintessence or phantom energy depending on the mixture of the two components. Both the dynamics of Λ and of X could be linked to high energy effects near the Planck scale. In the case of Λ it may be related to the running of this parameter under quantum effects, whereas X might be identified with some fundamental field (say, a dilaton) left over as a low-energy "relic" by e.g. string theory. We find that the dynamics of the ΛXCDM model can trigger a future stopping of the Universe expansion and can keep the ratio ρ D /ρ m (DE density to matter-radiation density) bounded and of order 1. Therefore, the model could explain the so-called "cosmological coincidence problem". This is in part related to the possibility that the present value of the cosmological term can be Λ 0 < 0 in this framework (the current total DE density nevertheless being positive). However, a cosmic halt could occur even if Λ 0 > 0 because of the peculiar behavior of X as "Phantom Matter". We describe various cosmological scenarios made possible by the composite and dynamical nature of ΛXCDM, and discuss in detail their impact on the cosmological coincidence problem.For instance, consider the so-called "cosmological coincidence problem" [40], to wit: why do we find ourselves in an epoch t = t 0 where the DE density is similar to the matter density (ρ D (t 0 ) ≃ ρ M (t 0 ))? In the ΛCDM model this is an especially troublesome problem because ρ Λ remains constant throughout the entire history of the Universe. In the ordinary dynamical DE models the problem has also its own difficulties. Thus, in a typical quintessence model the matter-radiation energy density ρ m = ρ M + ρ R decreases (with the expansion) faster than the DE density and we expect that in the early epochs ρ m ≫ ρ D , whereas at present and in the future ρ m ≪ ρ D . Therefore, why do we just happen to live in an epoch t 0 where the two functions ρ D (t 0 ) ≃ ρ m (t 0 )? Is this a mere coincidence or there is some other, more convincing, reason? The next question of course is: can we devise a model where the ratio ρ m /ρ D stays bounded (perhaps even not too far from 1) in essentially the entire span of the Universe lifetime? This is certainly not possible neither in the standard ΛCDM model, nor in standard quintessence models 2 . And it is also impossible for a model whose DE consists only of a running Λ [23]. However, in this paper we will show that we can produce a dynamical DE model with such a property. Specifically, we investigate a minimal realization of a composite DE model made out of just two components: a runnin...

A new class of Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological models with time-evolving fundamental parameters should emerge naturally from a description of the expansion of the universe based on the first principles of quantum field theory and string theory. Within this general paradigm, one expects that both the gravitational Newton's coupling G and the cosmological term Λ should not be strictly constant but appear rather as smooth functions of the Hubble rate H(t). This scenario ("running FLRW model") predicts, in a natural way, the existence of dynamical dark energy without invoking the participation of extraneous scalar fields. In this paper, we perform a detailed study of some of these models in the light of the latest cosmological data, which serves to illustrate the phenomenological viability of the new dark energy paradigm as a serious alternative to the traditional scalar field approaches. By performing a joint likelihood analysis of the recent supernovae type Ia data (SNIa), the Cosmic Microwave Background (CMB) shift parameter, and the Baryonic Acoustic Oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS), we put tight constraints on the main cosmological parameters. Furthermore, we derive the theoretically predicted dark-matter halo mass function and the corresponding redshift distribution of cluster-size halos for the "running" models studied. Despite the fact that these models closely reproduce the standard ΛCDM Hubble expansion, their normalization of the perturbation's power-spectrum varies, imposing, in many cases, a significantly different cluster-size halo redshift distribution. This fact indicates that it should be relatively easy to distinguish between the "running" models and the ΛCDM using realistic future X-ray and Sunyaev-Zeldovich cluster surveys.

Cosmologies with running cosmological term ρΛ and gravitational Newton's coupling G may naturally be expected if the evolution of the universe can ultimately be derived from the first principles of quantum field theory or string theory. For example, if matter is conserved and the vacuum energy density varies quadratically with the expansion rate as ρΛ(H) = n0 + n2 H2, with n0 ≠ 0 (a possibility that has been advocated in the literature within the QFT framework), it can be shown that G must vary logarithmically (hence very slowly) with H. In this paper, we derive the general cosmological perturbation equations for models with variable G and ρΛ in which the fluctuations δG and δρΛ are explicitly included. We demonstrate that if matter is covariantly conserved, the late growth of matter density perturbations is independent of the wavenumber k. Furthermore, if ρΛ is negligible at high redshifts and G varies slowly, we find that these cosmologies produce a matter power spectrum with the same shape as that of the ΛCDM model, thus predicting the same basic features on structure formation. Despite this shape indistinguishability, the free parameters of the variable G and ρΛ models can still be effectively constrained from the observational bounds on the spectrum amplitude.

While there is plentiful evidence in all fronts of experimental cosmology for the existence of a non-vanishing dark energy (DE) density \rho_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_D is so close to the matter energy density \rho_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_M, \delta\rho_D) 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 LXCDM model) which has been previously studied in the literature. By requiring that the coupled system of perturbation equations for \delta\rho_M and \delta\rho_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_D/\rho_M 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.Comment: Typos correcte

We consider on-center and off-center observers in an inhomogeneous, spherically symmetric, isocurvature (flat) concentration of dark energy with typical size of a few Gpc. Such a concentration could be produced e.g. by a recently formed global monopole with core size that approaches the Hubble scale. In this case we would have what may be called 'topological quintessence' in analogy with the well-known topological inflation. We show that the minimum comoving radius r0min of such a dark energy inhomogeneity that is consistent with the Union2 Type Ia supernovae (SnIa) data at the 3σ level is r0min ≃ 1.8 Gpc. As expected, the best-fit fractional dark energy density at the center, ΩX,in, approaches the corresponding ΛCDM value ΩX,in = 0.73 for large enough values of the inhomogeneity radius r0 (r0 > 4 Gpc). Using the Union2 data, we show that the maximum allowed shift r obs−max of the observer from the center of the inhomogeneity is about 0.7r0 which respects the Copernican principle. The model naturally predicts the existence of a preferred axis and alignment of the low CMB multipoles. However, the constraints on r obs−max coming from the magnitude of the CMB dipole remain a severe challenge to the Copernican principle and lead to r obs−max < 110 Mpc even for an inhomogeneity radius as large as r0 = 7 Gpc.