Javier Grande

Cosmic perturbations with running G and

Cosmologies with running cosmological term ρΛ and gravitational Newtons 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.

Composite dark energy: Cosmon models with running cosmological term and gravitational coupling

Abstract In the recent literature on dark energy (DE) model building we have learnt that cosmologies with variable cosmological parameters can mimic more traditional DE pictures exclusively based on scalar fields (e.g. quintessence and phantom). In a previous work we have illustrated this situation within the context of a renormalization group running cosmological term, Λ. Here we analyze the possibility that both the cosmological term and the gravitational coupling, G, are running parameters within a more general framework (a variant of the so-called “ΛXCDM models”) in which the DE fluid can be a mixture of a running Λ and another dynamical entity X (the “cosmon”) which may behave quintessence-like or phantom-like. We compute the effective EOS parameter, ω e , of this composite fluid and show that the ΛXCDM can mimic to a large extent the standard ΛCDM model while retaining features hinting at its potential composite nature (such as the smooth crossing of the cosmological constant boundary ω e = − 1 ). We further argue that the ΛXCDM models can cure the cosmological coincidence problem. All in all we suggest that future experimental studies on precision cosmology should take seriously the possibility that the DE fluid can be a composite medium whose dynamical features are partially caused and renormalized by the quantum running of the cosmological parameters.

What is there in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?

The coincidence problems and other dynamical features of dark energy are studied in cosmological models with variable cosmological parameters and in models with the composite dark energy. It is found that many of the problems usually considered to be cosmological coincidences can be explained or significantly alleviated in the aforementioned models.

Lambda-XCDM cosmologies: Solving the cosmological coincidence problem?

We explore the possibility of having a composite (self-conserved) dark energy (DE) whose dynamics is controlled by the quantum running of the cosmological parameters. We find that within this scenario it is feasible to find an explanation for the cosmological coincidence problem and at the same time a good qualitative description of the present data.

ΛXCDM cosmologies: solving the cosmological coincidence problem?

We explore the possibility of having a composite (self-conserved) dark energy (DE) whose dynamics is controlled by the quantum running of the cosmological parameters. We find that within this scenario it is feasible to find an explanation for the cosmological coincidence problem and at the same time a good qualitative description of the present data.