Effective growth of matter density fluctuations in the running LCDM and LXCDM models

TL;DR

This paper analyzes matter density fluctuations in running mbda models of dark energy, testing their consistency with large-scale structure data and exploring their potential to solve the cosmic coincidence problem.

Contribution

It introduces a method to constrain running mbda dark energy models using galaxy survey data and applies it to the LXCDM model to identify viable parameter regions.

Findings

The standard LCDM model's bias parameter is within 10% of unity.

The method effectively constrains the growth of matter perturbations in running mbda models.

The LXCDM model can be consistent with observations and address the cosmic coincidence problem.

Abstract

We investigate the matter density fluctuations \delta\rho/\rho for two dark energy (DE) models in the literature in which the cosmological term \Lambda is a running parameter. In the first model, the running LCDM model, matter and DE exchange energy, whereas in the second model, the LXCDM model, the total DE and matter components are conserved separately. The LXCDM model was proposed as an interesting solution to the cosmic coincidence problem. It includes an extra dynamical component, the "cosmon" X, which interacts with the running \Lambda, but not with matter. In our analysis we make use of the current value of the linear bias parameter, b^2(0)= P_{GG}/P_{MM}, where P_{MM} ~ (\delta\rho/\rho)^2 is the present matter power spectrum and P_{GG} is the galaxy fluctuation power spectrum. The former can be computed within a given model, and the latter is found from the observed LSS data (at small z) obtained by the 2dF galaxy redshift survey. It is found that b^2(0)=1 within a 10% accuracy for the standard LCDM model. Adopting this limit for any DE model and using a method based on the effective equation of state for the DE, we can set a limit on the growth of matter density perturbations for the running LCDM model, the solution of which is known. This provides a good test of the procedure, which we then apply to the LXCDM model in order to determine the physical region of parameter space, compatible with the LSS data. In this region, the LXCDM model is consistent with known observations and provides at the same time a viable solution to the cosmic coincidence problem.