Multi uid cosmology in f(G) gravity

Abstract

The treatment of 1+3 covariant perturbations in a multiuid cosmology with the consideration of f (G) gravity, G being the Gauss-Bonnet term, is done in the present paper. We define a set of covariant and gauge-invariant variables to describe density, velocity and entropy perturbations for both the total matter and component fluids. We then use different techniques such as scalar decomposition, harmonic decomposition, quasi-static approximation together with the redshift transformation to get simplified perturbation equations for analysis. We then discuss number of interesting applications like the case where the universe is filled with a mixture of radiation and Gauss-Bonnet fluids as well as dust with Gauss-Bonnet fluids for both short and long-wavelength limits. Considering polynomial f(G) model, we get numerical solutions of energy density perturbations and show that they decay with increase in redshift. This feature shows that under f(G) gravity, specifically under the considered f(G) model, one expects that the formation of the structure in the late Universe is enhanced.