Large-scale structure in non-standard cosmologies

Abstract

We study the growth of large scale structure in two recently proposed non-standard cosmological models: the brane induced gravity model of Dvali, Gabadadze and Porrati (DGP) and the Cardassian models of Freese and Lewis. A general formalism for calculating the growth of fluctuations in models with a non-standard Friedman equation and a normal continuity equation of energy density is discussed. Both linear and non-linear growth are studied, together with their observational signatures on higher order statistics and abundance of collapsed objects. In general, models which show similar cosmic acceleration at z ≃ 1, can produce quite different normalization for large scale density fluctuations,i.e. σ8, cluster abundance or higher order statistics, such as the normalized skewness S3, which is independent of the linear normalization. For example, for a flat universe with m ≃ 0.22, DGP and standard Cardassian cosmologies predict about 2 and 3 times more clusters respectively than the standard � model at z = 1.5. When normalized to CMB fluctuations the σ8 amplitude turns out to be lower by a few tens of a percent. We also find that, for a limited red-shift range, the linear growth rate can be faster in some models (eg modified polytropic Cardassian with q > 1) than in the Einstein-deSitter universe. The value of the skewness S3 is found to have up to ≃ 10% percent variations (up or down) from model to model.