1202.1637 (Spyros Basilakos)

A generalized growth index parametrization: Applications to the $Λ$CDM using the {\em WiggleZ} growth data    [PDF]

Spyros Basilakos

We re-examine the growth index of the concordance $\Lambda$ cosmology in the light of the latest {\em WiggleZ} data. In particular, we investigate five different models for the growth index $\gamma$, by comparing their cosmological evolution using observational data of the growth rate of structure formation at different redshifts, consistently scaled to the WMAP7 cosmology. Using a $\chi^2$ minimization procedure between the different growth rate parametrizations and the data we determine the free parameters of the models and we statistically quantify their ability to represent the observations. Using the combined growth data provided by the 2dF, SDSS and {\em WiggleZ} galaxy surveys and assuming a constant growth index we find that $\gamma=0.616^{+0.088}_{-0.083}$. Based on {\em Wiggle} data alone we obtain $\gamma=0.64^{+0.099}_{-0.094}$ %(within $1\sigma$ errors), which is somewhat greater, and almost $1\sigma$ away, of the theoretically predicted value of $\gamma\simeq 6/11$. Under the assumption that the growth index varies with time we find that the addition of the {\em WiggleZ} growth data in the likelihood analysis improves significantly the statistical results. As an example, in the case of $\gamma(z)=\gamma_{0}+\gamma_{1}z$ we find $\gamma_{0}=0.684^{+0.136}_{-0.124}$ and $\gamma_{1}=-0.331^{+0.507}_{-0.448}$, while without the {\em WiggleZ} we find that the parameters $(\gamma_{0},\gamma_{1})$ remain unconstrained.

View original: http://arxiv.org/abs/1202.1637