1306.2683 (Lixin Xu)

Growth Index after Planck    [PDF]

Lixin Xu

To investigate the possible deviation from the standard $\Lambda$CDM model and the Einstein's gravity theory in the dynamical perspectives, the growth index $\gamma_L$ was proposed. Recently, thanks to the measurement of the cosmic growth rate via the redshift-space distortion, one can understand the evolution of density contrast through $f\sigma_8(z)$, where $f(z)=d\ln \delta/d \ln a$ is the growth rate of matter and $\sigma_8(z)$ is the rms amplitude of the density contrast $\delta$ at the comoving $8h^{-1}$ Mpc scale. In this paper, we use the red-shift space distortion data points to investigate the growth index on the bases of the Einstein's gravity theory and a modified gravity theory under the assumption $f=\Omega_m(a)^{\gamma_L}$. To fix the background evolution, the cosmic observational data points from the type Ia supernovae SNLS3, cosmic microwave background radiation from {\it Planck} and baryon acoustic oscillation are used. Via the Markov Chain Monte Carlo method, the $\gamma_L$ values were obtained for the Einstein's gravity and a modified gravity theory in $1,2,3\sigma$ regions respectively: $0.676_{- 0.0667- 0.119- 0.153}^{+ 0.0603+0.127+0.170}$ and $0.555_{-0.0168- 0.0374-0.0532}^{+0.0190+0.0338+0.0436}$. In the Einstein's gravity theory, the values of growth index $\gamma_L$ show almost $2\sigma$ deviation from the theoretical prediction 6/11. However in the modified gravity framework, a deviation from the Einstein's relativity isn't probed in $1\sigma$ region.

View original: http://arxiv.org/abs/1306.2683