Growth index with the cosmological constant

We obtain the exact analytic form of the growth index at present epoch (a = 1) in a flat universe with the cosmological constant (i.e. the dark energy with its equation of state ωde = −1). For the cosmological constant, we obtain the exact value of the current growth index parameter γ = 0.5547, which is very close to the well known value 6/11. We also obtain the exact analytic solution of the growth factor for ωde = −1/3 or −1. We investigate the growth index and its parameter at any epoch with this exact solution. In addition to this, we are able to find the exact Ω m dependence of those observable quantities. Both growth index and its parameter are quite sensitive to Ω m at z = 0.15, where we are able to use 2dF observation. If we adopt 2dF value of growth index, then we obtain the constrain 0.11 ≤ Ω m ≤ 0.37 for the cosmological constant model. Especially, the growth index is quite sensitive to Ω0m around z ≤ 1. We might be able to obtain interesting observations around this epoch. Thus, the analytic solution for this growth factor provides the very useful tools for future observations to constrain the exact values of observational quantities at any epoch related to growth factor for ωde = −1 or −1/3. The background evolution equations in a flat Friedmann-RobertsonWalker universe (ρm + ρde = ρcr) are ( ȧ a )2 = 8πG 3 (ρm + ρde) = 8πG 3 ρcr , (1) 2 ä a + ( ȧ a )2 = −8πGωdeρde , (2)