# F(R) gravity and the Hubble parameter

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1. Feb 12, 2016

Dear all,

Considering Einstein Hilbert lagrangian, by using Einstein field equations one can get the form of Friedman equations and consequently the Hubble parameter.
I know that in f(R) models, Einstein equations get modified. However, what happens to the Friedman equation and the Hubble parameter?
I tried to solve them and get to the form of H, but it seems such a complicated equation.

Using the (00) component, I get

H^2=\frac{8\pi G}{3}\rho -\frac{6\alpha}{c^2}(\frac{\ddot{a}^2}{a^2} + H^4)

What should I do with the
$$\ddot{a}^2$$
in the first equation?!!
The (11) component just makes everything more complicated!!
I really appreciate any help or idea.
BTW, I am using FRW metric.

Last edited: Feb 12, 2016
2. Feb 12, 2016