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Matter density perturbations and effective gravitational constant in modified gravity models of dark energy

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arxiv 0705.1032 v4 pith:NOJX4TL4 submitted 2007-05-08 astro-ph gr-qchep-phhep-th

Matter density perturbations and effective gravitational constant in modified gravity models of dark energy

classification astro-ph gr-qchep-phhep-th
keywords energydarkdensitymodelsperturbationseinsteingravitylagrangian
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We derive the equation of matter density perturbations on sub-horizon scales for a general Lagrangian density f(R, phi, X) that is a function of a Ricci scalar R, a scalar field phi and a kinetic term X=-(nabla phi)^2/2. This is useful to constrain modified gravity dark energy models from observations of large-scale structure and weak lensing. We obtain the solutions for the matter perturbation delta_m as well as the gravitational potential Phi for some analytically solvable models. In a f(R) dark energy model with the Lagrangian density f(R)=alpha R^{1+m}-Lambda, the growth rates of perturbations exhibit notable differences from those in the standard Einstein gravity unless m is very close to 0. In scalar-tensor models with the Lagrangian density f=F(phi)R+2p(phi,X) we relate the models with coupled dark energy scenarios in the Einstein frame and reproduce the equations of perturbations known in the current literature by making a conformal transformation. We also estimate the evolution of perturbations in both Jordan and Einstein frames when the energy fraction of dark energy is constant during the matter-dominated epoch.

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