Radouane Gannouji

Galileon gravity and its relevance to late time cosmic acceleration

We consider the covariant Galileon gravity taking into account the third order and fourth order scalar field Lagrangians L{sub 3}({pi}) and L{sub 4}({pi}), consisting of three and four {pi}s with four and five derivatives acting on them, respectively. The background dynamical equations are set up for the system under consideration and the stability of the self-accelerating solution is demonstrated in a general setting. We extended this study to the general case of the fifth order theory. For the spherically symmetric static background, we spell out conditions for the suppression of fifth force effects mediated by the Galileon field {pi}. We study field perturbations in the fixed background and investigate the conditions for their causal propagation. We also briefly discuss metric fluctuations and derive an evolution equation for matter perturbations in Galileon gravity.

On the growth of linear perturbations

Abstract We consider the linear growth of matter perturbations in various dark energy (DE) models. We show the existence of a constraint valid at z = 0 between the background and dark energy parameters and the matter perturbations growth parameters. For ΛCDM γ 0 ′ ≡ d γ d z | 0 lies in a very narrow interval − 0.0195 ⩽ γ 0 ′ ⩽ − 0.0157 for 0.2 ⩽ Ω m , 0 ⩽ 0.35 . Models with a constant equation of state inside General Relativity (GR) are characterized by a quasi-constant γ 0 ′ , for Ω m , 0 = 0.3 for example we have γ 0 ′ ≈ − 0.02 while γ 0 can have a nonnegligible variation. A smoothly varying equation of state inside GR does not produce either | γ 0 ′ | > 0.02 . A measurement of γ ( z ) on small redshifts could help discriminate between various DE models even if their γ 0 is close, a possibility interesting for DE models outside GR for which a significant γ 0 ′ can be obtained.

Modified gravity a la Galileon: Late time cosmic acceleration and observational constraints

In this paper we examine the cosmological consequences of fourth order Galileon gravity. We carry out detailed investigations of the underlying dynamics and demonstrate the stability of one de Sitter phase. The stable de Sitter phase contains a Galileon field {pi} which is an increasing function of time ({pi}>0). Using the required suppression of the fifth force, supernovae, Baryon acoustic oscillations, and CMB data, we constrain parameters of the model. We find that the {pi} matter coupling parameter {beta} is constrained to small numerical values such that {beta}<0.02. We also show that the parameters of the third and fourth order in the action (c{sub 3},c{sub 4}) are not independent and with reasonable assumptions, we obtain constraints on them. We investigate the growth history of the model and find that the subhorizon approximation is not allowed for this model. We demonstrate strong scale dependence of linear perturbations in the fourth order Galileon gravity.

Dispersion of growth of matter perturbations in f ( R ) gravity

We study the growth of matter density perturbations

Neutron stars in the Starobinsky model

{\ensuremath{\delta}}_{m}

Constraining f(R) gravity models with disappearing cosmological constant

for a number of viable

Quasidilaton nonlinear massive gravity: Investigations of background cosmological dynamics

f(R)

Gauss-Bonnet inflation

gravity models that satisfy both cosmological and local gravity constraints, where the Lagrangian density

Background cosmological dynamics in

f

f(R)

is a function of the Ricci scalar