arXiv subjects

Radouane Gannouji

Publications and source records attributed to Radouane Gannouji.

44 records · Page 3Linked to original sources

arXiv2010

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_3(π) and L_4(π) consisting of three and four $π$'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 general setting. We extended this study to the general case of the fifth order theory. For spherically symmetric static background, we spell out conditions for suppression of fifth force effects mediated by the galileon field $π$. We study the field perturbations in the fixed background and investigate conditions for their causal propagation. We also briefly discuss metric fluctuations and derive evolution equation for matter perturbations in galileon gravity.

gr-qc↗

arXiv2010

Background cosmological dynamics in $f(R)$ gravity and observational constraints

In this paper, we carry out a study of viable cosmological models in $f(R)$-gravity at the background level. We use observable parameters like $Ω$ and $γ$ to form autonomous system of equations and show that the models under consideration exhibit two different regimes in their time evolution, namely, a phantom phase followed by a quintessence like behavior. We employ statefinder parameters to emphasize a characteristic discriminative signature of these models.

astro-ph.CO↗

arXiv2009

The dispersion of growth of matter perturbations in f(R) gravity

We study the growth of matter density perturbations delta_m for a number of viable f(R) gravity models that satisfy both cosmological and local gravity constraints, where the Lagrangian density f is a function of the Ricci scalar R. If the parameter m=Rf_{,RR}/f_{,R} today is larger than the order of 10^{-6}, linear perturbations relevant to the matter power spectrum evolve with a growth rate s=d (ln delta_m)/d (ln a) (a is the scale factor) that is larger than in the LCDM model. We find the window in the free parameter space of our models for which spatial dispersion of the growth index gamma_0= gamma(z=0) (z is the redshift) appears in the range of values 0.40< gamma_0<0.55, as well as the region in parameter space for which there is essentially no dispersion and gamma_0 converges to values around 0.40<gamma_0<0.43. These latter values are much lower than in the LCDM model. We show that these unusual dispersed or converged spectra are present in most of the viable f(R) models with m(z=0) larger than the order of 10^{-6}. These properties will be essential in the quest for f(R) modified gravity models using future high-precision observations and they confirm the possibility to distinguish clearly most of these models from the LCDM model.

astro-ph.CO↗

arXiv2009

Dispersion in the growth of matter perturbations

We consider the linear growth of matter perturbations on low redshifts in modified gravity Dark Energy (DE) models where G_eff(z,k) is explicitly scale-dependent. Dispersion in the growth today will only appear for scales of the order the critical scale ~ λ_{c,0}, the range of the fifth-force today. We generalize the constraint equation satisfied by the parameters γ_0(k) and γ'_0(k) \equiv \frac{dγ(z,k)}{dz}(z=0) to models with G_{eff,0}(k) \ne G. Measurement of γ_0(k) and γ'_0(k) on several scales can provide information about λ_{c,0}. In the absence of dispersion when λ_{c,0} is large compared to the probed scales, measurement of γ_0 and γ'_0 provides a consistency check independent of λ_{c,0}. This applies in particular to results obtained earlier for a viable f(R) model.

astro-ph.CO↗

arXiv2008

The growth of matter perturbations in some scalar-tensor DE models

We consider asymptotically stable scalar-tensor dark energy (DE) models for which the equation of state parameter $w_{DE}$ tends to zero in the past. The viable models are of the phantom type today, however this phantomness is milder than in General Relativity if we take into account the varying gravitational constant when dealing with the SNIa data. We study further the growth of matter perturbations and we find a scaling behaviour on large redshifts which could provide an important constraint. In particular the growth of matter perturbations on large redshifts in our scalar-tensor models is close to the standard behaviour $δ_m \propto a$, while it is substantially different for the best-fit model in General Relativity for the same parametrization of the background expansion. As for the growth of matter perturbations on small redshifts, we show that in these models the parameter $γ'_0\equiv γ'(z=0)$ can take absolute values much larger than in models inside General Relativity. Assuming a constant $γ$ when $γ'_0$ is large would lead to a poor fit of the growth function $f$. This provides another characteristic discriminative signature for these models.

astro-ph↗

arXiv2007

On the growth of linear perturbations

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 $Λ$CDM $γ'_0\equiv \frac{dγ}{dz}_0$ lies in a very narrow interval $-0.0195 \le γ'_0 \le -0.0157$ for $0.2 \le Ω_{m,0}\le 0.35$. 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 $γ'_0\approx -0.02$ 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.

astro-ph↗

arXiv2007

Conditions for the cosmological viability of f(R) dark energy models

We derive the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. We show that the cosmological behavior of f(R) models can be understood by a geometrical approach consisting in studying the m(r) curve on the (r, m) plane, where m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f with f_{,R}=df/dR. This allows us to classify the f(R) models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. The existence of a viable matter dominated epoch prior to a late-time acceleration requires that the variable m satisfies the conditions m(r) approx+0 and dm/dr>-1 at r approx-1. For the existence of a viable late-time acceleration we require instead either (i) m=-r-1, (sqrt{3}-1)/2 0 and n<-1 and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor a propto t^{1/2}. We also find that f(R) models can have a strongly phantom attractor but in this case there is no acceptable matter era.

gr-qc↗

arXiv2006

Scalar-Tensor Models of Normal and Phantom Dark Energy

We consider the viability of dark energy (DE) models in the framework of the scalar-tensor theory of gravity, including the possibility to have a phantom DE at small redshifts $z$ as admitted by supernova luminosity-distance data. For small $z$, the generic solution for these models is constructed in the form of a power series in $z$ without any approximation. Necessary constraints for DE to be phantom today and to cross the phantom divide line $p=-ρ$ at small $z$ are presented. Considering the Solar System constraints, we find for the post-Newtonian parameters that $γ_{PN}<1$ and $γ_{PN,0}\approx 1$ for the model to be viable, and $β_{PN,0}>1$ (but very close to 1) if the model has a significantly phantom DE today. However, prospects to establish the phantom behaviour of DE are much better with cosmological data than with Solar System experiments. Earlier obtained results for a $Λ$-dominated universe with the vanishing scalar field potential are extended to a more general DE equation of state confirming that the cosmological evolution of these models rule them out. Models of currently fantom DE which are viable for small $z$ can be easily constructed with a constant potential; however, they generically become singular at some higher $z$. With a growing potential, viable models exist up to an arbitrary high redshift.

astro-ph↗