Dynamics of linear perturbations inf(R)gravity

Abstract

We consider predictions for structure formation from modifications to general relativity in which the Einstein-Hilbert action is replaced by a general function of the Ricci scalar. We work without fixing a gauge, as well as in explicit popular coordinate choices, appropriate for the modification of existing cosmological code. We present the framework in a comprehensive and practical form that can be directly compared to standard perturbation analyses. By considering the full evolution equations, we resolve perceived instabilities previously suggested, and instead find a suppression of perturbations. This result presents significant challenges for agreement with current cosmological structure formation observations. The findings apply to a broad range of forms of fR for which the modification becomes important at low curvatures, disfavoring them in comparison with the CDM scenario. As such, these results provide a powerful method to rule out a wide class of modified gravity models aimed at providing an alternative explanation to the dark energy problem.

Viable Palatini-f(R)cosmologies with generalized dark matter

“…Let us though note that its presence stems from the new effective matter sources peculiar to the particular class of modified gravity models. It has been recently found that such a scale-dependent term is absent in various modified gravity theories within the metric formalism [39,40,41,42].…”

Models off(R)cosmic acceleration that evade solar system tests

“…First, we would like to narrow the parameter choices to yield expansion histories that are observationally viable, i.e. that deviate from ΛCDM in the effective equation of state (15) by no more than |1+ w eff | 0.2 during the acceleration epoch. This equates to choosing a value for the field at the present epoch…”

“…The addition of a non-linear function of the Ricci scalar R to the Einstein-Hilbert action has been demonstrated to cause acceleration for a wide variety of f (R) functions [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,…”

New constraints on parametrised modified gravity from correlations of the CMB with large scale structure

“…This requirement is twofold: at the background level, the predicted expansion history has to be coherent with distance measurements such as those from Supernovae (SNe), the position of the CMB peaks, and baryon acoustic oscillations (BAO); on the other hand, the modified predictions for structure formation need to pass the test of comparison with the observed large scale structure (LSS) of the Universe. It has been shown for several cases -for instance for f (R) [14][15][16][17][18] and DGP [19][20][21][22][23][24] -that the structure formation test can dramatically reduce the parameter space left unconstrained from the background test, and it is therefore instrumental to the quest for a truly viable model.…”