A cosmological exclusion plot: towards modelindependent constraints on modified gravity from current and future growth rate data
Abstract
Most cosmological constraints on modified gravity are obtained assuming that the cosmic evolution was standard ΛCDM in the past and that the present matter density and power spectrum normalization are the same as in a ΛCDM model. Here we examine how the constraints change when these assumptions are lifted. We focus in particular on the parameter Y (also called G_{eff}) that quantifies the deviation from the Poisson equation. This parameter can be estimated by comparing with the modelindependent growth rate quantity fσ_{8}(z) obtained through redshift distortions. We reduce the model dependency in evaluating Y by marginalizing over σ_{8} and over the initial conditions, and by absorbing the degenerate parameter Ω_{m,0} into Y. We use all currently available values of fσ_{8}(z). We find that the combination Ŷ=YΩ_{m,0}, assumed constant in the observed redshift range, can be constrained only very weakly by current data, Ŷ=0.28_{0.23}^{+0.35} at 68% c.l. We also forecast the precision of a future estimation of Ŷ in a Euclidlike redshift survey. We find that the future constraints will reduce substantially the uncertainty, Ŷ=0.30_{0.09}^{+0.08} , at 68% c.l., but the relative error on Ŷ around the fiducial remains quite high, of the order of 30%. The main reason for these weak constraints is that Ŷ is strongly degenerate with the initial conditions, so that large or small values of Ŷ are compensated by choosing nonstandard initial values of the derivative of the matter density contrast.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 February 2015
 DOI:
 10.1088/14757516/2015/02/001
 arXiv:
 arXiv:1408.3520
 Bibcode:
 2015JCAP...02..001T
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 15 pages, 8 figures