http://arxiv.org/abs/1811.02469
We provide explicit formulae for the effective fluid approach of $f(R)$ theories, such as the Hu-Sawicki and the designer models. Using the latter and simple modifications to the CLASS code, which we call EFCLASS, in conjunction to very accurate analytic approximations for the background evolution, we obtain competitive results in a much simpler and less error-prone approach. We also derive the initial conditions in matter domination and we find they differ from those already found in the literature for a constant $w$ model, e.g., the designer model even though it has $w=-1$ in the background, it has dark energy perturbations nonetheless. We also use the aforementioned models to derive constraints from the latest cosmological data, including supernovae, BAO, CMB, $H(z)$ and growth-rate data, and we find they are statistically consistent to the $\Lambda$CDM model. Finally, we find that the viscosity parameter $c_{vis}^2$ in realistic models is not constant as commonly assumed, but rather evolves significantly over several orders of magnitude, something which could affect forecasts of upcoming surveys.
R. Arjona, W. Cardona and S. Nesseris
Wed, 7 Nov 18
71/94
Comments: 23 pages, 12 figures, 5 tables. Comments welcome
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