Unraveling the effective fluid approach for $f(R)$ models in the sub-horizon approximation. (arXiv:1811.02469v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Arjona_R/0/1/0/all/0/1">Rubén Arjona</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cardona_W/0/1/0/all/0/1">Wilmar Cardona</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nesseris_S/0/1/0/all/0/1">Savvas Nesseris</a>
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.
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.
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