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Matter Power Spectra in Viable $f(R)$ Gravity Models with Massive Neutrinos

We investigate the matter power spectra in the power law and exponential types of viable $f(R)$ theories along with massive neutrinos. The enhancement of the matter power spectrum is found to be a generic feature in these models. In particular, we show that in the former type, such as the Starobinsky model, the spectrum is magnified much larger than the latter one, such as the exponential model. A greater scale of the total neutrino mass, $Σm_ν$, is allowed in the viable $f(R)$ models than that in the $Λ$CDM one. We obtain the constraints on the neutrino masses by using the CosmoMC package with the modified MGCAMB. Explicitly, we get $Σm_ν < 0.451 \ (0.214)\ \mathrm{eV}$ at 95% C.L. in the Starobinsky (exponential) model, while the corresponding one for the $Λ$CDM model is $Σm_ν < 0.200\ \mathrm{eV}$. Furthermore, by treating the effective number of neutrino species $N_{\mathrm{eff}}$ as a free parameter along with $Σm_ν$, we find that $N_{\mathrm{eff}} = 3.78^{+0.64}_{-0.84} (3.47^{+0.74}_{-0.60})$ and $Σm_ν = 0.533^{+0.254}_{-0.411}$ ($< 0.386) \ \mathrm{eV}$ at 95% C.L. in the Starobinsky (exponential) model.