Thermal relics as hot, warm and cold dark matter in power-law $f(R)$
  gravity
  We investigate the thermal relics as hot, warm and cold dark matter in
$\mathscr{L}=\varepsilon^{2-2\beta}R^\beta+{16\pi}m_{\text{Pl}}^{-2}\mathscr{L}_m$
gravity, where $\varepsilon$ is a constant balancing the dimension of the field
equation, and $1<\beta<(4+\sqrt{6})/5$ for the positivity of energy density and
temperature. If light neutrinos serve as hot/warm relics, the entropic number
of statistical degrees of freedom $g_{*s}$ at freeze-out and thus the predicted
fractional energy density $\Omega_\psi h^2$ are $\beta-$dependent, which
relaxes the standard mass bound $\Sigma m_\nu$. For cold relics, by exactly
solve the simplified Boltzmann equation in both relativistic and
nonrelativistic regimes, we show that the Lee-Weinberg bound for the mass of
heavy neutrinos can be considerably relaxed, and the 'WIMP miracle" for weakly
interacting massive particles (WIMPs) gradually invalidates as $\beta$ deviates
from $\beta=1^+$. The whole framework reduces to become that of GR in the limit
$\beta\to 1^+$.
