On the dark matter as a geometric effect in $f(\mathcal{R})$ gravity
  A mysterious type of matter is supposed to exist, because the observed
rotational velocity curves of particle moving around the galactic center and
the expected rotational velocity curves do not match. There are also a number
of proposals in the modified gravity for this discrepancy. In this contrast, in
$2008$, B$\ddot{\text{o}}$hmer et al. presented an interesting idea in
(Astropart Phys 29(6):386-392, 2008) where they showed that a $f(\mathcal{R})$
gravity model could actually explain dark matter to be a geometric effect only.
They solved the gravitational field equations in vacuum using generic
$f(\mathcal{R})$ gravity model for constant velocity regions and found that the
resulting modifications in the Einstein-Hilbert Lagrangian is of the form
$\mathcal{R}^{1+m}$, where $m=V_{tg}^2/c^2$; $V_{tg}$ being the tangential
velocity of the test particle moving around galactic dark matter region and,
$c$, the speed of light. From observations it is known that
$m\approx\mathcal{O}(10^{-6})$. In this article, we perform two things (1) We
show that the form of $f(\mathcal{R})$ they claimed is not correct. In doing
the calculations, we found that when the radial component of the metric for
constant velocity regions is a constant then the exact solutions for
$f(\mathcal{R})$ obtained is of the form of $\mathcal{R}^{1-\alpha}$ which
corresponds to a negative correction rather than positive, $\alpha$ is a
function of $m$. (2) We also show that we can not have an analytic solution of
$f(\mathcal{R})$ for all values of tangential velocity including the observed
value of tangential velocity $200-300$Km/s if the radial coefficient of the
metric which describes the dark matter regions is \emph{not a constant}. Thus,
we have to rely on the numerical solutions to get an approximate model for dark
matter in $f(\mathcal{R})$ gravity.
