The stability of the Einstein static state in $f(T)$ gravity
  The stability of the Einstein static universe against the homogeneous scalar
perturbations in $f(T)$ gravity is analyzed. Both the spatial closed and open
universes are considered. We find that the stable Einstein static solutions
exist in both cases. Considering a concrete $f(T)$ model and assuming that the
cosmic energy has a constant equation of state $w$, we obtain that, in the
closed case, $w<1/3$ is required. Thus, $f(T)$ theory gives a larger region of
$w$ than that in general relativity ($-1<w<-1/3$) to have the stable Einstein
static solution. For the open universe, $f(T)$ theory allows the stable
Einstein static solution, although this kind of solution is forbidden in
general relativity. Thus, a modification of gravity can play a crucial role in
stabilizing the Einstein static solution.
