G\"{o}del-type universes in f(R) gravity
  The $f(R)$ gravity theories provide an alternative way to explain the current
cosmic acceleration without a dark energy matter component. If gravity is
governed by a $f(R)$ theory a number of issues should be reexamined in this
framework, including the violation of causality problem on nonlocal scale. We
examine the question as to whether the $f(R)$ gravity theories permit
space-times in which the causality is violated. We show that the field
equations of these $f(R)$ gravity theories do not exclude solutions with
breakdown of causality for a physically well-motivated perfect-fluid matter
content. We demonstrate that every perfect-fluid G\"{o}del-type solution of a
generic $f(R)$ gravity satisfying the condition $df/dR > 0$ is necessarily
isometric to the G\"odel geometry, and therefore presents violation of
causality. This result extends a theorem on G\"{o}del-type models, which has
been established in the context of general relativity. We also derive an
expression for the critical radius $r_c$ (beyond which the causality is
violated) for an arbitrary $f(R)$ theory, making apparent that the violation of
causality depends on both the $f(R)$ gravity theory and the matter content. As
an illustration, we concretely take a recent $f(R)$ gravity theory that is free
from singularities of the Ricci scalar and is cosmologically viable, and show
that this theory accommodates noncausal as well as causal G\"odel-type
solutions.
