Acausality and Nonunique Evolution in Generalized Teleparallel Gravity
  We show the existence of physical superluminal modes and acausality in the
Brans-Dicke type of extension of teleparallel gravity that includes F(T)
gravity and teleparallel dark energy as special cases. We derive the
characteristic hypersurface for the extra degrees of freedom in the theory,
thereby showing that the time evolution is not unique and closed causal curves
can appear. Furthermore, we present a concrete disastrous solution in Bianchi
type I spacetime, in which the anisotropy in expansion can be any function of
time, and thus anisotropy can emerge suddenly, a simple demonstration that the
theory is physically problematic.
