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

Acausality and Nonunique Evolution in Generalized Teleparallel Gravity