  Spacetime curvature plays the primary role in general relativity but Einstein
later considered a theory where torsion was the central quantity. Just as the
Einstein-Hilbert action in the Ricci curvature scalar R can be generalized to
f(R) gravity, we consider extensions of teleparallel, or torsion scalar T,
gravity to f(T) theories. The field equations are naturally second order,
avoiding pathologies, and can give rise to cosmic acceleration with unique
features.
