Einstein’s other gravity and the acceleration of the Universe

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.