Non trivial frames for f(T) theories of gravity and beyond

Understand

Some conceptual issues concerning $f(T)$ theories --a family of modified gravity theories based on absolute parallelism-- are analyzed.

• Due to the lack of local Lorentz invariance, the autoparallel frames satisfying the field equations are evasive to an \emph{a priori} physical understanding.
• We exemplify this point by working out the vierbein (tetrad) fields for closed and open Friedmann-Robertson-Walker cosmologies.