Gravity and Unification: A review

Understand

We review various classical unified theories of gravity and other interactions that have appeared in the literature, paying special attention to scenarios in which spacetime remains four-dimensional, while an "internal" space is enlarged.

• The starting point for each such unification scenario is a particular formalism for General Relativity.
• We thus start by reviewing, besides the usual Einstein-Hilbert and Palatini formulations, the Einstein-Cartan, MacDowell-Mansouri and BF (both non-chiral and chiral) formulations.
• Each of these introduces some version of "internal" bundle and a dynamical variable that ties the internal and tangent bundles.