Exact Gravitational Gauge Structures and the Dirac Equation
  Exact gauge structures arise in the evolution of spin-1/2 particles in conformally flat space-times. The corresponding Berry potentials can be Abelian or non-Abelian depending on the mass degeneracy of the system considered. Examples include de Sitter universes and maximal acceleration. 
G. Lambiase and G. Papini