Unorthodox curvature interactions, black holes, and inflation (PDF) by Ron K. Unz
Physical Review D, December 15, 1984, pp. 2521-2527
The coupling of other fields to gravity is not uniquely determined if one removes the requirement that such interactions maintain gauge invariance. This paper discusses the possible consequences of such non-gauge-invariant couplings to gravity, and some of the possible implications for black holes and inflation.