Conformally coupled inflation
arXiv:1309.4900
Abstract
A massive scalar field in a curved spacetime can propagate along the light cone, a causal pathology, which can, in principle, be eliminated only if the scalar couples conformally to the Ricci curvature of spacetime. This property mandates conformal coupling for the field driving inflation in the early universe. During slow-roll inflation, this coupling can cause super-acceleration and, as a signature, a blue spectrum of primordial gravitational waves.
12 pages, no figures, invited paper to appear in "Cosmology with Fluid Components", special issue of "Galaxies" edited by I. Brevik