Beyond de Sitter: Longitudinal-Mode Instabilities and Spectral Evolution of Massive Vector Fields with Non-Minimal Curvature Coupling during Inflation

Abstract

We investigate the inflationary dynamics of a spectator massive vector field with a non-minimal curvature coupling in de Sitter (dS) and quasi-de Sitter (qdS) backgrounds, focusing on the transverse and longitudinal sectors. By solving the mode equations with Bunch-Davies initial conditions, we compute the spectral energy densities and quantify the deviations between the exact dS approximation and the slow-roll-corrected qdS evolution. We find that the coupling parameter xi significantly affects the evolution of the effective mass, the mass-crossing condition, and the behavior of the longitudinal mode. In particular, increasing xi suppresses the vector fluctuations and reduces transient tachyonic effects. We further follow the evolution of the modes into the radiation-dominated era, showing how the inflationary initial conditions determine the subsequent spectral energy densities. Our results identify the parameter regime in which the dS approximation provides an accurate description and determine when slow-roll corrections become relevant.