Nonminimal superheavy dark matter

Abstract We investigate the gravitational production of superheavy scalar fields with nonminimal coupling during and after inflation. Using the mode function solution in a de Sitter background, we derive analytical approximations and apply the steepest descent method. We study both positive and negative nonminimal couplings, demonstrating that their comoving number density spectra behave differently in the short and long wavelength regimes. We numerically compute the comoving number density spectra and dark matter abundance for a wide range of superheavy spectator fields, with masses exceeding the Hubble scale during inflation, with m χ > H I , and nonminimal couplings ranging from -300 ≤ ξ ≤ 300. When computing the allowed dark matter parameter space, we impose constraints from the maximum reheating temperature, Big Bang nucleosynthesis, and isocurvature perturbations. Our results show that the presence of a positive or negative coupling ξ expands the parameter space up to 3 orders of magnitude above the Hubble inflationary scale, allowing such dark matter candidates to be as heavy as ∼10 16 GeV in the Starobinsky model of inflation.