Non-minimally Coupled Ultra-light Axions as Cold Dark Matter

Abstract

We consider a non-minimally coupled scalar field as a potential cold dark matter candidate. These models are natural extensions of the ultra-light axion (ULA) models which are based on minimally coupled scalar fields. For a non-minimally coupled field, the scalar field energy density behaves as radiation at early times, which yields a bound on the coupling constant, ξ 10, from the primordial nucleosynthesis theory. The first-order perturbations of the non-minimally coupled field with adiabatic initial conditions cause the gravitational potential to decay on large scales. A comparison of the cosmological data with the theoretical matter power spectrum yields the following constraint on the coupling constant: ξ 0.01. We also consider isocurvature modes in our analysis. We argue that a mix of adiabatic and isocurvature initial conditions for a non-minimally coupled scalar field might allow one to obtain the usual adiabatic CDM power spectrum.