Non-minimally Coupled Ultra-light Axions as Cold Dark Matter
  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, $\xi
\lesssim 10$, 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: $\xi \lesssim 0.01$.
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.
