Lightish but clumpy: scalar dark matter from inflationary fluctuations

It has recently been shown [1] that light vector particles produced from inflationary fluctuations can give rise to the dark matter in the Universe. A similar mechanism has been discussed in [2] for a non-minimally coupled scalar enjoying a Higgs portal interaction. We discuss in detail how such a generation of dark matter works in a minimal setup of a massive scalar non-minimally coupled to gravity. For suitable values of the non-minimal coupling any initial constant value of the field is suppressed during inflation. At the same time, the quantum fluctuations acquired during inflation give rise to a peaked energy density power spectrum. Isocurvature constraints can be avoided since nearly all of the energy is concentrated influctuations too small to be observed in the CMB. For masses greater than or similar to eV and sufficiently high inflation scale the energy contained in these fluctuations is sufficient to account for the cold dark matter of the Universe. At small scales l(today) similar to 10(4) km root m/eV similar to 10(-4) AU root m/eV fluctuations are large and we therefore expect a rather clumpy nature of this form of dark matter.