Lightish but clumpy: scalar dark matter from inflationary fluctuations

TL;DR

This paper explores how a non-minimally coupled scalar field generated during inflation can produce a clumpy form of dark matter, avoiding isocurvature constraints and matching observed dark matter density.

Contribution

It provides a detailed mechanism for scalar dark matter production via inflationary fluctuations in a minimal non-minimally coupled gravity setup.

Findings

Scalar fluctuations can account for dark matter density.

Dark matter formed is highly clumpy at small scales.

Isocurvature constraints are avoided in this model.

Abstract

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 in fluctuations too small to be observed in the CMB. For masses $\gtrsim {\rm eV}$ 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 $\ell_{\rm today}\sim 10^{4}\,{\rm km}\sqrt{m/{\rm eV}}\sim 10^{-4}\,{\rm AU}\sqrt{m/{\rm eV}}$ fluctuations are large and we therefore expect a rather clumpy nature of this form of dark matter.