Suppressing structure formation at dwarf galaxy scales and below: late kinetic decoupling as a compelling alternative to warm dark matter

TL;DR

This paper explores how late kinetic decoupling of cold dark matter from radiation can suppress small-scale structure formation, offering an alternative to warm dark matter and potentially solving small-scale cosmological problems.

Contribution

It systematically classifies minimal models with dark matter and radiation allowing late kinetic decoupling, revealing new model classes and implications for dark matter self-interactions.

Findings

Identifies models where dark matter remains in thermal equilibrium until keV temperatures.

Shows late kinetic decoupling can suppress small-scale structures similar to warm dark matter.

Suggests models that naturally include dark matter self-interactions, addressing small-scale issues.

Abstract

Warm dark matter cosmologies have been widely studied as an alternative to the cold dark matter paradigm, the characteristic feature being a suppression of structure formation on small cosmological scales. A very similar situation occurs if standard cold dark matter particles are kept in local thermal equilibrium with a, possibly dark, relativistic species until the universe has cooled down to keV temperatures. We perform a systematic phenomenological study of this possibility, and classify all minimal models containing dark matter and an arbitrary radiation component that allow such a late kinetic decoupling. We recover explicit cases recently discussed in the literature and identify new classes of examples that are very interesting from a model-building point of view. In some of these models dark matter is inevitably self-interacting, which is remarkable in view of recent observational support for this possibility. Hence, dark matter models featuring late kinetic decoupling have the potential not only to alleviate the missing satellites problem but also to address other problems of the cosmological concordance model on small scales, in particular the cusp-core and too-big-too-fail problems, in some cases without invoking any additional input.