Modified Dust and the Small Scale Crisis in CDM

TL;DR

This paper introduces Modified Dust, an extension of dark matter models with higher derivative terms, which suppresses small-scale power and may address issues like the missing satellites and core-cusp problems in cosmology.

Contribution

The paper proposes a new Modified Dust model with non-zero sound speed, analyzing its linear perturbation evolution and potential to solve small-scale structure problems.

Findings

Suppression of power spectrum at large momenta

Enhanced perturbations at very short scales in radiation era

Potential to address small-scale structure issues

Abstract

At large scales and for sufficiently early times, dark matter is described as a pressureless perfect fluid---dust---non-interacting with Standard Model fields. These features are captured by a simple model with two scalars: a Lagrange multiplier and another playing the role of the velocity potential. That model arises naturally in some gravitational frameworks, e.g., the mimetic dark matter scenario. We consider an extension of the model by means of higher derivative terms, such that the dust solutions are preserved at the background level, but there is a non-zero sound speed at the linear level. We associate this {\it Modified Dust} with dark matter, and study the linear evolution of cosmological perturbations in that picture. The most prominent effect is the suppression of their power spectrum for sufficiently large cosmological momenta. This can be relevant in view of the problems that cold dark matter faces at sub-galactic scales, e.g., the missing satellites problem. At even shorter scales, however, perturbations of Modified Dust are enhanced compared to the predictions of more common particle dark matter scenarios. This is a peculiarity of their evolution in radiation dominated background. We also briefly discuss clustering of Modified Dust. We write the system of equations in the Newtonian limit, and sketch the possible mechanism which could prevent the appearance of caustic singularities. The same mechanism may be relevant in light of the core-cusp problem.