Formation and internal structure of superdense dark matter clumps and ultracompact minihaloes

TL;DR

This paper explores the formation, structure, and differences of superdense dark matter clumps and ultracompact minihaloes, emphasizing their origins, evolution, and observational implications in the universe.

Contribution

It systematically analyzes the formation mechanisms and internal structures of SDMC and UCMH, including effects of asphericity on their abundance and signals.

Findings

SDMC form from isocurvature or adiabatic perturbations.

Asphericity reduces SDMC abundance and gamma-ray signals.

SDMC can seed UCMH formation.

Abstract

We discuss the formation mechanisms and structure of the superdense dark matter clumps (SDMC) and ultracompact minihaloes (UCMH), outlining the differences between these types of DM objects. We define as SDMC the gravitationally bounded DM objects which have come into virial equilibrium at the radiation-dominated (RD) stage of the universe evolution. Such objects can be formed from the isocurvature (entropy) density perturbations or from the peaks in the spectrum of curvature (adiabatic) perturbation. The axion miniclusters (Kolb and Tkachev 1994) are the example of the former model. The system of central compact mass (e. g. in the form of SDMC or primordial black hole (PBH)) with the outer DM envelope formed in the process of secondary accretion we refer to as UCMH. Therefore, the SDMC can serve as the seed for the UCMH in some scenarios. Recently, the SDMC and UCMH were considered in the many works, and we try to systematize them here. We consider also the effect of asphericity of the initial density perturbation in the gravitational evolution, which decreases the SDMC amount and, as the result, suppresses the gamma-ray signal from DM annihilation.