Improved constraints on the primordial power spectrum at small scales from ultracompact minihalos

TL;DR

This paper derives new, stringent constraints on the primordial power spectrum at very small scales by analyzing the abundance and effects of ultracompact minihalos, which are more numerous than primordial black holes.

Contribution

It provides the first comprehensive set of constraints on small-scale primordial density fluctuations using ultracompact minihalos as probes.

Findings

Constraints are especially tight at small scales beyond current observational reach.

UCMHs could significantly impact gamma-ray observations and cosmic reionisation.

The study compiles the best constraints across a wide range of scales.

Abstract

For a Gaussian spectrum of primordial density fluctuations, ultracompact minihalos (UCMHs) of dark matter are expected to be produced in much greater abundance than, e.g., primordial black holes. Forming shortly after matter-radiation equality, these objects would develop very dense and spiky dark matter profiles. In the standard scenario where dark matter consists of thermally-produced, weakly-interacting massive particles, UCMHs could thus appear as highly luminous gamma-ray sources, or leave an imprint in the cosmic microwave background by changing the reionisation history of the Universe. We derive corresponding limits on the cosmic abundance of UCMHs at different epochs, and translate them into constraints on the primordial power spectrum. We find the resulting constraints to be quite severe, especially at length scales much smaller than what can be directly probed by the cosmic microwave background or large-scale structure observations. We use our results to provide an updated compilation of the best available constraints on the power of density fluctuations on all scales, ranging from the present-day horizon to scales more than 20 orders of magnitude smaller.