# Constraints on the primordial curvature power spectrum from primordial black holes

## TL;DR

This paper reviews how primordial black holes constrain the early universe's density fluctuations, introducing new observational limits and considering collapse uncertainties to refine the primordial curvature power spectrum constraints.

## Contribution

It presents the most stringent limits from PBHs on the primordial curvature power spectrum, including new observational constraints and collapse considerations.

## Key findings

- New observational constraints on primordial power spectrum amplitude
- Inclusion of uncertainties in collapse threshold and ellipsoidal collapse
- Most stringent PBH-based limits on small-scale primordial fluctuations

## Abstract

Large-amplitude density perturbations may have collapsed during the radiation dominated epoch of the Universe to form primordial black holes (PBHs). There are several constraints to the abundance of PBHs that stem from evaporation or gravitational effects. Due to the connection between primordial perturbations and the formation of PBHs, constraints on the present-day abundance of PBHs can be translated into limits on the primordial curvature power spectrum. We introduce several new observational and forecasted constraints to the amplitude of the primordial power spectrum and incorporate in our analysis uncertainties in the critical overdensity for collapse and considerations of ellipsoidal collapse. Our results provide the most stringent limits from PBHs on the primordial curvature power spectrum on small scales.