Primordial black hole formation from massless scalar isocurvature

TL;DR

This study numerically investigates primordial black hole formation driven by isocurvature perturbations of a massless scalar field in a radiation universe, revealing collapse conditions and late-time fluid dominance.

Contribution

It provides the first numerical analysis of PBH formation from scalar isocurvature perturbations, including second-order solutions and collapse criteria.

Findings

PBHs form when scalar field amplitude exceeds a threshold

Late-time collapse dynamics are dominated by fluid behavior

Scalar field influences initial conditions but not final collapse

Abstract

We numerically study the primordial black hole (PBH) formation by an isocurvature perturbation of a massless scalar field on super Hubble scales in the radiation-dominated universe. As a first step we perform simulations of spherically symmetric configurations. For the initial condition, we employ the spatial gradient expansion and provide the general form of the growing mode solutions valid up through the second order in this expansion. The initial scalar field profile is assumed to be Gaussian with a characteristic comoving wavenumber $k$; $\sim\exp(-k^2R^2)$, where $R$ is the radial coordinate. We find that a PBH is formed for a sufficiently large amplitude of the scalar field profile. Nevertheless, we find that the late time behavior of the gravitational collapse is dominated by the dynamics of the fluid but not by the scalar field, which is analogous to the PBH formation from an adiabatic perturbation in the radiation-dominated universe.