Critical collapse and black hole formation within an expanding perfect fluid

TL;DR

This paper investigates critical collapse and black hole formation in an expanding universe filled with a perfect fluid of varying sound speed, exploring different equations of state and the effects of a cosmological constant.

Contribution

It extends previous primordial black hole formation studies by analyzing the impact of different fluid equations of state and vacuum energy on critical collapse phenomena.

Findings

Critical collapse behavior is consistent across various w values.

Refined numerical code confirms previous results with improved accuracy.

The nature of critical collapse remains unchanged for different fluid parameters.

Abstract

Following on after three previous papers discussing the formation of primordial black holes in the early universe during the radiation dominated era, we present here related results considering the theoretical possibility of having a fluid with a different sound speed. In practice we vary the parameter w of the equation of state p = we between 0 and 1, where e is the energy density, and p the pressure of the fluid. We consider also the possibility of having a cosmological constant interpreted as a vacuum energy component (corresponding to w = -1) revising the results presented in our first paper of this series, having now a more refined numerical code available. To perform our simulations we have fine tuned our previous computer code allowing us to observe that the nature of the critical collapse is conserved for all the different values of w considered, in agreement with previous studies of ideal collapses done in a different context from PBHs. As already done in our previous works, we start the simulations with initial supra-horizon scale perturbations of a type which could have come from inflation, characterized by a pure growing mode component.