New thresholds for Primordial Black Hole formation during the QCD phase transition

TL;DR

This paper investigates how the critical density threshold for primordial black hole formation decreases during the QCD phase transition, potentially increasing PBH formation probability in the early universe.

Contribution

It models the evolution of the formation threshold during the QCD transition using three different models, revealing significant reductions that enhance PBH formation chances.

Findings

Threshold reduction up to 77% in the Bag Model

Potential PBH formation probability of ~10^{-10} during QCD epoch

Different models show varying impacts on the formation threshold

Abstract

Primordial Black Holes (PBHs) might have formed in the early Universe as a consequence of the collapse of density fluctuations with an amplitude above a critical value $\delta_{c}$: the formation threshold. Although for a radiation-dominated Universe $\delta_{c}$ remains constant, if the Universe experiences some dust-like phases (e.g. phase transitions) $\delta_{c}$ might decrease, improving the chances of PBH formation. We studied the evolution of $\delta_{c}$ during the QCD phase transition epoch within three different models: Bag Model (BM), Lattice Fit Model (LFM), and Crossover Model (CM). We found that the reduction on the background value of $\delta_{c}$ can be as high as $77\%$ (BM), which might imply a $\sim10^{-10}$ probability of PBHs forming at the QCD epoch.