Primordial Black Hole Formation in a Scalar Field Dominated Universe

Abstract: We present a numerical code that solves the Misner-Sharp system for a spherically symmetric cosmological model containing both a scalar field and a perfect fluid. While the code is capable of exploring general scenarios involving an minimally coupled scalar field and perfect fluid, we focus on the regime where the scalar field dominates the dynamics, particularly in the post-inflationary scalar field-dominated scenario, where the universe is governed by a rapidly oscillating scalar field for a period lasting a few $e$-folds. We analyse the threshold for PBH formation under quadratic and quartic potentials, evolving configurations from superhorizon scales. Our results confirm that a quartic potential behavior is similar to the radiation-dominated universe, resulting in a PBH formation threshold close to the well-established value in radiation backgrounds. Conversely, in the quadratic case, we observe a significant deviation from the expected dust-like behaviour, due to wave-like effects opposing the gravitational collapse. While numerical limitations prevent us from evolving a wide range of initial conditions to determine a precise threshold for PBH formation, our findings suggest that PBH formation may be suppressed with respect to the pure dust scenario, allowing the formation of stable solitonic structures instead. This study highlights the importance of properly accounting for wave dynamics in oscillating scalar fields when characterising PBH formation.