Formation of gravastars

Abstract

Regular black holes and horizonless black hole mimickers offer mathematically consistent alternatives to address the challenges posed by standard black holes. However, the formation mechanism of these alternative objects is still largely unclear and constitutes a significant open problem since understanding their dynamical formation represents a first step to assess their existence. We here investigate, for the first time and without invoking higher-curvature corrections, the dynamical formation of a well-known horizonless black hole mimicker, namely, a gravastar. More specifically, starting from the collapse of a uniform dust sphere as in the case of the Oppenheimer-Snyder collapse, we demonstrate that, under fine-tuned conditions, a gravastar can form from the nucleation and expansion of a de Sitter region with initial zero size at the center of the collapsing sphere. Furthermore, the de Sitter expansion naturally slows down near the Schwarzschild radius, where it meets the collapsing dust surface and gives rise to a static equilibrium. Interestingly, we also find a maximum initial compactness of the collapsing star of $\mathcal{C}= 3/8$, above which the collapse to a black hole is inevitable.