How a Gravastar Forms

A gravastar (gravitational vacuum condensate star) is a hypothetical black hole alternative with a dark energy interior and no singularity or event horizon. How one could actually form from gravitational collapse has been an open problem — until recently.

Image: Phys.org - Collapsing stars could spawn mini-universes, offering new path to gravastars

Physicists Daniel Jampolski and Luciano Rezzolla at Goethe University Frankfurt have now published the first model of gravastar formation within standard general relativity, with a preprint on arXiv. Starting from the classic Oppenheimer-Snyder collapse of a uniform dust sphere, they show that a gravastar can form if a de Sitter region (a bubble of dark energy with negative pressure) nucleates at zero size in the center and expands outward. The expansion naturally slows as it approaches the Schwarzschild radius, where it meets the infalling dust surface and settles into static equilibrium.

The outcome depends on initial conditions. Too little energy density or spatial curvature in the de Sitter bubble and the collapse produces a black hole. Too much and the result is a non-equilibrium, non-compact configuration. Gravastar formation sits on the boundary between these two outcomes, requiring fine-tuned but physically plausible parameters.

The paper also derives a compactness limit: if the initial dust sphere has mass-to-radius ratio , collapse to a black hole is inevitable. Below that threshold, a gravastar remains possible.

One open question, noted by Physics Forums users, is what physically triggers the de Sitter bubble — the authors suggest quantum fluctuations or conformal anomaly effects as candidates.

Sources: arXiv, Physical Review D, Physics Forums