Formation of gravastars

[Sagittarius A-Star]

TL;DR

Big Bang inside a Star: How a Gravastar forms

The following paper was just published:

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 𝒞 =3/8, above which the collapse to a black hole is inevitable.

Source:
https://journals.aps.org/prd/abstract/10.1103/c6lw-nx7k

preprint archive:
https://arxiv.org/abs/2509.15302

via:
https://www.uni-frankfurt.de/en/new...nern-eines-sterns-wie-ein-gravastern-entsteht

 

[berkeman]

Wow, weird stuff. https://en.wikipedia.org/wiki/Gravastar

 

[Sagittarius A-Star]

From one of the authors of the paper:

Video: Seminar entitled "Three exotic (but not crazy) ideas in classical gravity", given by Luciano Rezzolla (Goethe University Frankfurt, Germany), September 24th, 2025, during the VII Amazonian Symposium on Physics:

 

[Ibix]

I'm not quite sure where the de Sitter region comes from. The FLRW region is the star itself, but they just seem to add the de Sitter region by hand. They argue that it's plausible by integrating backwards from their final state and showing that the de Sitter region goes to a zero size volume, but I'm not clear on the causation before that event. Perhaps I missed something.

 

[Sagittarius A-Star]

they just seem to add the de Sitter region by hand. ... Perhaps I missed something.

The paper does not describe, how a dark energy vacuum region is created by the collapse. They refer to other papers:

In our case, since we eventually want to obtain a static gravastar, we need to add an expanding de Sitter region inside the collapsing FLRW spacetime, noting that the appearance of a de Sitter solution represents a common feature of most “bouncing” solutions in gravitational collapse (see, e.g., [11–13, 15]).

sources 11 to 13:
https://arxiv.org/abs/2009.12057
https://arxiv.org/abs/2301.01309
https://arxiv.org/abs/2309.14912

source 15:
https://arxiv.org/abs/2412.02742

 

[Sagittarius A-Star]

The following video links (at t=26m14s) Gravastars to a (controversial) paper, which was discussed here in PF.

 

[PAllen]

Hey, Luciano Rezzola is one of my favorite GR researchers! I’ll want to take a close look at these.

 

[sbrothy]

Wow, weird stuff. https://en.wikipedia.org/wiki/Gravastar

Wow, yeah. I was about to it call it bunk but if you people think there's something there I guess I'll have to take it seriously. Weird indeed!

 

[sbrothy]

The paper does not describe, how a dark energy vacuum region is created by the collapse. They refer to other papers:



sources 11 to 13:
https://arxiv.org/abs/2009.12057
https://arxiv.org/abs/2301.01309
https://arxiv.org/abs/2309.14912

source 15:
https://arxiv.org/abs/2412.02742

I'll have to spare my likes because most of this stuff goes over my head, but the universe have shown us it's weird before, so why not now?

 

[Sagittarius A-Star]

Hey, Luciano Rezzola is one of my favorite GR researchers! I’ll want to take a close look at these.

I found an older paper from him:

we conclude it is not possible to model the measured ringdown of GW150914 as due to a rotating gravastar.
...
We conclude with two final remarks. First, there is no contradiction between our results and those of Ref. [32] as our conclusions refer to gravastars that are thick and have a surface at a small but not infinitesimal distance from the putative event horizon.

Source:
https://arxiv.org/abs/1602.08759

The reference [32] is:

In other words, universal ringdown waveforms indicate the presence of light rings, rather than of horizons.

Source:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.171101