<p>A regular black hole has a Schwarzschild metric &quot;all the way in&quot;.  This has a couple of interesting features, one is a spherical event horizon, which separates two regions of spacetime (inside and outside) so that events &quot;inside&quot; cannot pass information to the &quot;outside&quot;.  And a singularity at r=0. That's a bit of an embarrassment to the theory, since singularities might not be physically &quot;real&quot;.</p>
<p>So finding an alternative is of interest to many people.  The Gravitational Condensate Star is one such proposal.</p>
<p>The proposal of Mazur and Mottola (from 2006) is that the interior of a collapsing star should develop negative pressure (in a way that they analogise to the the formation of a Bose-Einstein condensate) and form a region of spacetime that is expanding:  a de-Sitter solution. In this region the pressure = -density.  This region prevents matter from forming an event horizon and has no singularity.  Matter that falls towards the star, instead of falling past an event horizon, instead forms a thin shell at a radius that is just more than the Schwarzschild event horizon. In the shell, pressure = density.  Beyond this dense shell is the vacuum of space, and it has a Schwarzschild metric (density=pressure=0).  There's nothing special about the Schwarzschild metric beyond the event horizon, the spacetime around the Earth is well approximated by this metric, at a large radius, it is well approximated by Newtonian gravity.</p>
<p>The de-Sitter metric inside a gravatar would have to be very strong, since the density of a collapsed star is very high, the negative pressure is very negative, far more so than the de-Sitter spacetime of the expanding universe.</p>
<p>However there is very limited evidence for the existence of gravatars. It was hoped that Ligo and other gravitational wave detectors might be able to distinguish them from regular black holes, but the results of Ligo are consistent with black holes. Moreover the mechanism for the formation of a region of negative pressure is speculative, it doesn't correspond to anything observed. So at the moment, gravatars are probably best considered &quot;interesting theory&quot; rather than &quot;probably true&quot;.</p>
<p>If you want more maths, you can read the paper <a href="https://www.mdpi.com/2218-1997/9/2/88" rel="noreferrer">Gravitational Condensate Stars: An Alternative to Black Holes
by Pawel O. Mazur and Emil Mottola </a></p>

CC BY-SA 4.0
2024-05-28T16:02:07.513Z
"However there is very limited evidence for the existence of gravatars." That is a very generous way of saying, "no evidence whatsoever".
TimRias
CC BY-SA 4.0
2024-05-28T11:10:41.417Z
2024-05-28T11:10:41.417Z
James K
<p>The Wikipedia article on gravastar says that it has a 'regular' black hole metric (Schwarzschild, I presume?) on the outside, but a de Sitter metric on the inside....</p>
<p>What exactly does that mean?</p>
<p>Don't all black holes, and our universe itself, have a de Sitter metric?
Isn't that why people criticize AdS/CFT correspondence for being unrealistic, because our universe and its black holes are de Sitter?</p>

CC BY-SA 4.0
2024-05-28T16:56:51.847Z
related https://astronomy.stackexchange.com/questions/38690/what-are-geodes https://astronomy.stackexchange.com/questions/25449/is-a-gravastar-with-no-horizon-with-ultracompact-and-extreme-density-possible
Mithoron
<black-hole><gravity><cosmology><general-relativity>
What exactly is a 'gravastar'? How is it different from a 'regular' black hole?
Kurt Hikes
# What exactly is a 'gravastar'? How is it different from a 'regular' black hole?

(Asked by: username_0 on 2024-05-27)

The Wikipedia article on gravastar says that it has a 'regular' black hole metric (Schwarzschild, I presume?) on the outside, but a de Sitter metric on the inside....

What exactly does that mean?

Don't all black holes, and our universe itself, have a de Sitter metric?
Isn't that why people criticize AdS/CFT correspondence for being unrealistic, because our universe and its black holes are de Sitter?

**Tags:** black-hole, gravity, cosmology, general-relativity
> **Comments:**
> **username_1**: related https://astronomy.stackexchange.com/questions/38690/what-are-geodes https://astronomy.stackexchange.com/questions/25449/is-a-gravastar-with-no-horizon-with-ultracompact-and-extreme-density-possible  

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## 1 answers

### Answer by username_2 on 2024-05-28. Score: 8

A regular black hole has a Schwarzschild metric "all the way in". This has a couple of interesting features, one is a spherical event horizon, which separates two regions of spacetime (inside and outside) so that events "inside" cannot pass information to the "outside". And a singularity at r=0. That's a bit of an embarrassment to the theory, since singularities might not be physically "real".

So finding an alternative is of interest to many people. The Gravitational Condensate Star is one such proposal.

The proposal of Mazur and Mottola (from 2006) is that the interior of a collapsing star should develop negative pressure (in a way that they analogise to the the formation of a Bose-Einstein condensate) and form a region of spacetime that is expanding: a de-Sitter solution. In this region the pressure = -density. This region prevents matter from forming an event horizon and has no singularity. Matter that falls towards the star, instead of falling past an event horizon, instead forms a thin shell at a radius that is just more than the Schwarzschild event horizon. In the shell, pressure = density. Beyond this dense shell is the vacuum of space, and it has a Schwarzschild metric (density=pressure=0). There's nothing special about the Schwarzschild metric beyond the event horizon, the spacetime around the Earth is well approximated by this metric, at a large radius, it is well approximated by Newtonian gravity.

The de-Sitter metric inside a gravatar would have to be very strong, since the density of a collapsed star is very high, the negative pressure is very negative, far more so than the de-Sitter spacetime of the expanding universe.

However there is very limited evidence for the existence of gravatars. It was hoped that Ligo and other gravitational wave detectors might be able to distinguish them from regular black holes, but the results of Ligo are consistent with black holes. Moreover the mechanism for the formation of a region of negative pressure is speculative, it doesn't correspond to anything observed. So at the moment, gravatars are probably best considered "interesting theory" rather than "probably true".

If you want more maths, you can read the paper [Gravitational Condensate Stars: An Alternative to Black Holes
by Pawel O. Mazur and Emil Mottola](https://www.mdpi.com/2218-1997/9/2/88)

> **Comments:**
> **username_3**: "However there is very limited evidence for the existence of gravatars." That is a very generous way of saying, "no evidence whatsoever".  

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