Kurzgesagt – In a Nutshell 

Sources – Gravastars

We thank our experts for their feedback:
 

• Emil Mottola

Professor of Physics and Astronomy at New Mexico University

• Pawel O. Mazur

Professor of Physics and Astronomy at the University of South Carolina

—The star's shell rushes in, bounces against the collapsing core and explodes, shining brighter than whole galaxies. 

#Behte, Hans A.; Brown, Gerald (1985):  “How a Supernova Explodes” Scientific American, vol. 252, 5, 60-87

https://astro.uconn.edu/wp-content/uploads/sites/2830/2020/10/Bethe_Brown_How-a-Supernova-Explodes.pdf 

Quote:  “Collapse of the stellar core begins when the mass of iron exceeds the Chandrasekhar mass, which is between 1.2 and 1.5 solar masses. At this point the pressure of electrons can no longer resist gravitational contraction. Early in the collapse the inward movement is accelerated by electron capture, which converts a proton and an electron into a neutron and a neutrino. The loss of the electron reduces the electron pressure and hence the Chandrasekhar mass. When the density reaches 4 X 1011 grams per cubic centimeter, matter becomes opaque to neutrinos, which are therefore trapped in the core. By this time the Chandrasekhar mass is less than one solar mass and its significance has also changed: it is now the largest mass that can collapse homologously, or as a unit. When the collapse is complete, the central part of the homologous core is converted into nuclear matter. The nuclear matter is compressed beyond its equilibrium density and then rebounds, launching a powerful shock wave. As the shock wave plows through the outer core, iron nuclei "evaporate" to form a gas of nucleons.”

The finer details of the mechanism of a stellar-collapse supernova are an area of active research:

#Burrows, Adam; Vartanyan, David (2021):  “Core-Collapse Supernova Explosion Theory”, Nature, vol. 589, 29–39

https://www.nature.com/articles/s41586-020-03059-w
https://arxiv.org/abs/2009.14157
Quote:  “The supernova explosions of these massive stars, the so-called core-collapse supernovae (CCSNe), have been theoretically studied for more than half a century and observationally studied even longer. Yet, the mechanism of their explosion has only recently come into sharp focus. A white dwarf is birthed in these stars as well, but before their outer envelope can be ejected this white dwarf achieves the “Chandrasekhar mass”1 near ∼1.5 M. This mass is gravitationally unstable to implosion. After a life of perhaps ∼10-40 million years, the dense core of this star implodes within less than a second to neutron-star densities, at which point it rebounds like a spherical piston, generating a shock wave in the outer imploding core.

 [...]

What has emerged recently in the modern era of CCSN theory is that the structure of the progenitor star, turbulence and symmetry-breaking in the core after bounce, and the details of the neutrino-matter interaction are all key and determinative of the outcome of collapse.”

Supernovas can have luminosities comparable to those of entire galaxies:

#NASA/ESA Hubble Wordbank: “Supernova” (retrieved 2024)

https://esahubble.org/wordbank/supernova/ 

Quote: “As the core collapses, the outer layers are blasted outwards in a supernova, the biggest explosion known to occur in the Universe. At its peak, a supernova can be brighter than an entire galaxy and can reach a diameter several light-years across.”

—Depending on how massive the star was, there are two possible outcomes – either the core compresses into a super dense neutron star, or it kind of breaks reality and collapses into a singularity: an infinitely dense point with no size or dimensions at all. 

Depending on the initial mass and, more weakly, the metallicity of a star with an initial mass above 9 solar masses, it is generally considered it can end up as a neutron star or undergo “complete” gravitational collapse and become a black hole. In this video, we comment on an alternative on what could happen to stars that undergo “complete” gravitational collapse. 

#Heger, Alexander et al. (2003): “How Massive Single Stars End Their Life” The Astrophysical Journal, vol. 591, 1, 288

https://iopscience.iop.org/article/10.1086/375341/meta 

—A place where the laws of the universe stop making sense and time and space are reversed – a black hole. 

In a certain sense, the roles of space an time are swapped inside a black hole

#Max Planck Institute for Gravitational Physics: “Changing places – space and time inside a black hole”, Einstein Online (retrieved 2024)
https://www.einstein-online.info/en/spotlight/changing_places/ 

—Gravastars are a third, even weirder option.

#Mazur, Pawel O.; Mottola, Emil (2004):  “Gravitational Vacuum Condensate Stars” Proceedings of the National Academy of Sciences, vol. 101, 26, 9545-9550

https://www.pnas.org/doi/epdf/10.1073/pnas.0402717101
https://www.mdpi.com/2218-1997/9/2/88#fn002-universe-09-00088  

Quote: “A new final endpoint of complete gravitational collapse is proposed. By extending the concept of Bose–Einstein condensation to gravitational systems, a static, spherically symmetric solution to Einstein’s equations is obtained, characterized by an interior de Sitter region of 𝑝=−𝜌 gravitational vacuum condensate and an exterior Schwarzschild geometry of arbitrary total mass M. These are separated by a phase boundary with a small but finite thickness ℓ, replacing both the Schwarzschild and de Sitter classical horizons. The resulting collapsed cold, compact object has no singularities, no event horizons, and a globally defined Killing time.”

#Mottola, Emil (2023): “Gravitational Vacuum Condensate Stars”, Ch. 8 of “ Regular Black Holes: Towards a New Paradigm of Gravitational Collapse”, edited by Cosimo Bambi, Springer, 283-352. 

https://arxiv.org/abs/2302.09690 

Quote: “Gravitational vacuum condensate stars, proposed as the endpoint of gravitational collapse consistent with quantum theory, are reviewed. Gravastars are cold, low entropy, maximally compact objects characterized by a surface boundary layer and physical surface tension, instead of an event horizon.”

—Atoms and particles are crushed so hard that they transform into pure energy. A sort of mini universe if you want – and just like our universe, this bubble violently wants to expand and grow. 

As the exterior of the star collapses, a bubble of infinite pressure forms at its center and grows outwards. The matter inside this bubble of infinite pressure no longer acts like ordinary matter and instead stretches space like dark energy. 

#Mazur, Pawel O.; Mottola, Emil (2015):  “Surface Tension and Negative Pressure Interior of a Non-Singular ‘Black Hole’” Classical and Quantum Gravity,, vol. 32, 21 https://iopscience.iop.org/article/10.1088/0264-9381/32/21/215024 

https://arxiv.org/abs/1501.03806 

Quote: “As the star is compressed further and its radius approaches the Schwarzschild radius R → Rs+  from outside, (2.22) shows that the radius of the sphere where the pressure diverges and f(R0) = 0 moves from the origin to the outer edge of the star, i.e. R0 → Rs- , and in that limit the interior solution with negative pressure comes to encompass the entire interior region 0 ≤ r < R, excluding only the outer boundary at R = Rs. Finally, and most remarkably of all, since in this limit H2R2 = Rs/R → 1 inspection of (2.18) shows that the entire interior solution then has constant negative pressure

p = −ρ , for r < R = R0 = Rs = 2GM (2.23)

 with

 f(r) = 1/4 (1 − H2r2 ) = 1/4 h(r) = 1/4 (1 − r2 /Rs2)  , H = 1/ Rs (2.24)

corresponding to a patch of pure de Sitter space in static coordinates.” 

—In a fraction of second, the bubble smashes into the collapsing star around it. 

We thank Professor Pawel O. Mazur for the following comment on the scale of time it takes to form a gravastar:

Quote: “[T]he Schwarzschild radius of [a] 10 solar masses super-compact completely collapsed object will be 30 km. The speed of light is 300,000 km per second. The crossing time is 30/300,000 seconds or 10-4 seconds. This number sets the time scale  for how fast the gravastar forms.”

—Just like black holes, a gravatar can have any mass, but a typical one would  be about the size of the London metropolitan area and as massive as 10 suns. 

Typical black holes formed by the collapse of a star have masses between 3 and 20 times the mass of our Sun. 

#COSMOS - The SAO Encyclopedia of Astronomy: “Stellar Black Hole” (retrieved 2024)

https://astronomy.swin.edu.au/cosmos/S/Stellar+Black+Hole 

Since gravastars form also by the collapse of heavy stars, we expect a similar range for their mass. Let us consider a gravastar with 10 times the mass of the Sun.

Given the mass of a traditional black hole, we can calculate its Schwarzschild radius  

#Toth, Viktor T: “Hawking Radiation Calculator” (used 2024)
https://www.vttoth.com/CMS/hawking-radiation-calculator 

For a black hole of 10 solar masses, this means its diameter is approximately the width of London.

#Google Maps: Distance Measuring tool (used 2024)
https://www.google.com/maps 

A gravastar has only a shell thinner than an atom outside the Schwarzschild radius, so the comparison naturally stands. 

—The shell of the gravastar is utterly dark and the coldest thing in the universe, only a billionth of a degree above absolute zero. If we look at it in deep infrared, even the cosmic microwave background glows bright in comparison. 

Typical black holes formed by the collapse of a star have masses between 3 and 20 times the mass of our Sun.

#COSMOS - The SAO Encyclopedia of Astronomy: “Stellar Black Hole” (retrieved 2024)

https://astronomy.swin.edu.au/cosmos/S/Stellar+Black+Hole 

Since gravastars form also by the collapse of heavy stars, we expect a similar range for their mass. Let us consider a gravastar with 10 times the mass of the Sun. 

The temperature of the gravastar’s surface is of the order of the Hawking radiation: 

#Mazur, Pawel O.; Mottola, Emil (2004):  “Gravitational Vacuum Condensate Stars” Proceedings of the National Academy of Sciences, vol. 101, 26, 9545-9550

https://www.pnas.org/doi/epdf/10.1073/pnas.0402717101 

Quote: “Because of the absence of an event horizon, the GBEC star does not emit Hawking radiation. Since w is of the order unity in the shell whereas r≌rs , the local temperature of the fluid within the shell is of the order TH~ℏ/kBGM.”

Which, for our typical black hole of 10 solar masses is:

TH = 6.2 × 10 -9 K

#Toth, Viktor T: “Hawking Radiation Calculator” (used 2024)

https://www.vttoth.com/CMS/hawking-radiation-calculator 

For comparison, the cosmic microwave background is about 2.7 K.

This temperature is below even the coldest temperature ever measured in outer space

#NASA/ESA Hubble: “The Boomerang Nebula - the coolest place in the Universe?” (retrieved 2024)
https://esahubble.org/images/heic0301a/ 

— It is made from an entirely new, unique and extreme matter that is at the very limit of what is physically possible in nature and doesn’t have a name yet. Actually, the shell is so incredibly thin that atoms seem truly gigantic next to it. 

Typical black holes formed by the collapse of a star have masses between 3 and 20 times the mass of our Sun.

#COSMOS - The SAO Encyclopedia of Astronomy: “Stellar Black Hole” (retrieved 2024)

https://astronomy.swin.edu.au/cosmos/S/Stellar+Black+Hole 

Since gravastars form also by the collapse of heavy stars, we expect a similar range for their mass. Let us consider a gravastar with 10 times the mass of the Sun.

 #Mazur, Pawel O.; Mottola, Emil (2004):  “Gravitational Vacuum Condensate Stars” Proceedings of the National Academy of Sciences, vol. 101, 26, 9545-9550

https://www.pnas.org/doi/epdf/10.1073/pnas.0402717101 

Quote: “We can estimate the size of ℓ and ε by consideration of the expectation value of the quantum stress tensor in the static exterior Schwarzschild space-time. [...]  With this semiclassical estimate for we find ℓ ≌ (LPIrs)1/2 ≌ 3 × 10 -14 (M/MSol)1/2cm”

For our typical black hole of 10 solar masses this yields a thickness of:

ℓ ≌ 3 × 10 -14 × (10)½  cm = 9.5 × 10 -14 cm = 9.5 × 10 -16 m

For comparison, the typical radius of an atom is of the order of 10 -10 m

— the shell is incredibly tight. So tight that material is so incredibly tight that if you wanted to stretch the whole shell by just one meter, you would need the energy of an entire supernova. 

Typical black holes formed by the collapse of a star have masses between 3 and 20 times the mass of our Sun.

#COSMOS - The SAO Encyclopedia of Astronomy: “Stellar Black Hole” (retrieved 2024)

https://astronomy.swin.edu.au/cosmos/S/Stellar+Black+Hole 

Since gravastars form also by the collapse of heavy stars, we expect a similar range for their mass. Let us consider a gravastar with 10 times the mass of the Sun.

The energy needed to stretch a surface a certain amount is given by its surface tension. For the shell of a gravstar:

#Mazur, Pawel O.; Mottola, Emil (2015): “Surface tension and negative pressure interior of a non-singular 'black hole'”  Classical and Quantum Gravity, vol. 32,  21

https://iopscience.iop.org/article/10.1088/0264-9381/32/21/215024/meta

Quote: “The redshifted surface tension of the condensate star surface is given by 

𝜏s=Δ𝜅/8𝜋G, where  Δ𝜅=𝜅+-𝜅-=2𝜅=1/Rs is the difference of equal and opposite surface gravities between the exterior and interior Schwarzschild solutions.“
  

In SI units, the surface tension of a gravastar is given by 𝜏=c4/(8𝜋RG) where R is the Schwarzschild radiusof the gravastar, c is the velocity of light and G is the universal gravitational constant. This means that, broadly, the energy required to stretch the shell one meter across the equator of the gravastar is

E ~ 𝜏 × A = (c4/(8𝜋RG)) × (2𝜋R × 1) ~ c4/G ~ (108)4/10-11= 1043

For comparison, a typical supernova releases 1044 J of kinetic energy. 

#Hartmann, Dieter H. (1999): “Afterglows from the largest explosions in the universe”,” Proceedings of the National Academy of Sciences of the United States of America, vol.96, 9, 4752–4755.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC33568/

Quote: “The formation of a new black hole or compact neutron star is heralded by an energy release of ≈1053 erg [= 1046 J ]. More than 99% of this energy is carried away by neutrinos within a few seconds. Roughly 1% (1051 erg = 1 foe[= 1044J]) of the energy is converted into the kinetic energy of the ejected stellar envelope, and even less (0.01 foe) is emitted as optical light on a time scale of months.”

But supernovas come in a range of sizes, so stretching the surface of the gravstar by just one meter would require the amount of kinetic energy liberated by a small supernova. 

—Well it only gets weirder – The interior of a gravastar is perfectly simple. Because it is sort of… empty. Completely empty. 

The interior of the gravstar is a “de Sitter condensate”, an ever-expanding vacuum like that of empty space. 

#Mazur, Pawel O.; Mottola, Emil (2004):  “Gravitational Vacuum Condensate Stars” Proceedings of the National Academy of Sciences, vol. 101, 26, 9545-9550

https://www.pnas.org/doi/epdf/10.1073/pnas.0402717101 

Quote: “If we require that the origin is free of any mass singularity then the interior is determined to be a region of de Sitter space-time in static coordinates” 

—Vacuum fluid is everywhere in the universe – the room you are in is 99.98% vacuum between the air particles bouncing around. 

Most of air is nitrogen (N2), which in good approximation acts like an ideal gas, and a mole of air occupies 22.4L or 0.0224 m3.

Considering that a nitrogen molecule has a volume of approximately twice the volume of a typical atom of radius 10-10 m, it occupies:

2 × 4ℼ × (10-10 m)3/3 = 8 ×10-30 m3

Since each mole has 6.022 × 10²³ molecules in it, the total space occupied by the molecules is: 

 (8 ×10-30 m3) × (6.022 × 10²³) =5×10-6 m3

So the percentage of vacuum, by volume, is:

100 × (0.0224 m3-5×10-6 m3)/(0.0224 m3) = 99.98% 

—But the superdense vacuum inside a gravastar has almost a billion trillion, trillion, trillion times more energy per cubic centimeter than the vacuum outside the star. 

Here, we compare the average density of dark matter in the universe, the “density of the vacuum outside the gravastar”, to the effective density of the gravastar.

The density of dark matter is of around 7×10−27 kg/m3:

#Steinhardt, Paul J.; Turok,Neil (2006): “Why the cosmological constant is small and positive” Science, vol. 312 1180-1182

https://arxiv.org/abs/astro-ph/0605173 

Quote: “One of the greatest challenges in physics today is to explain the small positive value of the cosmological constant or, equivalently, the energy density of the vacuum. The observed value, 7 × 10−30 g/cm3 , is over one hundred twenty orders of magnitude smaller than the Planck density, 1093 g/cm3, as the universe emerges from the big bang, yet its value is thought to be set at that time.”

#COSMOS - The SAO Encyclopedia of Astronomy: “Dark Energy” (retrieved 2024)

https://astronomy.swin.edu.au/cosmos/D/Dark+Energy 

The effective density of the gravastar is its mass M divided by the total volume inside the shell, which is at the Schwarzschild radius R of a black hole of the same mass. 

Since the shell is a sphere of radius R, the volume of the gravstar is V=4𝜋R3/3, so the effective density is:

⍴=M/(4𝜋R3/3)

Typical black holes formed by the collapse of a star have masses between 3 and 20 times the mass of our Sun.

#COSMOS - The SAO Encyclopedia of Astronomy: “Stellar Black Hole” (retrieved 2024)

https://astronomy.swin.edu.au/cosmos/S/Stellar+Black+Hole 

Since gravastars form also by the collapse of heavy stars, we expect a similar range for their mass. Let us consider a gravastar with 10 times the mass of the Sun. Then, the effective density is: 

⍴=M/(4𝜋R3/3)= 3×10×(1.98×1030 kg)/(4×ℼ×(2.95×104 m)3) = 1.8×1017 kg/m3

#Toth, Viktor T: “Hawking Radiation Calculator” (used 2024)

https://www.vttoth.com/CMS/hawking-radiation-calculator 

#Kurzgesagt – In a Nutshell (2021): “What If You Fall into a Black Hole?”

https://www.youtube.com/watch?v=QqsLTNkzvaY  

#Kurzgesagt – In a Nutshell (2017): “Why Black Holes Could Delete The Universe – The Information Paradox”

https://www.youtube.com/watch?v=yWO-cvGETRQ 

#Kurzgesagt – In a Nutshell (2021): ”The Largest Black Hole in the Universe - Size Comparison”

https://www.youtube.com/watch?v=0FH9cgRhQ-k 

#Kurzgesagt – In a Nutshell (2023): ”What Happens If You Destroy A Black Hole?”

https://www.youtube.com/watch?v=cFslUSyfZPc 

#Kurzgesagt – In a Nutshell (2024): “This Black Hole Could be Bigger Than The Universe” 

https://www.youtube.com/watch?v=71eUes30gwc 

—If you fell onto a gravastar, you would be extremely dead before you even hit the surface, ripped apart and ground down by the gravitational forces. 

From the outside, a gravastar behaves basically like a black hole until the point you have already almost hit the surface. 

#Mottola, Emil (2023): “Gravitational Vacuum Condensate Stars”, Ch. 8 of “ Regular Black Holes: Towards a New Paradigm of Gravitational Collapse”, edited by Cosimo Bambi, Springer, 283-352. 

https://arxiv.org/abs/2302.09690 

Quote: “A gravitational condensate star is both cold and dark, and hence in its appearance to distant observers and in its external geometry, in most respects indistinguishable from a BH.”

#Mazur, Pawel O.; Mottola, Emil (2004):  “Gravitational Vacuum Condensate Stars” Proceedings of the National Academy of Sciences, vol. 101, 26, 9545-9550

https://www.pnas.org/doi/epdf/10.1073/pnas.0402717101 

Quote: “Since the exterior space-time is Schwarzschild until distances of the order of the diameter of an atomic nucleus from r =rs, a gravastar cannot be distinguished from a black hole by present observations of x-ray bursts.” 

This means that a gravastar born from the collapse of a star will cause the same “spaghettification” effect as a stellar black hole.   

#NASA SpaceMath: “Black holes and tidal forces” (retrieved 2024)

https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf 

—And once your scattered remains touch the shell, the atoms you were once made of would probably break down and dissolve completely, only to be converted into the vacuum energy of the interior – making the gravastar an infinitesimal bit bigger and the inside an infinitesimal bit more massive.

We thank the expert Emil Mottola for the following quote: 

Quote: “But if you fell onto a gravastar you not fly through an imaginary surface to be crushed in an interior singularity, but hit a real surface, where interactions would likely be able to convert your mass into the vacuum energy of the interior—and the whole gravastar would just incorporate your mass and become a bit larger.”

Unlike in a traditional black hole, where a falling observer would not notice anything special crossing the event horizon, in a gravastar, one would be crushed against its surface due to its extreme gravity at the same distance from the center.

#Mottola, Emil (2023): “Gravitational Vacuum Condensate Stars”, Ch. 8 of “ Regular Black Holes: Towards a New Paradigm of Gravitational Collapse”, edited by Cosimo Bambi, Springer, 283-352. 

https://arxiv.org/abs/2302.09690 

Quote: “Occasional statements that free fall through the event horizon implied by continuation of worldlines to r < rM is required by the Equivalence Principle are incorrect, just because of the possibility of sources and discontinuities on the horizon, which would interrupt that free fall. The Equivalence Principle does not prevent Einstein’s elevator from coming to an abrupt end of its free fall at the surface of the earth, or a surface at r = rM , if a surface exists there.”

#Mottola, Emil (2023): “Gravitational Vacuum Condensate Stars”, Ch. 8 of “ Regular Black Holes: Towards a New Paradigm of Gravitational Collapse”, edited by Cosimo Bambi, Springer, 283-352. 

https://arxiv.org/abs/2302.09690 

Quote: “Moreover, in order for the gravastar proposal to be a viable alternative for a BH of any mass, a gravastar must be able to absorb accreting baryonic matter and convert it to the interior condensate, thereby growing its mass to any larger value.”

—Black holes were suggested more than a century ago as an abstract solution to equations of gravity. For more than 50 years they were considered mathematically valid but too absurd to be real. Few believed they actually existed.

Earlier, non-relativistic “black holes” had been suggested by Michell and Laplace in the late 18th century but their properties and the significance of the event horizon as a one-way causality border could not be understood. 

#Max-Plank Institute for the History of Science: “A Brief History of Black Holes”

https://www.mpiwg-berlin.mpg.de/feature-story/brief-history-black-holes 

The fist relativistic description of a black hole is commonly attributed to Schwarzchild:

#Schwarzschild, Karl (1916): “Über das Gravitationsfeld einer Kugel aus inkompressibler Flüssigkeit nach der Einsteinschen Theorie”, Sitzungsberichte der Deutschen Akademie der Wissenschaften zu Berlin, vol. 23, 224-234. 

Original (in German): https://www.jp-petit.org/Schwarzschild-1916-interior-de.pdf
English Translation: https://arxiv.org/abs/physics/9905030 

The idea had important detractors at first, including Einstein himself:

#Einstein, Albert (1939): “On a Stationary System with Spherical Symmetry consisting of many gravitating masses”, The Annals of Mathematics, vol. 40, 4, 922-936

https://static1.squarespace.com/static/532a9587e4b085a89f267c62/t/551f7768e4b019d7121d5338/1428125544775/Einstein+Annals+of+Mathematics+1939.pdf

Quote: “The essential result of this investigation is a clear understanding as to why the "Schwarzschild singularities" do not exist in physical reality. Although the theory given here treats only clusters whose particles move along circular paths it does not seem to be subject to reasonable doubt that mote general cases will have analogous results. The "Schwarzschild singularity" does not appear for the reason that matter cannot be concentrated arbitrarily. And this is due to the fact that otherwise the constituting particles would reach the velocity of light.” 

—And then we saw stars being thrown around by invisible titans.  We saw light stretching around invisible gaps in the sky. And as our technology and theories improved, we even sort of took a picture of them.

We now have a few observations of stars orbiting or being deflected by dark compact objects generally interpreted to be black holes. One of the first was the binary system Cygnus X-1, in which a star and a dark compact object orbit around their common center of mass

#Encyclopaedia Britannica: “Cygnus X-1” (retrieved 2024)  

https://www.britannica.com/topic/Cygnus-X-1 

Perhaps the most spectacular example of an observations of stars being deflected by a dark compact object is the movie of stars swinging around Sagittarius A*, generally considered a supermassive black hole at the center of our galaxy.

#UCLA Galactic Center Group: “Animations”  (retrieved 2024)

https://www.astro.ucla.edu/~ghezgroup/gc_edit/currentMay22/animations.html

#ESO (2021): “Animated sequence of the VLTI images of stars around the Milky Way’s central black hole”

https://www.eso.org/public/videos/eso2119b/ 

We have even taken pictures of objects of this kind. They show a glowing accretion disk of “light stretching around invisible gaps in the sky”: 

#Event Horizon Telescope (2019): “Astronomers Capture First Image of a Black Hole”

https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole 

Note that the central shadow in the picture is not an “event horizon”, but rather a “shadow” with a radius of around two and a half times bigger than the one the horizon would have. 

#Event Horizon Telescope (2019): “Astronomers Capture First Image of a Black Hole”

https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole 

Quote: “The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across.” 

These findings are normally interpreted to be confirmation of the existence of black holes. However, since the gravitational effects of black holes and gravastars are identical from the outside, they could be gravastars instead.

#Abramowicz, Marek A.; Kluzniak,  Wlodek; Lasota, Jean-Pierre (2002): “No observational proof of the black-hole event-horizon” Astronomy & Astrophysics, vol.396, 3, L31-L34 

https://www.aanda.org/articles/aa/pdf/2002/48/aaeg151.pdf 

Quote: “Recently, several ways of verifying the existence of black-hole horizons have been proposed. We show here that most of these suggestions are irrelevant to the problem of the horizon, at best they can rule out the presence of conventional baryonic matter in the outer layers of black-hole candidates. More generally, we argue that it is fundamentally impossible to detect in electromagnetic radiation direct evidence for the presence of a black-hole horizon. This applies also to future observations, which would trace very accurately the details of the space-time metric of a body suspected of being a black hole. Specific solutions of Einsteins’s equations lack an event horizon, and yet are indistinguishable in their electromagnetic signature from Schwarzschild black holes. 

[...]

There is no observational way to distinguish what may seem to be a Schwarzschild black-hole from a gravastar.”

—they seem to delete information, which should not be possible. 

The information paradox is one of the best well-known open problems in physics:

#Engelhardt, Netta (2023): “The Black Hole information paradox: A resolution on the horizon” MIT Physics Annual

https://physics.mit.edu/wp-content/uploads/2023/09/PhysicsAtMIT_2023_Engelhardt_Feature.pdf
Quote: “The black hole information paradox is a conflict between two apparently incontrovertible facts: first, that semiclassical gravity is valid on scales where gravitational and quantum effects are more or less separate; second, that quantum mechanics is “unitary” and thus all quantum processes are in principle, though not necessarily in practice, reversible. What precisely do we mean by “reversible”? How is a black hole different from a fire? Consider the following thought experiment: you write a message—classical information—on a notepad, which you then toss into a fire in some sealed chamber. Once the fire has consumed the notepad, the information appears to be destroyed: how can we possibly reverse the fire and read the message? Well, if we had arbitrarily powerful machinery that could track every molecule and collect all of the fine-grained information about the fire, and we knew the exact equations describing the behavior of every molecule as it interacts with other molecules, we could in principle recreate the message written on the notepad from the ash. This is the fundamental difference between a black hole and a fire. A 1975 calculation by Stephen Hawking showed that if semiclassical gravity is approximately valid at the event horizon of a black hole, then black holes can evaporate. The black hole evaporation process appears to create an unprecedented problem: it is in principle impossible to reverse-engineer the information that went into a black hole that has evaporated. Even if we knew the exact equations of motion of the universe and the exact state of the universe after evaporation, we still would not be able to ascertain the information that went into the black hole. The radiation emitted by the black hole as it evaporates must be thermal, and indistinguishable between any two evaporated black holes—even if they were originally formed from two very different stars!”

We have also made a video explaining the Information Paradox in detail: 

#Kurzgesagt – In a Nutshell (2017): “Why Black Holes Could Delete The Universe – The Information Paradox”
https://www.youtube.com/watch?v=yWO-cvGETRQ 

—Gravastars are a relatively new idea without any of those problems. They don’t need singularities that break physics or delete information.

#Mottola, Emil (2023): “Gravitational Vacuum Condensate Stars”, Ch. 8 of “ Regular Black Holes: Towards a New Paradigm of Gravitational Collapse”, edited by Cosimo Bambi, Springer, 283-352. 

https://arxiv.org/abs/2302.09690 

Quote: “Because of the global timelike Killing field K(t) = ∂t and absence of either an event horizon or an interior singularity, a gravitational condensate star shows no loss of unitarity, information paradox, or any conflict with either quantum theory or general principles of statistical mechanics”

—But they, too, create new problems. Like weird exotic matter for their incredibly cold, and tight shell; super dense nothing to make a super massive empty core. 

Besides the exotic types of matter described in the original papers

#Mazur, Pawel O.; Mottola, Emil (2004):  “Gravitational Vacuum Condensate Stars”, Proceedings of the National Academy of Sciences, vol. 101, 26, 9545-9550

https://www.pnas.org/doi/epdf/10.1073/pnas.0402717101
https://www.mdpi.com/2218-1997/9/2/88#fn002-universe-09-00088  

other concerns about gravastars raised by the scientific community involve the stability of rapidly rotating gravastars.

#Cardoso, Vitor; et al. (2008): “Ergoregion Instability of Ultracompact Astrophysical Objects”, American Physical Society Physical Review D, vol.77,12, 124044

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=2844&context=phys_facwork 

However, not all rotating gravastars would be unstable.
 

#Chirenti, Cecilia B.M.H.; Rezzolla, Luciano (2008): “On the ergoregion instability in rotating gravastars”,  American Physical Society Physical Review D, vol.78, 8, 084011

https://inspirehep.net/literature/794042 

—The collision of two black holes should “sound” like a bass drum, a deep thumb that stops quickly. But two gravastars colliding should sound like a gong, leaving subtle echoes behind. 

#Mottola, Emil (2023): “Gravitational Vacuum Condensate Stars”, Ch. 8 of “ Regular Black Holes: Towards a New Paradigm of Gravitational Collapse”, edited by Cosimo Bambi, Springer, 283-352. 

https://arxiv.org/abs/2302.09690 

Quote: “The observation of gravitational waves (GWs) by LIGO/LSC has opened up a new window on the universe that among many other interesting possibilities provides perhaps the best opportunity for observational tests of the gravastar proposal. The GW data is not yet accurate enough to test the prediction of a discrete spectrum of ringdown modes from a non-singular gravastar with a surface made in . Indeed it was quickly realized that sensitivity to the nature of a very compact QBH with ε ≪ 1 is obtained only some delay time after the initial GW merger signal, in the ringdown phase [47], where the signal/noise ratio is very much lower. Nevertheless a regular QBH such as a gravastar could produce a GW ‘echo’ at multiples of the characteristic time ∆t ∼ 2GM ln(1/ε) after the compact object merger event . These may be observable with the improved sensitivities of Advanced LIGO and future detectors.”