A shell collapsar is a proposed alternative explanation for the observations of

black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

candidates. In this concept, matter collapses onto the

event horizon In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside observer. Wolfgang Rindler coined the term in the 1950s. In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive c ...

, forming a compact shell of ultra-high density. Being of neutron star size (~11km), this hypothetical object strongly deflects light rays like a black hole, but has no central point-like singularity. The term ''shell collapsar'' was first proposed by Trevor W. Marshall in 2009 and 2012, then more firmly established in 2016. It predicts the existence of

neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...

masses beyond the

Tolman–Oppenheimer–Volkoff limit The Tolman–Oppenheimer–Volkoff limit (or TOV limit) is an upper bound to the mass of cold, non-rotating neutron stars, analogous to the Chandrasekhar limit for white dwarf stars. Stars more massive than the TOV limit collapse into a black hol ...

of , thus providing an alternative to stellar-mass black holes for the highly compact bodies that merge to produce

gravitational wave Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...

signals. In 2011, Jun Ni explored the theoretical framework of neutron stars within the context of

general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

. He discusses solutions to the field equations that do not impose a maximum mass limit on neutron stars, which challenges the conventional understanding that there is a maximum mass beyond which a neutron star would collapse into a black hole. Ni introduced a new class of neutron stars that can exist beyond this traditional limit, suggesting that these stars could have significantly higher masses than previously thought. He coined the term "超重星" (Chōjūsei) in Japanese, which translates to "supermassive star" in English, to describe these neutron stars that defy the established mass constraints. Luboš Neslušan suggested such a solution with the gravitation potential minimum in the shell of the neutron star instead of the centre and references Ni. deLyra shows similar models of shell-like compact gas objects. New research from Matthew R. Edwards even suggest a ''shell universe'' model based on Ni's solutions in order to reduce the

Hubble Tension Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faste ...

. A shell collapsar may be

void Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...

inside as time dilation delays indefinitely the entry of mass-particles. Marshall's solution however, predicts intense gravitational field energy there, arising from Einsteinian gravitation in the highly non-linear regime. As

time dilation Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity between them (special relativity), or a difference in gravitational potential between their locations (general relativity). When unsp ...

is extreme on the collapsar surface (just outside the event horizon), accreting neutron matter can be viewed as freezing onto the outer shell, as described in the ‘frozar’ model. Z. Zakir conceives that outward gravitational forces arise in the interior and push inner matter into the ‘frozar’ shell. The shell collapsar is a special case of a

gravastar In astrophysics, a gravastar (a blend word of "gravitational vacuum star") is an object hypothesized in a 2001 paper by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has the usual black hole metric outside of ...

. With the gravastar, an exotic form of matter stabilizes the object with the equation of state of dark energy inside. The shell collapsar comes to a similar result with ordinary neutron star matter and simply

Einstein's field equations In the general theory of relativity, the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of spacetime to the distribution of matter within it. The equations were published by Albert Einstein in 1915 in th ...

describing intense gravitational energy density, comparable as E/c² to the neutron matter density.

References

{{Stellar core collapse Hypothetical astronomical objects