Naked singularity
In generic black holes, this is not a problem, as an outside viewer cannot observe the spacetime within the event horizon.
Astronomical observations of black holes indicate that their rate of rotation falls below the threshold to produce a naked singularity (spin parameter 1).
If loop quantum gravity is correct, naked singularities may be possible in nature.
This holds true across a diverse range of physically plausible scenarios within the framework of the general theory of relativity.
The Oppenheimer–Snyder–Datt (OSD) model illustrates the collapse of a spherical cloud composed of homogeneous dust (pressureless matter).
[4][5] In this scenario, all the matter converges into the spacetime singularity simultaneously in terms of comoving time.
Notably, the event horizon emerges before the singularity, effectively covering it.
This leads to two distinct potential outcomes arising from the collapse of generic dust: the formation of a black hole, characterized by the horizon preceding the singularity, and the emergence of a naked singularity, where the horizon is delayed.
In the case of a naked singularity, this delay enables null geodesics or light rays to escape the central singularity, where density and curvatures diverge, reaching distant observers.
[6][7][8] In exploring more realistic scenarios of collapse, one avenue involves incorporating pressures into the model.
The consideration of gravitational collapse with non-zero pressures and various models including a realistic equation of state, delineating the specific relationship between the density and pressure within the cloud, has been thoroughly examined and investigated by numerous researchers over the years.
[9] They all result in either a black hole or a naked singularity depending on the initial data.
From concepts drawn from rotating black holes, it is shown that a singularity, spinning rapidly, can become a ring-shaped object.
This results in two event horizons, as well as an ergosphere, which draw closer together as the spin of the singularity increases.
When the outer and inner event horizons merge, they shrink toward the rotating singularity and eventually expose it to the rest of the universe.
A singularity rotating fast enough might be created by the collapse of dust or by a supernova of a fast-spinning star.
[11] Intriguingly, it is recently reported that some spinning white dwarfs can realistically transmute into rotating naked singularities and black holes with a wide range of near- and sub-solar-mass values by capturing asymmetric dark matter particles.
[12] Similarly, the spinning neutron stars could also be transmuted to the slowly-spinning near-solar mass naked singularities by capturing the asymmetric dark matter particles, if the accumulated cloud of dark matter particles in the core of a neutron star can be modeled as an anisotropic fluid.
[14] [15] Mathematician Demetrios Christodoulou, a winner of the Shaw Prize, has shown that contrary to what had been expected, singularities which are not hidden in a black hole also occur.
[17] Disappearing event horizons exist in the Kerr metric, which is a spinning black hole in a vacuum.
Specifically, if the angular momentum is high enough, the event horizons could disappear.
In this case, "event horizons disappear" means when the solutions are complex for
), i.e. the spin exceeds what is normally viewed as the upper limit of its physically possible values.
Disappearing event horizons can also be seen with the Reissner–Nordström geometry of a charged black hole.
), i.e. the charge exceeds what is normally viewed as the upper limit of its physically possible values.
A naked singularity could allow scientists to observe an infinitely dense material, which would under normal circumstances be impossible according to the cosmic censorship hypothesis.
[20] The cosmic censorship hypothesis says that a gravitational singularity would remain hidden by the event horizon.
[21] Some research has suggested that if loop quantum gravity is correct, then naked singularities could exist in nature,[22][23][24] implying that the cosmic censorship hypothesis does not hold.
