Naked singularities
Variations on a theme – alternatives to black hole formation: the naked singularity
Researchers have recently found a wide variety of stellar collapse scenarios in which an event horizon in fact does not form, so that the singularity remains exposed to our view. This phenomenon is known as a naked singularity. Matter and radiation can both fall in and come out. Whereas visiting the singularity inside a black hole would be a one-way trip, you could in principle come as close as you like to a naked singularity and return to tell the tale.
In contrast to the situation during the formation of a black hole, where the star’s density is uniform during the process of collapse and varies only in time, there are other stellar collapse scenarios where the density increases with distance from the centre. In effect, the star has an onionlike structure of concentric shells of matter, and the strength of gravity acting on each shell depends on the average density of matter within that shell. Because the denser inner shells feel a stronger pull, they collapse faster than the outer ones, and the entire star does not collapse to a singularity simultaneously. The innermost shells collapse first and the outer shells pile on, one by one.
The resulting delay can postpone the formation of an event horizon, and if density increases with distance too rapidly the dense inner shells may not have enough mass to trap light. The singularity when it forms will be naked. Thus, there is a threshold: if the degree of inhomogeneity is very small, below a critical limit, a black hole will form; with sufficient inhomogenity, a naked singularity arises[1]. A similar process of delayed collapse may also result in the formation of what is known as a black star, where the gravitational field around the star is identical to that around a black hole, but the star’s interior is full of matter and no even horizon forms[2].
[1] For this elaboration on naked singularities, see Pankaj S Joshi, “Naked Singularities”, Scientific American, op cit, August 2013, 74 at 79,80. See also "Using black holes to test Einstein’s theory of gravity" in the segment on black holes.
[2] Carlos Barcelo et al, “Black stars, not holes”, Scientific American, op cit, August 2013, 74 at 82, at 87. Here, the authors also avail themselves of the onion ring analogy in their comment that “If a black star could be peeled layer by layer like an onion, at each stage the remaining core would be a smaller black star, also emitting radiation”.
Researchers have recently found a wide variety of stellar collapse scenarios in which an event horizon in fact does not form, so that the singularity remains exposed to our view. This phenomenon is known as a naked singularity. Matter and radiation can both fall in and come out. Whereas visiting the singularity inside a black hole would be a one-way trip, you could in principle come as close as you like to a naked singularity and return to tell the tale.
In contrast to the situation during the formation of a black hole, where the star’s density is uniform during the process of collapse and varies only in time, there are other stellar collapse scenarios where the density increases with distance from the centre. In effect, the star has an onionlike structure of concentric shells of matter, and the strength of gravity acting on each shell depends on the average density of matter within that shell. Because the denser inner shells feel a stronger pull, they collapse faster than the outer ones, and the entire star does not collapse to a singularity simultaneously. The innermost shells collapse first and the outer shells pile on, one by one.
The resulting delay can postpone the formation of an event horizon, and if density increases with distance too rapidly the dense inner shells may not have enough mass to trap light. The singularity when it forms will be naked. Thus, there is a threshold: if the degree of inhomogeneity is very small, below a critical limit, a black hole will form; with sufficient inhomogenity, a naked singularity arises[1]. A similar process of delayed collapse may also result in the formation of what is known as a black star, where the gravitational field around the star is identical to that around a black hole, but the star’s interior is full of matter and no even horizon forms[2].
[1] For this elaboration on naked singularities, see Pankaj S Joshi, “Naked Singularities”, Scientific American, op cit, August 2013, 74 at 79,80. See also "Using black holes to test Einstein’s theory of gravity" in the segment on black holes.
[2] Carlos Barcelo et al, “Black stars, not holes”, Scientific American, op cit, August 2013, 74 at 82, at 87. Here, the authors also avail themselves of the onion ring analogy in their comment that “If a black star could be peeled layer by layer like an onion, at each stage the remaining core would be a smaller black star, also emitting radiation”.