Black holes are certainly among the most curious objects in the cosmic catalog. Although they have been studied for decades, some aspects of them still elude physicists. This is particularly the question of the interior of black holes. Point of matter concentrated in a gravitational singularity or passage in space-time? Several hypotheses are possible.
Following the publication of the theory of general relativity by Albert Einstein more than 100 years ago, physicists set out to study and better understand black holes, which constitute solutions to the equations of the gravitational field. According to general relativity, the interior of a black hole is occupied by a gravitational singularity. However, today physicists know that singularities have no physical reality. So what can the inside of a black hole contain?
It may be that at the center of a black hole matter is not crushed to an infinitesimally small point. Instead, there could be the smallest possible configuration of matter, on the smallest possible volume.
It's called a Planck star, and it's a theoretical possibility envisioned by loop quantum gravity (LQG), which itself is a highly hypothetical theory of quantum gravity. In the world of LQG, space and time are quantified; that is, at the microscopic scale, spacetime is discrete, made up of tiny subunits.
This theoretical fragmentation of spacetime offers a definite advantage:it is impossible for singularities to form inside black holes. As matter smashes under the immense gravitational pull of a collapsing star, it encounters resistance. Spacetime discretization prevents matter from reaching anything smaller than the Planck length (about 1.68 ×10 -35 meters).
All the material that has fallen into the black hole is compressed into a mass the length of Planck. This continued compressive strength eventually forces the material to rebound, making black holes temporary objects. But due to the extreme time dilation effects around black holes, from our vantage point in the outer Universe, it takes billions, if not hundreds of billions, of years before they explode.
Another attempt to eradicate the singularity — that is not based on untested theories of quantum gravity — is known as gravastar. The difference between a black hole and a gravastar is that instead of a singularity, the gravastar is filled with dark energy. Dark energy is a substance that pervades space-time, causing the accelerating expansion of the Universe.
When matter falls on a gravastar, it is not able to actually penetrate the event horizon (due to the dark energy contained within) and so it crashes into its surface and merges with it, constituting a Bose-Einstein condensate. But outside of this surface, gravastars look and act like normal black holes.
However, recent observations of black hole mergers with gravitational wave detectors have potentially ruled out the existence of gravastars, as merging gravastars would give a different signal than black holes.
The idea of a single point of infinite density comes from our conception of stationary, non-rotating, non-charged black holes. The most realistic black holes are rotating. The rotation of a black hole stretches the singularity into a ring. And according to the mathematics of Einstein's theory of general relativity, once you pass through the annular singularity, you enter a wormhole and exit through a white hole in a different area of the Universe.
However, the interiors of rotating black holes are extremely unstable according to these same calculations. The singularity, stretched into a ring, spins at such high speed that it possesses incredible centrifugal force. And in general relativity, fairly strong centrifugal forces act like antigravity:they repel instead of attract.
This creates a boundary inside the black hole, called the inner horizon. Outside this region, radiation falls inward toward the singularity, constrained by the extreme gravitational pull. But the radiation is pushed by antigravity near the annular singularity, the boundary of which is the inner horizon. If you encountered the inner horizon, you would face an infinitely energetic wall of radiation.
