In General Relativity, a black hole is defined as a region of space time with gravitational field powerful enough to prevent light from escaping said region of space time. The mass, size and lifetime of a black hole could vary depending on how it formed. While the primary cause for the formation of a black hole is the gravitational collapse of massive stars, this is not the only possible cause for the formation of a black hole. A black hole may form because of the collapse of massive stars, initial density perturbation during early epoch of the universe and high-energy collision.
Gravitational Collapse of Massive Stars
During the lifetime of a star, the heated gas and plasma pressure exerted by the nuclear reaction inside the core is at constant battle against the gravity of the star itself. This act of balance, however, requires the availability of fusion fuels in the core of the star.
As the star exhausted its fusion fuel supply, the outward pressure can’t balance against the gravity of the star. This is going to make the star collapse against its own gravity. If the star has no more internal pressure to balance the gravity, the only thing capable of halting the collapse is degeneracy pressure.
The first degeneracy pressure mechanism to halt the formation of singularity is the neutron degeneracy pressure. Stars supported by neutron degeneracy pressure from collapsing under its own gravity are called neutron stars. If the neutron degeneracy pressure is not enough to halt the collapse, the next degeneracy pressure mechanism to halt it is quark degeneracy pressure. Stars supported by quark degeneracy pressure from collapsing are called quark stars. If the mass of the star is bigger than the quark degeneracy pressure can hold, the star is going to form singularity and become a black hole.
Since the temperature of a star is not going to drop suddenly, it is theorized that a star massive enough to form a black hole is going to become a black hole step-by-step. First, the normal star is going to undergo supernova and become a neutron star. As the temperature drop, the neutron star undergo quark-nova and become a quark star. If the temperature drop further the quark star is going to become black hole.
Black holes formed during early epoch of the universe
The cosmological epoch we are living at the moment is called the Stelliferous Era. During the Stelliferous Era, the only region of space-time known to have enough density to undergo gravitational collapse can only be found in stars. During earlier epochs of the universe however, the density of the universe was much higher. If any region of the universe have higher density than the average during this earlier epoch, a primordial black hole may form.
Black holes formed during earlier epochs of the universe may range in size from Planck-sized up to several hundred thousands solar mass. Various black holes may formed during earlier epoch of the universe, but only the stellar ones are going to survive until the Stelliferous Era. Smaller black holes are going to undergo evaporation by emitting Hawking Radiation.
Micro black holes formed by high-energy collision
Other than gravitational collapse, black holes could also formed in high energy collision. If a high energy collision can create a region with sufficient density, light will not be able to escape from said region until that region evaporate by emitting Hawking Radiation.
To date, no evidence of micro black holes could be found in any particle accelerator experiment. This suggests that the previous theory – stating that micro black holes could form no matter how low the energy involved in the collision – is somehow incorrect or incomplete.
Unlike most people’s beliefs, micro black holes are not dangerous. The black hole will completely evaporate by Hawking Radiation before it can gain any more mass. Micro black holes could be generated by not only the high-energy collision experiments, but also cosmic rays’ collision with the earth atmosphere.
