Before describing the importance of black holes and quasars in galactic formation and dynamics, a brief digression into the history of astrophysics.
In the late 18th century, English natural philosopher and geologist John Michell and French mathematician and astronomer Pierre-Simon Laplace contemplated the prospect of an object with gravity too strong for light to escape: a black hole.
German physicist Karl Schwarzschild mathematically conjectured simple black holes in 1915, the same year Einstein introduced general relativity. The Schwarzschild radius is the size of the event horizon for a simplified abstraction of black holes: massive, non-rotating, and spherically symmetric objects. A black hole’s event horizon defines the rim of no return for matter/energy, as the gravitational pull approaches infinity.
Einstein was pleasantly surprised to learn of Schwarzschild’s exact solutions for general relativity’s field equations, as he could only produce an approximate solution.
Whereas Einstein had used a rectangular coordinate system to approximate the gravitational field near the black hole mathematical construct, Schwarzschild developed a polar, spherical coordinate system, which afforded more elegant mathematical expression.
(Schwarzschild’s triumphal equations were created while he was in the German army during World War I. Schwarzschild was suffering from a painful autoimmune disease (pemphigus) which he developed while at the Russian front, yet he managed to write 3 outstanding physics papers in 1915: 2 on relativity theory, and 1 on quantum theory. Schwarzschild died the following year.)
Einstein considered black holes purely a mathematical construct. He did not think that black holes could actually form. Following Einstein’s lead, mainstream physicists disregarded all results to the contrary, though a minority maintained that black holes were possible. It was not until the close of the 1960s that consensus conviction turned toward acceptance that black holes existed.
Via quantum mechanics, not general relativity, English theoretical physicist Stephen Hawking predicted in 1974 that black holes must emit radiation (Hawking radiation), though at a temperature inversely proportional to the mass of the black hole.
There is no escape from a black hole in classical theory, but quantum theory enables energy and information to escape. ~ Stephen Hawking
30 years later, Hawking had convinced himself that black holes do not exist. Hawking’s repudiation stemmed from a paradoxical conundrum.
The equivalence principle of relativity assumes that the laws of physics are identical everywhere. Someone falling into a black hole would feel the same as if floating free in space, at least until ripped apart by gravitational intensity.
At the quantum level, approaching a singularity would be very energetic, with excited particles bustling about. Someone entering the event horizon would be fried to a crisp by sizzling subatomics.
Such a quantum firewall poses serious problems. It violates the relativity axiom of equivalence. And it breaks the mathematical symmetry of quantum theory.
So, Hawking proposed that a black hole isn’t really a black hole. Instead, it is a cosmological shredder, which merely mangles matter before releasing it; an utterly unsupported hypothesis.
A different resolution of the paradox is proposed, namely that gravitational collapse produces apparent horizons but no event horizons behind which information is lost. ~ Stephen Hawking, concerned about the cosmic integrity of information
By trading an event horizon for one which is only apparent, Hawking’s proposal attempts to leave both relativity and quantum theories intact. Instead, it denies what a black hole must be.