One of the most popular theories about the formation of massive black holes links them with the first generation of stars. However, recent simulations suggest these stars were no more than a few dozen times the sun's mass, Volonteri said – too small to easily achieve the mass of the largest black holes.
One alternative scenario proposes that massive black holes originated from stars up to 1 million times the sun's mass born from gas that rapidly accumulated over less than about 2 million years. Another model suggests the kernels of massive black holes were born from dense clusters of stars in the centers of galaxies that merged to form stars up to a few thousand times the sun's mass, which in turn collapsed to create black holes.
Curiously, astronomers recently discovered that billion-solar-mass black holes existed when the cosmos was less than a billion years old. Scientists are at a loss to satisfyingly explain how such massive black holes could have formed so early in the universe's history.
After a certain point known as the Eddington limit, the energy released by matter rushing toward a black hole should halt the flow feeding that black hole, restricting how large it can grow. Although it is possible that billion-solar-mass black holes could have formed in less than a billion years after the Big Bang, "all odds must be favorable to the black hole, so only lucky black holes can make it," Volonteri said.
Scientists are now exploring whether or not black holes can overcome the Eddington limit and grow at so-called super-Eddington rates. "That would make black hole growth easier," Volonteri said.
It remains a hotly debated question whether these massive black holes dominate how their galaxies grow or vice versa. Perhaps the energy these black holes release alters the overall evolution of galactic structures, or perhaps galaxies control how much gas falls into these black holes and thus regulate how large they grow. Another possibility is that massive black holes and their galaxies develop symbiotically.