How Early Black Holes Grew to Be Billions of Times the Mass of Our Sun
Supermassive black holes at the ends of our universe may have grown extremely rapidly after the Big Bang, allowing them to grow to a monstrous size in a relatively short period of cosmic time. Researchers from the Weizmann Institute of Science have proposed a model that would allow these infant black holes to accrue mass much more rapidly in the early universe than we observe in modern black holes.
Supermassive black holes accrue mass by feeding on interstellar gas, which emits light upon being crushed by the event horizon of a black hole. In the case of very old black holes at the edges of the observable universe, by the time this light (called a quasar) reaches us, the resulting images are from only a billion years after the Big Bang (which is thought to have occurred 13.8 billion years ago). Even though these black holes are infants, several of them had already reached titanic proportions in their "baby pictures" that are comparable to many modern black holes, which have had many more billion years to grow. The researchers hypothesized that they were able to grow very rapidly directly after the Big Bang as a result of a mechanism that interfered with the development of a feature called an accretion disk, which is essential to hindering the exponential growth of a black hole.
From the paper: "We propose a dynamical mechanism that can trigger supra-exponential accretion in the early universe, when a BH seed is trapped in a star cluster fed by the ubiquitous dense cold gas flows... The low-mass BH's random motions suppress the formation of a[n]... accretion disk. Supra-exponential growth can thus explain the puzzling emergence of supermassive BHs that power luminous quasars so soon after the Big Bang."
Modern black holes feature accretion disks, which are disks or gas and dust that are consumed by the black hole and "take up space," so to speak, preventing the black hole from accruing mass too rapidly. They also generate heat when they are swallowed, which repels more gas and dust from falling into the black hole. "Black holes don't actively suck in matter - they are not like vacuum cleaners," said astrophysicist and lead study author Tal Alexander. "A star or a gas stream can be on a stable orbit around a black hole, exactly as the Earth revolves around the sun, without falling into it. It is actually quite a challenge to think of efficient ways to drive gas into the black hole at a high enough rate that can lead to rapid growth."
By simulating a model using a black hole 10 times the size of the sun, they approximated what would have happened to nascent black holes in the early universe. The early universe was much colder and denser than the universe is today, so they fed the simulation streams of cold, dense gas. They found that the gravitational pull of neighboring stars "causes it to zigzag randomly, and this erratic motion prevents the formation of a slowly draining accretion disk," said Alexander. Without the accretion disk, gas would have tumbled in from all sides, since the matter would not have a chance to settle into a slowly rotating disk. The bigger the black hole got, the more gas it would consume, leading to the supra-exponential growth. As a result, the black holes could have grown to approximately 10,000 times the mass of our Sun by a mere 10 million years after the Big Bang, when the growth would have slowed to a more leisurely pace. Even with a plateau, the black holes could easily have grown to be up to 10 billion times the mass of our Sun just one billion years after the Big Bang.
Alexander said, "This theoretical result shows a plausible route to the formation of supermassive black holes very soon after the Big Bang."