Hungry Heart: Role of the Universe's Earliest Black Holes

First black holes born starving

The first black holes in the universe had dramatic effects on their surroundings despite the fact that they were small and grew very slowly, according to recent supercomputer simulations carried out by astrophysicists Marcelo Alvarez and Tom Abel of the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, and John Wise, formerly of KIPAC and now of NASA Goddard Space Flight Center. Several popular theories posit that the first black holes gorged themselves on gas clouds and dust in the early universe, growing into the supersized black holes that lurk in the centers of galaxies today. However, the new results, published in The Astrophysical Journal Letters, point to a much more complex role for the first black holes.

"I'm thrilled that we now can do calculations that start to capture the most relevant physics, and we can show which ideas work and which don't," said Abel. "In the next decade, using calculations like this one, we will settle some of the most important issues related to the role of black holes in the universe."

To make their discovery, the researchers created the most detailed simulations to date of the first black holes in the universe that formed from the collapse of stars. The simulations started with data taken from observations of the cosmic background radiation—the earliest view of the structure of the universe. The researchers then applied the basic laws that govern the interaction of matter, allowing the early universe in their simulation to evolve as it did in reality.

In the simulation, clouds of gas left over from the Big Bang slowly coalesced under the force of gravity, and eventually formed the first stars. These massive, hot stars burned bright for a short time, emitting so much energy in the form of starlight that they pushed nearby gas clouds far away. Yet these stars could not sustain such a fiery existence for long, and they soon exhausted their internal fuel. This caused one of the stars in the simulation to collapse under its own weight, forming a black hole located in a pocket of emptiness. With very little matter in the near vicinity, this black hole was essentially "starved" of food on which to grow.

"Quasars [extremely strong sources of radiation] powered by black holes a billion times more massive than our sun have been observed in the early universe, and we have to explain how these behemoths could have grown so big so fast," said Alvarez. "Their origin remains among the most fundamental unanswered questions in astrophysics."

One explanation for the existence of supermassive black holes in the early universe postulates that the first black holes were 'seeds' that grew into much larger black holes by gravitationally attracting and then swallowing matter. But in their simulation, Alvarez, Abel and Wise found that such growth was negligible, with the black hole in the simulation growing by less than one percent of its original mass over the course of a hundred million years.

Although the simulations do not yet completely rule out the theory, this makes it less likely that these first black holes could have grown directly into the supermassive black holes observed to have existed less than a billion years later, Alvarez said.

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