The existence of gigantic black holes in the very early universe challenges our assumptions of how black holes form and grow. New research suggests that these monsters may have found their origins in the earliest epochs of the Big Bang.
For years astronomers have been troubled by observations of fully grown supermassive black holes before the universe was even a billion years old. This is challenging because as far as we know the only way to make black holes is through the deaths of massive stars. And the only way for them to grow is either through mergers or the accumulation of material. Following these known mechanisms it’s extremely difficult to build the observed black holes, which have masses hundreds of millions of times that of the Sun, so quickly.
And so astronomers have been long attempting to find some other way to explain how these giant black holes arrive on the cosmic scene. In a new paper, a team of researchers point to an seemingly unlikely scenario: the first microseconds of the Big Bang.
In the 1970s Stephen Hawking hypothesized that the tumultuous epochs of the incredibly early universe would cause random fluctuations of matter to spontaneously collapse to form black holes. These primordial black holes might even persist to the present day, and astronomers have even gone so far as to propose that these black holes explain dark matter.
But observations have placed considerable constraints on the populations of primordial black holes. They simply can’t be a major constituent of the universe, otherwise we would have seen evidence for them by now.
But in the new paper the researchers point out that they don’t need to be common to form the seeds of supermassive black holes. They can be incredibly rare, making up less than 1% of all the mass in the universe. But if they are formed in the early universe, then slowly over time they can accrete new material and merge with each other, especially in the first few hundred million years as galaxies are first forming.
This scenario would mean that giant black holes would form not after the appearance of the first stars, but in parallel with them. Then by the time stars and galaxies appear the black holes are already fully grown.
The researchers were able to find a scenario that could explain the observed population of giant black holes in the young universe. However, this is only the first step in the research. The next is to fine-tune these models and incorporate them in more detailed simulations of the evolution of the early universe to see just how plausible this scenario is.
