Black holes are spots in the universe where the gravitational pull is so strong that not even light can escape. One property of black holes that confounds astronomers is just how big they can be. Researchers have described one category of black holes ranging from 100,000 to over 10,000,000,000 times as massive as the Sun, known as supermassive black holes. These black holes are so huge that they’re bigger than entire galaxies! Supermassive black holes commonly exist at the centers of galaxies, including our own Milky Way, though ours is only a modest 4,000,000 Suns in mass. Scientists wonder how supermassive black holes got so big in a universe that formed only 13.7 billion years ago during the Big Bang.
Nearly 14 billion years might seem like plenty of time for huge celestial objects to grow, considering the Earth is only 4.5 billion years old. But once they form, black holes can only get bigger by merging with others or by waiting for stuff to fall into them. Astronomers estimate that many black holes merge every year, but supermassive black holes only merge rarely. Astronomers using mathematical models of these events calculate that it could take more time than the universe has existed for supermassive black holes to fully merge.
Gas and dust falling onto black holes, a process known as luminous accretion, also happens too slowly to explain the growth of supermassive black holes. For example, our galaxy’s supermassive black hole only grows by 1 Sun’s worth of mass every 3,000 years. However, assuming black holes grow at a constant rate, the most gargantuan ones would have needed to grow by more than a Sun’s worth of mass every year since the Big Bang.
To solve this problem, astronomers theorize how black holes originated in the early universe. Supermassive black holes need a head start compared to their conventional black hole cousins, which are typically 10s to 100s of Suns in mass. So, astronomers hypothesize that black holes many 100s to 100,000s of Suns in mass must have formed early in the universe. They call these early black holes seeds. Broadly speaking, astronomers have proposed 2 potential origins for black hole seeds: huge clouds of dust collapsing directly into black holes and Population III stars exploding.
An astronomer from Columbia University recently explored how supermassive black holes emerged by calculating how big their seeds must have been to grow to the size they are today. The astronomer’s first step was to find an appropriate formula to calculate the initial mass of a black hole based on how big and how old it is now. Researchers have shown that black hole growth is almost, but not perfectly, exponential. So, this astronomer started with a modified exponential growth formula, similar to calculating compound interest. He took the derivative of this formula to determine its rate of change, or how fast the black holes grew. The astronomer assumed that the seeds of all supermassive black holes formed between 100 and 200 million years after the Big Bang.
The astronomer selected 132,539 supermassive black holes with well-measured masses and used his new formula to calculate the properties of their seeds. He found that 54% of the seeds were smaller than 350 times the mass of the Sun and could have resulted from Population III stars exploding. Another 40% were between 350 and 2,000 times the mass of the Sun, and only around 6% were between 2,000 and 30,000 times the mass of the Sun. He suggested the latter 2 categories could result from smaller seeds merging together quickly after they formed. He noted that these results did not exclude the collapse of dust directly into the black holes, they only showed it’s not necessary to explain the supermassive black holes we see today.
The astronomer suggested the growth rates of these supermassive black holes also showed that they accumulated most of their mass extremely quickly, likely in the first 1.5 billion years of the universe’s existence. He explained that the universe was much denser back then. Then, galaxies were closer together, so huge quantities of material could fall on black holes constantly. He concluded that supermassive black holes are ultimately a relic of the primordial universe, which looked and behaved in very alien ways compared to today’s orderly and distinct stars, dust clouds, and galaxies.
