40 quintillion stellar-mass black holes are lurking in the universe, new study finds
Scientists have estimated the number of “small” black holes in the universe. And no surprise: It’s a lot.
This number may appear impossible to calculate; after all, detecting black holes is not the easiest process. Because they are as black as the space they inhabit, light swallowing cosmic goliaths can only be identified in the most extraordinary circumstances, such as when they are bending the surrounding light, snacking on unfortunate gases and stars that stray too close, or spiraling toward enormous collisions that unleash gravitational waves.
But that hasn’t stopped scientists from designing some innovative methods for estimating the number. A team of astrophysicists has invented a new estimate for the number of stellar-mass black holes — those with masses 5 to 10 times that of the sun — in the universe using a new method described in The Astrophysical Journal.
According to the new estimate, 40,000,000,000,000,000,000, or 40 quintillion, stellar-mass black holes fill the observable universe, accounting for around 1% of all normal stuff.
So, how did the scientists come up with that figure? They estimated how often stars in our universe will change into black holes by studying the evolution of stars in our universe, said first author Alex Sicilia, an astronomer at the International School of Advanced Studies (SISSA) in Trieste, Italy.
“This is one of the first, and one of the most robust, ab initio [ground up] computation[s] of the stellar black hole mass function across cosmic history,” Sicilia said in a statement.
To create a black hole, you must first create a massive star with a mass five to ten times that of the sun. As massive stars near the end of their life, they begin to fuse heavier and heavier elements, such as silicon or magnesium, inside their raging cores. However, once the fusion process begins to generate iron, the star is doomed to rapid self-destruction. Iron absorbs more energy than it emits, leading the star to lose its capacity to push back against the strong gravitational forces caused by its massive mass. It collapses in on itself, packing first its core, and later all the matter close to it, into a point of infinitesimal dimensions and infinite density — a singularity. The star becomes a black hole, and beyond a boundary called the event horizon, nothing — not even light — can escape its gravitational pull.
To arrive at their estimate, the astrophysicists modeled not only the lifetimes of the universe’s stars, but also their pre-lives. Using known statistics of various galaxies, such as their sizes, the elements they contain, and the sizes of the gas clouds stars would form in, the team constructed a model of the universe that accurately reflected the different sizes of stars that would be formed, as well as how frequently they would be formed.
After determining the rate of formation for stars that could eventually transform into black holes, the researchers modeled their lives and deaths, using data such as mass and metallicity — the abundance of elements heavier than hydrogen or helium — to determine the percentage of candidate stars that would transform into black holes. The researchers guaranteed that they weren’t double-counting any black holes in their survey by also looking at stars paired into binary systems and calculated the rate at which black holes can meet and merge. They also determined how these mergers, combined with black hole snacking on neighboring gas, would change the size distribution of black holes observed throughout the universe.
With these calculations in hand, the researchers created a model that tracked the population and size distribution of stellar-mass black holes through time, yielding their jaw-dropping figure. The researchers then proved that their model was in good agreement with the data by comparing it to data from gravitational waves, or ripples in space-time caused by black hole and binary star mergers.
Astrophysicists hope to use the new estimate to investigate some puzzling questions raised by observations of the very early universe, such as how the early universe became so quickly populated by supermassive black holes – often with masses millions, if not billions, of times greater than the stellar-mass holes studied in this study – so soon after the Big Bang.
Because these huge black holes were generated by the merging of smaller, stellar-mass black holes — or black hole ‘seeds’ — the researchers expect that a better understanding of how small black holes evolved in the early universe could aid in the discovery of their supermassive counterparts.
“Our work provides a robust theory for the generation of light seeds for supermassive black holes at high redshift [further back in time], and can constitute a starting point to investigate the origin of “heavy seeds”, that we will pursue in a forthcoming paper,” Lumen Boco, an astrophysicist at SISSA, said in the statement.