A team of astrophysicists has calculated the number of stellar-mass black holes in the observable universe to be 40 quintillion, accounting for 1% of the total ordinary matter in the universe.
The researchers focus on stellar-mass black holes, the smallest-known variety, but note that their calculations could help address the longstanding mystery of how supermassive black holes proliferated. Their research is published in the Astrophysical Journal Letters.
For a long time, black holes were only theorised to exist and had never been observed — as their name suggests, they don’t let light escape their gravitational pull. But astronomers have figured out that black holes are at the centre of large concentrations of light-emitting matter (our own Milky Way features a supermassive black hole at its centre). More recently, black holes mergers have been detectable thanks to gravitational wave detectors like the LIGO-Virgo Collaboration.
But counting all the black holes in the observable universe, which stretches some 90 billion light-years across, is a daunting task. To get to the 40 quintillion sum (that’s 40 billion billions, or 40,000,000,000,000,000,000) the research team coupled a new star evolution code called SEVN and with data on the metallicity, star formation rates, and stellar sizes in known galaxies.
“The innovative character of this work is in the coupling of a detailed model of stellar and binary evolution with advanced recipes for star formation and metal enrichment in individual galaxies,” said Alex Sicilia, an astrophysicist at SISSA in Italy and the paper’s lead author, in an institute release. “This is one of the first, and one of the most robust, ab initio computation of the stellar black hole mass function across cosmic history.”
The research is the first in a series of works that is attempting to model black hole masses, from star-sized ones up to supermassive black holes. Stellar-mass black holes are the smallest-known of the bunch, generally weighing in at few to a few hundred times the mass of the Sun. Intermediate black holes are notoriously absent from the observational record, but supermassive black holes reside at the centre of most galaxies and accrete matter around them, pulling stars, planets, and gases close with their ridiculous gravitational might.
In the paper, the researchers also investigated how black holes of varying sizes might form. Stellar-mass black holes arise from the collapsed cores of dead stars, but the origins of supermassive black holes are more of a mystery. Lumen Boco, also an astrophysicist at SISSA and co-author of the paper, said in the same release that the team’s calculations “can constitute a starting point to investigate the origin of ‘heavy seeds’, that we will pursue in a forthcoming paper.”
The new study doesn’t address so-called primordial black holes, hypothetical objects left over from the beginning of the universe that could be much, much smaller than any known black holes. There’s no evidence that these actually exist, but some physicists have suggested them as a potential solution to the mystery of dark matter. One team actually proposed that a bowling ball-size black hole could be Planet Nine, a theoretical body in the outer solar system affecting the orbits of distant objects.
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