More than 40 quintillion black holes are in the observable universe, new estimate suggests
A team of astrophysicists has calculated the number of starsmass of black holes in the observable universe at 40 quintillion, representing 1% of the total ordinary matter of the universe.
The researchers focus on stellar-mass black holes, the smallest known variety, but note that their calculations could help solve the long-standing mystery of supermassive black hole proliferation. Their research is published in the letters of the Astrophysical Journal.
For a long time, black holes were only theorized to exist and had never been observed – as their name suggests, they don’t let light escape their gravitational pull. But astronomers have discovered that black holes are at the center of large concentrations of light-emitting matter (our own Milky Way has a supermassive black hole at its center). More recently, black hole mergers have been detectable using gravitational wave detectors like the LIGO-Virgo collaboration.
But counting all the black holes in the observable universe, which spans some 90 billion light-years, is a daunting task. To arrive at the sum of 40 quintillion (i.e. 40 trillion, or 40,000,000,000,000,000,000), the research team coupled a new star evolution code called SEVN and data on metallicity, star formation rates and stellar sizes in known galaxies.
“The novelty of this work lies in coupling a detailed model of stellar and binary evolution with advanced recipes for star formation and metal enrichment in individual galaxies,” said astrophysicist Alex Sicilia. at SISSA in Italy and main author of the article, in an institute Release. “This is one of the earliest and most robust ab initio calculations of the stellar black hole mass function throughout cosmic history.”
The research is the first in a series of works that attempt to model the masses of black holes, star-sized ones to supermassive black holes. Stellar-mass black holes are the smallest known of the group, typically weighing a few to a few hundred times the mass of the Sun. Intermediate black holes are notoriously absent from observations, but supermassive black holes reside at the center of most galaxies and accumulate matter around them, pulling nearby stars, planets and gas with their ludicrous gravitational pull.
In the paper, the researchers also studied how black holes of different sizes could form. Stellar-mass black holes arise from the collapsed cores of dead stars, but the origins of supermassive black holes are more mysterious. Lumen Boco, also an astrophysicist at SISSA and co-author of the paper, said in the same statement that the team’s calculations “may provide a starting point for investigating the origin of the ‘heavy seeds’, which we we will continue in a future article.”
The new study does not 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 hole. There is no evidence that these actually exist, but some physicists have suggested them as a potential solution to the dark matter mystery. One team actually proposed that a A black hole the size of a bowling ball could be Planet Nine, a theoretical body in the outer solar system affecting the orbits of distant objects.
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