40 Quintillion Black Holes are Lurking in the Universe, Study Projects

"Scientists have estimated the number of 'small' black holes in the universe," reports Live Science. "And no surprise: It's a lot."Using a new method, outlined January 12 in The Astrophysical Journal, a team of astrophysicists has produced a fresh estimate for the number of stellar-mass black holes - those with masses 5 to 10 times that of the sun - in the universe. And it's astonishing: 40,000,000,000,000,000,000, or 40 quintillion, stellar-mass black holes populate the observable universe, making up approximately 1% of all normal matter, according to the new estimate. So how did the scientists arrive at that number? By tracking the evolution of stars in our universe they estimated how often the stars - either on their own, or paired into binary systems - would transform into black holes, said first author Alex Sicilia, an astrophysicist 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 arrive at their estimate, the astrophysicists modeled not just the lives, but the pre-lives of the universe's stars. 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 built a model of the universe that accurately reflected the different sizes of stars that would be made, and how often they would be created.... [T]he researchers designed a model that tracked the population and size distribution of stellar-mass black holes over time to give them their eye-watering number. Then, by comparing the estimate with data taken from gravitational waves, or ripples in space-time, formed by black hole and binary star mergers, the researchers confirmed that their model was in good agreement with the data. Astrophysicists hope to use the new estimate to investigate some perplexing questions that arise from observations of the very early universe - for instance, how the early universe became so quickly populated by supermassive black holes - often with masses millions, or even billions, of times greater than the stellar-mass holes the researchers examined in this study - so soon after the Big Bang.

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