40 quintillion stellar-mass black holes are lurking in the universe, new study finds

 

A black hole and its event horizon are depicted in this image. (Photo courtesy of Future Publishing/Nick Forder)

One percent of the mass in the cosmos is thought to be made up of "small" black holes.

The number of "small" black holes in the cosmos has been calculated by scientists. And, unsurprisingly, there's a lot of it.

This figure may appear hard to calculate; after all, detecting black holes is not an easy task. The light-sucking cosmic goliaths can only be seen when they're bending the light around them, nibbling on the unfortunate gases and stars that venture too close, or hurtling toward huge collisions that unleash gravity waves, because they're as pitch-black as the space they lurk in.

However, this hasn't stopped scientists from devising some innovative methods for estimating the number. A team of astrophysicists has established 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 on Jan. 12.

According to the revised estimate, there are 40,000,000,000,000,000,000, or 40 quintillion, stellar-mass black holes in the observable universe, accounting for around 1% of all normal stuff.

So, how did the scientists come up with that figure? They calculated 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.

In a statement, Sicilia said, "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 over cosmic history."

To create a black hole, you must start with a massive star, around five to ten times the mass of the sun. As massive stars approach the end of their life, their scorching cores begin to combine larger and heavier elements, such as silicon or magnesium. The star, however, is on the verge of cataclysmic self-destruction once the fusion process begins to generate iron. The star loses its power to push out against the huge gravitational forces caused by its enormous mass because iron takes in more energy to fuse than it gives out. It collapses in on itself, cramming its centre first, then all nearby matter, into an infinitely small point with infinite density - a singularity. Nothing — not even light — can escape the gravitational pull of a black hole beyond a threshold known as the event horizon.

The astrophysicists calculated this estimate by modelling not only the lives, but also the pre-lives of the universe's stars. The team built a model of the universe that accurately reflected the different sizes of stars that would be created, as well as how often they would be created, 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 researchers used data such as their mass and a trait called metallicity — the abundance of elements heavier than hydrogen or helium — to find the percentage of candidate stars that would transform into black holes after determining the rate of formation for stars that could eventually transform into black holes. The researchers made sure they didn't double-count any black holes in their survey by looking at stars coupled into binary systems and calculated the rate at which black holes can collide and merge. They then calculated how these mergers, in combination with black holes feeding on neighbouring gas, might influence the size distribution of black holes observed throughout the universe.

The researchers used these calculations to create a model that tracked the population and size distribution of stellar-mass black holes through time to arrive at their staggering figure. The researchers then proved that their model was in good accord 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 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.

Because these huge black holes are the result of the merger of smaller stellar-mass black holes — or black hole'seeds,' the researchers expect that gaining a better understanding of how small black holes developed in the early universe may aid in the discovery of their supermassive counterparts.

In a statement, Lumen Boco, an astrophysicist at SISSA, said, "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," which we will pursue in a forthcoming paper."

References:

• The Astrophysical Journal AlexSicilia et al 2022 ApJ 924 56 .