The universe could contain 40 trillion black holes

A team of researchers from the Higher International School for Advanced Studies in Trieste, Italy, has just made the most accurate estimate yet of how many stellar-mass black holes (those formed after the gravitational collapse of very massive stars) exist in the universe. Universe. And the result is a really huge number, so much so that it may seem impossible to calculate. 40 trillion, that is, a 4 followed by 19 zeros: 40,000,000,000,000,000,000. And that is equivalent to 1% of all the baryonic matter (the ‘normal’ matter, neither dark nor exotic), from which the planets and stars of the Universe are made.

In an article published a few days ago in ‘The Astrophysical Journal’, the scientists explain the ingenious method that has allowed them to reach that figure.

By definition, a black hole cannot be seen, since no light or radiation can escape its enormous gravity. In fact, astronomers can only directly observe supermassive black holes, with masses equivalent to billions of suns, since these monsters cause the matter they attract to rotate at high speeds around them, start to shine and thus reveal the outline of the black hole.

But with those of stellar mass, between five and ten times the Sun, things are quite different. Sometimes it is possible to reveal their presence by the local gravitational effects that these black holes cause in their environment, but the vast majority are absolutely invisible. And no one so far had managed to estimate their number. How did the Trieste team do it then?

Tracking stellar evolution
According to astrophysicist Alex Sicilia, first signer of the study, what they did was track the evolution of stars in our universe, that is, estimate how often stars, whether single or binary, become black holes. “This is one of the first and one of the strongest ab initio [from scratch] calculations of the stellar black hole mass function in cosmic history,” Sicilia said.

To ‘make’ a black hole, the first thing that is needed is a large star with a mass between five and ten times that of the Sun. As is well known, as stars reach the end of their lives, they begin to merge heavier and heavier elements within their fiery cores. But when the star reaches iron, if it has the right mass, its days are already numbered. Iron, in fact, consumes more energy to fuse than it emits, which causes it to stop ‘pushing’ and opposing the gravitational forces generated by its own mass and trying to compress it. In the end, gravity wins the battle and mercilessly crushes the star, ‘packing’ all its mass closer and closer until it becomes a single point of microscopic dimensions and infinite density: a singularity. The star becomes a black hole from which nothing, not even light, can ever escape again.

A statistical model
To arrive at their estimate of 40 trillion, the researchers used known statistics from various galaxies, such as their sizes, the elements they contain, and the sizes of the gas clouds in which stars form, and built a model of the universe. which accurately reflected the different sizes of stars that would form, and how often they would be created.

The next step was to pin down how many stars could eventually become black holes, and to model what their lives and deaths would look like using data such as their mass and metallicity, a trait that indicates the abundance of elements heavier than hydrogen or helium. In this way, Sicilia and her colleagues found what percentage of those candidate stars would eventually become black holes. By also looking at paired stars in binary systems and calculating the rate at which black holes can find each other and merge, the researchers ensured that they weren’t double-counting any black holes in their survey.

With these calculations in hand, the scientists designed a third model capable of revealing the distribution and size of stellar-mass black holes over time. The data showed that their number was really huge: 40 trillion. To be sure, the team compared their estimates with data collected by gravitational-wave observatories.