How many black holes are there in the universe?
To create a black hole, you need to create stars, because black holes come from the death of stars. . The first step is to model the evolution of galaxies over the billions of years of cosmic history. Galaxies are the homes of stars, after all, and their overall evolution affects the number of stars that appear within them. Astronomers have taken known observations of galaxy statistics across cosmic time, noting the general trend in galactic melting rates and demographics. Another key factor is the so-called "metallicity" of a galaxy, which is a measure of the amount of elements other than hydrogen and helium inside a galaxy (astronomers call these "metals"). Larger galaxies will have more gas, which allows them to form more stars. But more metals can improve gas cooling, which in turn helps galaxies churn out new stars efficiently.
With these building blocks, astronomers had a model of the stellar population within galaxies, telling them how many small stars, medium stars and large stars appear in the universe. And then they needed to trace the evolution - and, above all, the deaths - of those stars. To do this, they turned to simulations, which link the properties of a particular star (its mass and metallicity) to its life and eventual disappearance. Only a fraction of the largest stars produce black holes, and those simulations tell astronomers what percentage of stars in a galaxy are lighting up each year.
Next, astronomers had to track the evolution of binary systems, as black holes can feed on sister stars, becoming engorged with their gas in the process. Thus a black hole formed in a binary system will eventually be larger than a black hole born on its own.
As black holes age, they continue to feed on any surrounding gas, which astronomers have also estimated . Finally, black holes occasionally find each other in the darkness of interstellar space and merge together. Then, to produce an accurate survey, the astronomers had to estimate the merger rate of black holes within each galaxy.
By putting all the pieces together, the astronomers were able to track the population of black holes over billions of years. They produced what is called a "mass function," which is a kind of astronomical census, reporting how many of each size of black hole exist at any given time. Unsurprisingly, larger black holes, called supermassive black holes, are much rarer than their smaller cousins. The researchers found that in every cubic megaparsec of space (where a megaparsec is one million parsecs, or 3.26 million light years), our universe is home to approximately 50 million solar masses of black holes. If each black hole is a couple of times the mass of the Sun, this translates into about 10 million individual black holes in the same volume. To put it into perspective, the total amount of mass contained in black holes is about 10% of the mass contained in stars. So for all the stars you see in the night sky, there are many black holes lurking.
