By peering inside globular cluster NGC 6397, many astronomers were expecting to find an intermediate-mass black hole. Instead, only a cluster of smaller-mass black holes were found, as illustrated in this artist’s impression. Astronomers, at last, are closing in on the mass function and distribution of black holes within the Universe. (Credit: ESA/Hubble, N. Bartmann)

For the first time, astronomers have created a data-driven estimate for how many black holes are in our Universe: more than anyone expected.

Starts With A Bang!

Black holes are wondrous objects, but how many are out there?

Made famous by the movie Interstellar, this depiction of a black hole seen edge-on with respect to its accretion disk in a highly-curved spacetime shows the substantial spacetime-bending power of a black hole. Close to the event horizon but still outside of it, time passes at a tremendously different rate for an observer at that location than for an observer far away and outside of the main gravitational field. The number of black holes in the Universe, as well as the black hole mass function, is still under investigation. (Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman)

Most black holes form when high-mass stars end their lives.

Imaged in the same colors that Hubble’s narrowband photography would reveal, this image shows NGC 6888: the Crescent Nebula. Also known as Caldwell 27 and Sharpless 105, this is an emission nebula in the Cygnus constellation, formed by a fast stellar wind from a single Wolf-Rayet star. The fate of this star: supernova, white dwarf, or a direct collapse black hole, is not yet determined. (Credit: J-P Metsävainio (Astro Anarchy))

Those stars die in core-collapse supernova events.

The anatomy of a very massive star throughout its life, culminating in a Type II (core-collapse) Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars. (Credit: Nicolle Rager Fuller/NSF)

Some leave neutron stars behind, but the more massive ones leave remnant black holes.

Supernovae types as a function of initial star mass and initial content of elements heavier than Helium (metallicity). Note that the first stars occupy the bottom row of the chart, being metal-free, and that the black areas correspond to direct collapse black holes. For modern stars, we are uncertain as to whether the supernovae that create neutron stars are fundamentally the same or different than the ones that create black holes, and whether there is a ‘mass gap’ present between them in nature. However, the formation of black holes is a plausible end result in nearly all supernova scenarios. (Credit: Fulvio314 / Wikimedia Commons)

Neutron star mergers supplement the black hole population.

We knew that when two neutron stars merge, as simulated here, they can create gamma-ray burst jets, as well as other electromagnetic phenomena. But perhaps, above a certain mass threshold, a black hole is formed where the two stars collide in the second panel, and then all the additional matter-and-energy gets captured, with no escaping signal. (Credit: NASA/AEI/ZIB/M. Koppitz and L. Rezzolla)

Occasionally, stars also directly collapse: (probably) leaving black holes behind.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. (Credit: NASA/ESA/C. Kochanek (OSU))

Although we’ve quantified star-formation throughout cosmic history, the black hole “fraction” remained uncertain.

This 20-year time-lapse of stars near the center of our galaxy comes from the ESO, published in 2018. Note how the resolution and sensitivity of the features sharpen and improve toward the end, all orbiting our galaxy’s (invisible) central supermassive black hole. Practically every large galaxy, even at early times, is thought to house a supermassive black hole, but only the one at the center of the Milky Way is close enough to see the motions of individual stars around it, and to thereby accurately determine the black hole’s mass. The actual number density of black holes in the Universe, and their number density as a function of mass, remains only poorly estimated, with large uncertainties remaining. (Credit: ESO/MPE)

All of this changed, however, since the dawn of gravitational wave astronomy.


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