It Takes Two Black Holes to Tango

Avi Loeb
4 min readDec 10, 2023


A simulation of the light emitted by hot gas in a merger of a pair of supermassive black holes. (Credit: NASA)

Just like humans, black holes come in singles and pairs. And when a third black hole joins an existing pair, the three-body dynamics is often chaotic. Again, just as in human relationships.

Most galaxies host a supermassive black hole at their center. This is the case for our Milky-Way galaxy and its nearest neighbor, the Andromeda galaxy. These two galaxies are on a collision course. In a few billion years, before the Sun will die, the two galaxies will merge. In 2008, I wrote with my former postdoc T.J. Cox the first paper forecasting this merger to a bigger galaxy which I labeled “Milkomeda”.

The supermassive black holes from the Milky-Way and Andromeda are predicted to sink to the center of Milkomeda, each dressed up with a dense cluster of stars. Dynamical friction on surrounding stars and gas is expected to bring the black holes closer down to a distance smaller than the size of the solar system, where the emission of gravitational radiation will cause them to merge. The spacetime ripples by the two black holes are inevitable, akin to the waves stirred on the surface of a pond by two sticks in a circular motion.

Mergers are not only found locally but also in the infant universe. Last week, a new preprint announced the Webb telescope’s discovery of an offset quasar, likely a member of a merging black-hole pair, 740 million years after the Big Bang. As I demonstrated in 17 papers with my former graduate student, Laura Blecha, one expects an abundant population of black hole pairs as a result of galaxy mergers throughout cosmic history.

A black-hole pair in a galaxy like Milkomeda could be interrupted by a third galaxy joining the pair and kicking the lightest black hole out. A decade ago, I calculated in a paper with my former postdoc, Girish Kulkarni, that a substantial fraction of supermassive black-hole pairs could be interrupted by a third black-hole in galactic nuclei. In an earlier paper with my former graduate student, Loren Hoffman, we calculated the chaotic dynamics of the triple black hole system and used it in another paper to explain the characteristics of a specific kicked quasar.

The fundamental question is whether pairs of black holes at large separations would be able to emit their full power in gravitational radiation before they merge. Alternatively, the final merging phase could be accelerated by friction on a large body of gas and stars or be avoided altogether by the lack of any such material that could bring them to the gravitational emission phase. In either case, the emission of gravitational waves will be suppressed compared to the case where all pairs complete their final phase of merging through the emission of gravitational radiation.

Does the census of all black hole pairs at large separations match the expected cosmic background of gravitational waves? Interestingly, both quantities were gauged recently.

First, a paper analyzing Webb telescope data suggested last month that tens of percent of all quasars are in pairs. This large fraction implies that quasars shine at the same time during a merger, like synchronized swimmers, or else we would only notice one of them at a time. This inference is natural given that galaxy mergers are expected to shove large quantities of gas towards each of the merging black holes at the same time. The duration of the merging phase when the feeding is effective — because both black holes are fed from the same gas reservoir, must be comparable to quasar lifetime in order for the fraction of dual quasars to be high.

Second, recent studies used an array of pulsars — spinning neutron stars — as clocks to measure the cumulative gravitational-wave background from all such pairs throughout cosmic history. Does this measurement agree with the census of black hole pairs at large separations?

In collaboration with the brilliant astrophysicist Hamsa Padmanabhan, we showed that the cosmic background of gravitational waves from pulsar timing arrays is weaker than expected based on the census of black hole pairs at large separations from the Webb telescope. This suggests that pairs spend less time in their final phase of merging. The conclusion is not surprising given the tumultuous dynamics and feedback in merging galaxies.

In addition, black hole pairs also act like pinball machines by ejecting stars to outer space. Some of these stars reach the speed of light, as I calculated in a paper with my former postdoc, James Guillochon. If any of these relativistic stars host habitable planets, the travel tickets for riding them must be sold at the highest prices by interstellar travel agencies.

These rides start from the center of a merging galaxy like Milkomeda, and continue through interstellar space within that galaxy, culminating in amazing views of the galaxy from outside in intergalactic space. Sometimes nature offers a spectacle far grander than any of our imagined spaceships.


Image credit: Chris Michel (October 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".