What Caps the Mass of the Most Massive Black Holes?

Avi Loeb
5 min readMar 22, 2024


A black hole accreting gas from its environment (Credit: NASA/ESA/CSA).

A quasar is a point source, a quasi-star, with broad emission lines from gas clouds moving at speeds of thousands of kilometers per second around it. These speeds are an order of magnitude higher than the characteristic values inside galaxies.

Quasars were first discovered in 1963 by Maarten Schmidt and explained theoretically in 1964 by Ed Salpeter in the West and Yakov Zel’dovich in the East, as supermassive black holes which are accreting gas from their host galaxies. The black holes do not emit light, but the gas approaches them at a fraction of the speed of light and dissipates about a tenth of its rest mass into radiation near the event horizon. As a result, some quasars appear as the brightest sources of light in the early Universe. With the Webb telescope, we can detect these bright beacons of light all the way out to the time when the Universe was merely a few percent of its current age, 400 million years after the Big Bang.

The seeds of the first black holes could have been massive stars of up to 100 solar masses, or supermassive stars of up to a million solar masses — which collapse directly to a black hole, as I proposed with Volker Bromm in a 2003 paper. The growth of the early seeds is exponential in time with a doubling-period of order 400 million years, independent of the black hole mass. That this doubling-period equals the age of the Universe for the earliest known quasars, implies that the first black holes had to accrete all the time in order to grow in the infant universe. By today, the cosmic age of 13.8 billion years is 30 times longer than the doubling-period, making bright quasars a rare episode in the history of the vast majority of galaxies.

We can derive an upper limit on the highest luminosity that any black hole of mass M can have, by dividing its rest-mass energy E=Mc² by the time it takes light to cross its smallest horizon scale, GM/c³, where c is the speed of light and G is Newton’s constant. No energy can be released at a faster time and E represents the maximum energy available in the system. The resulting limit on the luminosity, (c⁵/G) = 4x10^{52} Watts, is ten billion times brighter than the highest luminosity quasar.

Remarkably, the highest-luminosity quasars appear to reflect black holes with a few tens of billions of solar masses at all cosmic times over the past 90% of cosmic history. This is puzzling because one would expect a 10 billion solar mass black hole at a redshift of 6 to grow by at least a factor of ten until today, even if it were to maintain the same growth rate as it exhibited in the first billion years of its existence. But for some unknown reason, there appears to be a cap to black hole growth and this cap does not evolve with redshift. What is the physical origin of this cap?

One way to figure it out is by observing the environments of the most massive black holes in the present-day universe. These black holes reside at the centers of clusters of galaxies. The cluster cores are filled with hot X-ray-emitting gas. The cooling time of this hot gas is shorter than the age of the Universe and should have delivered a huge amount of cold gas to feed the central black hole. The expected inflow rates of a solar mass per day are similar to the accretion rate inferred for the brightest quasars in the early Universe. However, detailed observations by the Chandra X-ray telescope did not reveal the expected cold flows, suggesting that heating by conduction, turbulent mixing or jets from the central black hole suppress the inflow of cold gas and the resulting black hole growth. This suppression is responsible for the absence of black holes with hundreds of billions of solar masses in the local Universe. The most massive black holes are starved by the lack of cooling flows around them. This also suppresses star formation inside the central galaxy of X-ray clusters. As a result, these environments end up as clusters of galaxies containing mostly hot gas, rather than super-galaxies with more stars.

At the event horizon of the most massive black holes, the gravitational tide measures about two percent of the tide raised on Earth by the Moon. Clearly, this weak gravitational tide would have a negligible effect on the body of astronauts on their way into the black hole horizon. For a few days after crossing the event horizon, these astronauts would enjoy the trip until their body would inevitably be ripped apart near the singularity. In the last few days of my life, I would love to take this one-way trip for fun. The decision would be tougher for my students who subscribe to social media, because they wouldn’t be able to share their experience through tweets or Instagram posts to the outside world once they enter the event horizon. The benefit of not subscribing to social media is that there is no downside to entering the event horizon of the most massive black hole in the universe.

Do supermassive black holes impact faraway life in the cosmos? In a paper with my former postdoc, John Forbes, we showed that about a tenth of all habitable planets in the Universe would have suffered significant mass loss from their atmospheres as a result of quasar illumination from the center of their host galaxies. It is remarkable that life on Earth could have been affected by our Galactic center before we came to exist. As of now, the 4 million solar mass black hole there is fortunately starved.


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 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".