The Biggest Mouth in the Universe

Avi Loeb
5 min readDec 8, 2023
Artist’s rendering of a quasar (Credit: ESO/M. Kornmesser).

A black hole is the ultimate prison. It is impossible to exit from the interior of its event horizon even at the speed of light. For those freedom-lovers who are worried about the risk of entering a region where no escape is possible, there is one existential question: how large is the mouth of the most massive black hole in the universe?

The record holder is the quasar TON 618 with a mass of forty billion suns, detected at a redshift of 2.2 when the universe was merely 3 billion years old. A quasar is a supermassive black hole that shines brightly as a result of the viscous heating of gas that drains into it. The mouth of TON 618 is a thousand times bigger than the Earth-Sun separation.

If astronauts approached the horizon of TON 618, they would experience a gravitational acceleration of about 40 “gee” — the value on the surface of Earth. But the difference in the acceleration between the feet and head of the astronauts would be a hundred trillion times smaller. This is five orders of magnitude smaller than the material stress imposed on our bodies by Earth, suggesting a comfortable entry into the event horizon of TON 618. An astronaut’s body would not sense that the invisible horizon was crossed and continue on its free-fall journey towards the black hole singularity for 4 days. The horror will start as soon as the gravitational tide exceeds the material strength of the astronaut’s body close to the singularity. Afterwards, the body will be ripped apart and compressed to extreme densities. The fate of the body’s material is unknown. My personal conjecture is that it will join the surface of a dense object that replaces the singularity of Einstein’s theory of gravity in the ultimate quantum theory of gravity. This central object has the maximum possible density, known as the Planck density — which is roughly 93 orders of magnitude larger than the initial density of the astronaut’s body.

What an amazing 4-day journey! If I knew the timing of my death, I would gladly sign up for this trip in my last 4 days.

How did the mouths of the biggest monsters in the universe grow so large? They started small and grew in mass by swallowing gas and stars from their immediate environment.

Small black hole seeds form naturally when a massive star, tens of times larger than the sun, consumes its nuclear fuel and collapses. This formation scenario was conceived by Robert Oppenheimer and his student Hartland Snyder in a paper titled “On Continued Gravitational Contraction” that they published in 1939. The LIGO experiment confirmed their theoretical expectation through the detection of gravitational radiation from collisions of such stellar-mass black holes.

Some of these seeds reside at the centers of galaxies, where large gas reservoirs could feed them, resulting in a runaway growth to supermassive proportions. During their growth, the light emitted by the resulting quasars heats the surrounding gas and suppresses star formation in the host galaxy.

The runaway growth of a supermassive black hole is exponential over a timescale of order a billion years, dictated only by fundamental constants — like the mass and charge of electrons and protons, Newton’s constant and the speed of light. Coincidentally, this timescale happens to be the age of the Universe at a redshift of 6. The coincidence in time implies that the population of runaway quasars shine most of the time as they grow at higher redshifts, when the universe is younger than a billion years. As a result, star formation is expected to be choked in the host galaxies at very high redshifts by the heating feedback from the growing black holes. As the universe gets older, the active phase of quasars occupies a smaller fraction of the available cosmic time, allowing the gas in galaxies to cool and form stars. Once stars consume the cold gas, less of it is available for the growth of supermassive black holes.

Together with my postdoctoral fellow, Fabio Pacucci, we discovered that the latest data from the Webb telescope indeed supports the above sequence of events. Indeed, black holes are overmassive relative to the stars in early galaxies compared to their counterparts in the present-day universe.

The Sun formed 4.6 billion years ago, after the peak in the quasar activity. Just as with the dinosaurs on Earth, we missed the action when the biggest beasts were active in our environment.

The largest black hole mouths in the universe are larger than the extent of planets in the solar system. Their early growth inhibited the formation of stars around them. Viewed one way, we should feel lucky that life-as-we-know-it was not harmed by a quasar like TON 618. On the other hand, it would have been exciting to be a “fly on the wall” of cosmic history and watch these giant mouths consume matter around them. Well, you cannot eat the cake and have it too.


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".