Cosmic Waterfalls in Spacetime Cliffs
Waterfalls are sites where gravity pulls water down a steep cliff. This setup endows the water with rapid motion as it falls down the gravitational potential well.
In Albert Einstein’s theory of General Relativity, gravity is the curvature of spacetime and matter could reach the speed of light in steeply distorted spacetimes.
There is no steeper cliff of spacetime than the vicinity of a black hole singularity, where infalling matter reaches the speed of light and the gravitational tide can rip apart any object. Ironically, the heating caused by the infall of matter makes some black hole environments the brightest sources of light in the Universe. They are observed all the way to the edge of the observable Universe, when the cosmic clock showed less than a billion years after the Big Bang — merely 7 percent of the current time.
The earliest supermassive black hole was discovered by the Webb Telescope 500 million years after the Big Bang in the form of the source CAPERS-LRD-z9 at a cosmological redshift of 9.288. This black hole has a mass of about 40 million solar masses (as reported here). Surrounding galactic gas appears to fall into this black hole like water going down a waterfall. As the gas heats up, it brightens. The resulting luminous quasar appears as a point source of light, so bright that it outshines its host galaxy.
The quasar phase at the centers of all galaxies is short lived, lasting for just 10–100 million years. In the 13.8-billion years movie of cosmic history, quasars appear as explosive flashes of light at the centers of galaxies like decorative sprinkles. Their enormous luminosities are sufficient to heat the surrounding gas and eventually expel it out of the gravitational potential well of their host galaxies. The situation resembles a baby eating more and more food from the table until it eventually becomes energetic enough to push the food off the table. This feedback leads to a self-imposed starvation, which truncates the growth of the central black hole. As a result, supermassive black holes grow up to a limiting mass that correlates with the depth of the gravitational potential well of the host galaxy in which they are embedded.
Lower mass black holes form as a result of the collapse of the core of a massive star after it consumes its nuclear fuel and loses pressure support against gravity. The cosmic waterfall generated by this sudden collapse is immense, often amounting to the consumption of a solar mass per second. In a recent paper that I co-authored (reported here), we discovered a massive star in the neighboring Andromeda galaxy that faded into darkness with the telltale signatures of a black hole relic.
The birth of a stellar-mass black hole can also be seen out to great cosmological distances, if it leads to the formation of a pair of opposing jets which channels some of the infalling matter into a collimated outflow. The jets drills through the envelope of the host star. Collisions of shells within the jets generate a bright burst of gamma-rays for an observer aligned with the jets’ axis. Subsequent slowdown of the jets by the ambient interstellar medium generates an afterglow at longer wavelengths. Long-duration gamma-ray bursts are observed out to redshifts comparable to those of quasars.
Altogether, the massive cosmic waterfall created by the infall of matter down the cliffs of steeply distorted spacetimes result in the brightest sources of light in the cosmos: quasars and gamma-ray bursts. It is ironic that the brightest sources of light announce the birth of the darkest objects in the cosmos: black holes.
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In this essay, I featured four amazing watercolors from a series of celestial waterfalls created by the accomplished artist, Greg Wyatt. These watercolors include inspiring statements by Boethius, John Milton and Aristotle. This is the second in a sequence of essays, where Greg and I collaborate on the interface between art and science. The first essay in this series, titled “Music of the Cosmic Spheres,” appeared here.
ABOUT THE AUTHOR
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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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