There is a particle accelerator at the center of our Milky-Way galaxy which radiates photon energies at least ten times larger than the energy record of the Large Hadron Collider at CERN, the best collider constructed by humans on Earth.
Recently, the HAWC Gamma-Ray Observatory detected ultra-high-energy photons with energies over a hundred thousand times larger than the proton rest-mass energy (114 TeV). The total luminosity carried by gamma-rays up to a given photon energy scales roughly inversely with that energy. What is the source of this powerful radiation?
The authors of the paper consider emission from high-energy electrons or protons injected by astrophysical accelerators. But there is a more exotic source that they did not consider, namely primordial black holes.
Mini black holes could have been produced through the collapse of rare radiation bubbles during a phase transition in the first millionth of an attosecond (10 to the power of -24 seconds) after the Big Bang. Stephen Hawking predicted in 1974 that a primordial black hole with an asteroid-scale mass of 2.4 billion tons would evaporate over the age of the Universe by emitting gamma-rays.
A cosmic population of relic black holes would respond to gravity in the same way that elementary dark-matter particles do. They are expected to cluster near the center of the Milky-Way, at the bottom of the Galactic gravitational-potential well.
The Galactic center may be a mixed bag, containing a collection of different dark particles and dark objects. Even if the evaporating mini black-holes make a tiny fraction of dark matter, they could still make a substantial contribution to the high-energy gamma-rays emanating from the Milky-Way center. Could they be the source of the observed ultra-high-energy gamma-rays?
It is important to keep in mind that black holes which evaporate over roughly the age of the universe would decay to a population of lower-mass black holes with an even shorter lifetime. The present-day abundance of secondary black holes of a given mass below 2.4 billion tons, would scale as the ratio between their lifetime and the age of the Universe.
The Hawking evaporation-time of a black hole scales as its mass cubed whereas the Hawking luminosity scales inversely with mass squared. This implies that at any given time, low-mass black-holes would contribute a luminosity fraction in proportion to their mass. Since the Hawking temperature scales inversely with mass, I figured out before my morning jog that the population of exploding black holes would radiate gamma-rays with a luminosity that is inversely proportional to the limiting photon energy, similar to the gamma-ray spectrum observed by the HAWC observatory from the Galactic center.
What is the mass fraction of dark matter needed to account for the HAWC gamma-ray flux? My calculation indicates that it is smaller than a millionth, a tiny fraction which is permitted by other astrophysical constraints.
Following the black hole population down in mass (`turtles all the way down’) to the Planck mass implies an injection of a power-law distribution of energetic gamma-rays, electrons, positrons, protons and neutrinos up to the Planck energy. As much as they are of interest to theories of quantum-gravity, Planck particles account for an undetectable fraction of a tenth of a quintillionth (10 to the power of -19) of the total luminosity from the population of evaporating black holes.
A few days ago, I gave a lecture at the Harvard Astronomy department about my recent work on primordial black holes. I suggested that extraterrestrial civilizations could use them as engines with up to 100% efficiency for converting rest-mass to usable energy, an idea that I published in a recent paper. My brilliant colleague, Doug Finkbeiner, noted in a follow-up email that such black-hole engines could also be used to propel rockets by channeling their energy output through an exhaust. Doug wrote: “Apart from giant lasers pushing from far away, the usual paradigm for getting rockets to go fast is positronium annihilation to convert mass to thrust with decent efficiency. Even then it is hard to get to 0.2c. That has led me to think that one would search for interstellar commerce in the bulge by looking for a 511 keV excess. But if one could build black-hole rockets, maybe one should look for a GeV gamma excess in the bulge. The fact that both exist already is amusing. Admittedly both of these approaches have formidable engineering challenges, but engineers in the bulge could easily have a billion-year head start on us. They might be extraordinarily talented engineers, at least if they avoid destroying their civilization first by voting for the wrong leaders!”
Doug and I are currently discussing a possible collaborative paper. We agreed from the get-go that aliens might be wiser than humans, and so the content of our paper will not be affected by the results of the U.S. Presidential election tomorrow.
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.
