The early universe could have given birth to primordial black holes. If rare regions had an enhanced mass density such that their self-gravity reversed cosmic expansion on the scale of the cosmic horizon, then they would have collapsed to black holes. In such a case, the black hole horizon would be similar in size to the cosmic horizon, the distance travelled by light since the Big Bang. The light crossing-time of the horizon radius for a non-spinning black hole with the mass of the Sun is ten microseconds. This implies that primordial black holes of a solar mass could have formed when the Universe was ten microseconds old. The cosmic formation time of smaller black holes is proportional to their mass, because the horizon radius scales linearly with black hole mass.
It is possible that dark matter, which makes 85% of the cosmic matter budget, is simply a collection of primordial black holes. The black holes that make dark matter are allowed to have asteroid masses between 10^{16}-10^{22} grams. Lower mass black holes would have overproduced the cosmic background of X-ray and gamma-ray radiation through their Hawking radiation, an effect predicted in 1974 by Stephen Hawking. Higher mass black holes would have magnified the brightness of background stars beyond the limits set by microlensing surveys.
Alternatively, dark matter might be composed of weakly-interacting elementary particles. If the particle mass is comparable to that of the proton, then about a hundred billion such particles are passing through our body every second. The passage is harmless since the particles are sterile. In fact, a quadrillion neutrinos from the Sun are passing every second through our body, each carrying as much energy as the rest-mass of the electron. They are harmless because they only couple to our flesh and blood through the weak interaction.
However, the passage of a single primordial black hole through the human body is a whole different story.
In a new paper, my brilliant collaborator Bob Scherrer showed that an asteroid-mass black hole would generate a harmful supersonic shock wave along its path through the human body, destroying biological tissue along the way. The shock wave would be similar to that produced by a bullet passing through the human body, a well-studied phenomenon. Bob calculated that the injury would be fatal for primordial black holes with a mass larger than 10^{17} grams. In addition, the gravitational tide from the black hole would tend to tear apart cells in the body. Tidal tearing becomes significant for masses above 10^{19} grams.
I pointed out to Bob that primordial black holes below 10^{17} grams would be lethal because of their Hawking radiation. Black holes with a mass of 10^{17} grams radiate about 36 kilowatts in energetic gamma-rays with a typical photon energy comparable to the electron rest mass. Such radiation could be fatal. The Hawking luminosity scales inversely with the square of the black hole mass and the Hawking temperature scales inversely with mass. This implies that the passage of lower-mass black holes is devastating to the human body.
As they approach us, the lower-mass black holes would not be black at all. They would look like bright fireballs, millions of times hotter and ten times faster than the fireballs produced by meteors in the Earth’s atmosphere. The radiation in meteor fireballs is powered by the friction of the impactor with the surrounding air. The radiation of primordial black holes is emitted intrinsically by a quantum-mechanical process. As Hawking showed, a black hole cannot confine particles with a wavelength larger than the size of its horizon. As a result, these particles leak out of the vacuum and the black hole evaporates in an explosive process that accelerates as its mass declines.
Death by a primordial black hole with a mass above 10^{17} grams would feel like an encounter with a dark ghost, tearing your body apart by invisible forces. Death by a black hole with a mass below 10^{17} grams would feel like being hit by a bright fireball that burns your body from the inside. Either way, encountering primordial black holes is not good for your health.
But there is no need to lose any sleep on this health hazard, even if dark matter is composed of asteroid-mass black holes. The good news is that the probability of being hit by a primordial black hole is negligible.
Even if we optimistically imagine that all 100 billion Sun-like stars in the Milky-Way galaxy host a population of eight billion human-like beings, only one human-like being will be killed by a 10^{17} gram primordial black hole every two decades throughout the galaxy. Medical insurance companies can ignore this risk, even if they were to open branches on all habitable exoplanets and serve the entire Milky-Way galaxy.
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.
