In the infant universe, a substantial enhancement in the radiation density on the scale of the cosmic horizon could have made some small regions behave as a closed universe and sealed their fate in isolated collapses to black holes.
The typical variations that are actually observed in the cosmic microwave background radiation had an initial amplitude that is a 100,000 times smaller than needed to make black holes. But these variations can only be observed on large spatial scales. It is possible that rare density enhancements of a much larger amplitude were generated on very small scales as a result of new physics at high energies. Although existing cosmological data just allows for that, there is added motivation to consider this hypothetical possibility because of the existence of dark matter.
Most of the matter in the universe is dark, and despite searches for signatures of related elementary particles on the sky or in laboratory experiments, none were found so far. Primordial black holes (PBHs) could potentially make the dark matter. Various astrophysical constraints rule out PBHs as the dark matter if they have either low or high masses, but allow for a range of masses between a billionth and a thousandth of the mass of the moon—similar to asteroids with a size ranging between one and a hundred miles.
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Sixty-six million years ago, an asteroid in this size range impacted the Earth and killed the dinosaurs as well as three quarters of all life forms. This is a sober reminder that even the sky is a source of risks. We could protect ourselves from future asteroid impacts by searching for reflected sunlight from their surfaces upon their approach to Earth. In 2005, the U.S. Congress tasked NASA to find 90 percent of all hazardous objects larger than 140 meters, about a hundred times below the size of the Chicxulub impactor that killed the dinosaurs.
This led to the construction of survey telescopes like Pan STARRS and the forthcoming Vera C. Rubin Observatory, which can fulfil two thirds of the congressional goal. These surveys take advantage of the sun as a lamppost that illuminates the dark space near us. An early alert would allow us to deflect dangerous asteroids away from Earth. But PBHs do not reflect sunlight and cannot be identified this way ahead of impact. They do glow faintly in Hawking radiation, but their luminosity is lower than a mini light bulb of 0.1 watt for masses above a millionth of the mass of the moon. Is this invisibility a reason for concern?
In particular, if PBHs in the allowed mass range make up the dark matter, one may wonder whether they pose a threat to our life. An encounter of a PBH with a human body would represent a collision of an invisible relic from the first femtosecond after the big bang with an intelligent body—a pinnacle of complex chemistry made 13.8 billion years later. Although this constitutes a meeting of an extraordinary kind between the early and late universe, we would not wish it upon ourselves.
Let me explain.
To illustrate, I will focus on the upper end of the allowed mass window, at which the dark matter is made of PBHs with a thousandth of the mass of the moon. Smaller PBHs could be more common, but their effect is weaker. The horizon size of such a PBH is merely 1,000 times larger than the size of an atom.
One would naively expect that such a small object passing through our body would only result in a minor injury confined to a limited cylindrical trail of microscopic width. This would be the case for an energetic particle, like a cosmic ray, passing like a miniature projectile through our body. But this expectation ignores the long-range influence of gravity. The attractive gravitational force induced by a PBH of the abovementioned mass would shrink our entire body by several inches during its quick passage. The pull would be impulsive, lasting 10 microseconds for the typical PBH speed of 100 miles per second in the dark matter halo of the Milky Way galaxy. The resulting pain would feel as if a tiny vacuum cleaner with a tremendous suction power went quickly through our body and shrunk its mussels, bones, blood vessels and internal organs. The dramatic bodily distortion would create severe damage and cause immediate death. How likely is it for us to experience a fatal event of this type during our life?
Gladly, a back-of-the-envelope estimate relieves all worries. If PBHs of the above mass make the dark matter, the chance of a PBH passing through our body during our entire lifetime is miniscule, only one part in 1026. This translates to a small probability of order 10–16 for a single death in the entire population of eight billion people living on Earth at present. The likelihood of one death increases to 10–9 if the current population size persists for another billion years, after which the expanding sun is expected to boil off all oceans on Earth. And if we assume similar statistics concerning stars within other galaxies, then only up to a trillion people in the entire observable volume of the universe might be killed by the passage of PBHs through their bodies. It is extremely safe to assume that any of us will not be one of these people. The total number of deaths might be larger in the multiverse if it contains many more volumes with similar conditions, and if even more dangerous types of dark matter exist in parts of it.
Nevertheless, it is possible that rare, invisible objects in the outskirts of the solar system, like the hypothesized Planet Nine, are PBHs. In a recent paper that I wrote with my student Amir Siraj, we showed that PBHs could be detected with the Vera C. Rubin Observatory throughout the entire solar system by the flares they generate when they encounter rocks from the Oort cloud.
Obviously, the risks for life on Earth from other catastrophes like asteroid impacts are far greater as the dinosaurs learned from firsthand experience. The above numbers imply that we should not lose sleep or upgrade our medical insurance coverage over concerns about invisible PBHs that may be lurking in the Milky Way’s halo. During these days of looming risks from pandemics and climate change, this is a refreshingly positive message from Mother Nature that we should happily embrace.
This is an opinion and analysis article.