Death by the primordial black hole
In the nascent universe, a substantial increase in the radiation density on the scale of the cosmic horizon could have caused certain small regions to behave like a closed universe and seal their fate in isolated black hole collapses.
The typical variations that are actually observed in the cosmic microwave background the radiation had an initial amplitude a hundred thousand times smaller than necessary to make black holes. But these variations can only be observed at large spatial scales. It is possible that rare increases in density of a much larger amplitude were generated at very small scales as a result of new high energy physics. While the existing cosmological data allows just that, there is additional motivation to consider this hypothetical possibility due to the existence of dark matter.
Most of matter in the universe is dark, and despite searches for signatures of elementary particles on the sky or in laboratory experiments, none have been found so far. Primordial black holes (PBH) could potentially make dark matter. Various astrophysical constraints eliminate PBHs as dark matter if they have low or high masses, but allow a mass range of one billionth to one thousandth of the moon’s mass, similar to asteroids between one and one hundred miles in size.
Sixty-six million years ago, a asteroid in this size range impacted Earth and killed dinosaurs as well as three-quarters of all life. It’s a sober reminder that even the sky is a source of risk. We could protect ourselves from future asteroid impacts by looking for sunlight reflected off their surfaces as they approach Earth. In 2005, the United States Congress instructed NASA to find 90 percent of all dangerous objects over 140 meters, about one hundred times below the size of the Chicxulub impactor that killed the dinosaurs.
This led to the construction of survey telescopes like Pan STAR and the next one Vera C. Rubin Observatory, which can achieve two-thirds of the Congress target. These readings take advantage of the sun like a lamppost that illuminates the dark space near us. Early warning would allow us to deflect dangerous asteroids far from Earth. But PBHs do not reflect sunlight and cannot be identified in this way prior to impact. They shine faintly Hawking radiation, but their brightness is less than a 0.1 watt mini bulb for masses greater than one millionth of the mass of the moon. Is this invisibility a cause for concern?
In particular, if PBHs in the allowable mass range constitute dark matter, it is questionable whether they pose a threat to our lives. An encounter of a PBH with a human body would represent a collision of an invisible relic of the first femtosecond after the big bang with an intelligent body – a peak of complex chemistry achieved 13.8 billion years later. Although this constitutes an encounter of an extraordinary kind between the early and late universes, we ourselves would not wish it.
Let me explain.
To illustrate, I will focus on the upper end of the allowable mass window, at which dark matter is PBH with one thousandth of the moon’s mass. Smaller PBHs might be more common, but their effect is weaker. The size of the horizon of such a PBH is simply a thousand times the size of an atom.
One would naively expect that such a small object passing through our body would result in only 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 aforementioned mass would reduce our whole body by several centimeters during its rapid passage. The attraction 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 make it seem like a small vacuum cleaner with enormous suction power was rapidly passing through our body and narrowing its molds, 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 such a fatal event in our lifetime?
Gladly, an estimate on the back of the envelope relieves all worries. If the PBHs in the mass above make dark matter, the likelihood that a PBH will pass through our bodies in our lifetime is tiny, only one in 1026. This results in a low probability of order 10–16 for just one death out of the entire population of eight billion people currently living on Earth. The probability of death increases to 10–9 if the current population size persists for another billion years, after which the expanding sun is expected boil all of the Earth’s oceans. And if we assume similar statistics for stars in other galaxies, then only up to a trillion people in the entire observable volume of the universe could be killed by the passage of PBH through their bodies. It’s extremely safe to assume that none of us will be one of those people. The total number of deaths could 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 and invisible objects on the periphery of the solar system, such as the hypothetical planet Nine, are PBHs. In a recent paper that I wrote with my student Amir Siraj, we have shown that PBHs can be detected with the Vera C. Rubin Observatory throughout the solar system by the flares they generate when they encounter rocks in the Oort cloud.
Obviously, the risks to life on Earth from other disasters like asteroid impacts are much greater than dinosaurs have learned from first-hand experience. The numbers above imply that we shouldn’t lose sleep or improve our medical insurance coverage due to concerns about invisible PBHs that might be lurking in the Milky Way halo. In these days of looming pandemic and climate change risk, this is a refreshing and positive message from Mother Nature that we should happily embrace.
This is an opinion and analysis article.