In the infant universe, a significant increase in radiation density on the cosmic horizon scale could have caused some small regions to behave like a closed universe and seal their fate in isolated collapses into black holes.
The typical variations actually observed in the cosmic microwave background radiation had an initial amplitude that is a hundred thousand times smaller than that needed to create black holes. However, these variations can only be observed on large spatial scales. It is possible that as a result of new physics, at high energies on very small scales, rare increases in density with a much larger amplitude were produced. While existing cosmological data only permit this, the existence of dark matter gives additional motivation to consider this hypothetical possibility.
Most of the matter in the universe is dark, and despite searching for signatures of related elementary particles in the sky or in laboratory experiments, none has yet been found. Primordial black holes (PBHs) could potentially create dark matter. Various astrophysical restrictions exclude PBHs as dark matter if they have either low or high masses but allow a mass range between a billionth and a thousandth the mass of the moon – similar to asteroids between one and a hundred miles in size.
An asteroid of this size struck Earth 66 million years ago, killing dinosaurs and three quarters of all life forms. This is a sober reminder that even heaven is a source of risk. We could protect ourselves from future asteroid impacts by looking for reflected sunlight from their surfaces as they approach Earth. In 2005, the US Congress commissioned NASA to find 90 percent of all dangerous objects larger than 140 meters, about a hundred times smaller than the Chicxulub impactor that killed the dinosaurs.
This led to the construction of survey telescopes such as Pan STARRS and the upcoming Vera C. Rubin Observatory that can meet two-thirds of the congressional goal. These surveys use the sun as a lamp post illuminating the dark room near us. An early warning would allow us to deflect dangerous asteroids away from Earth. However, PBHs do not reflect sunlight and cannot be identified in this way prior to impact. They glow faintly in Hawking radiation, but their luminosity is less than that of a miniature light bulb of 0.1 watt for masses over a millionth of the mass of the moon. Is this invisibility a cause for concern?
In particular, when PBHs make up dark matter in the permissible mass range, one can wonder whether they pose a threat to our lives. A PBH’s encounter 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 created 13.8 billion years later. Although this is an extraordinary kind of encounter between the early and late universes, we would not wish it.
Let me explain.
To illustrate, I’ll focus on the upper end of the allowed mass window, where dark matter is made up of PBHs one-thousandth the mass of the moon. Smaller PBHs might be more common, but their effect is weaker. The horizon size of such a PBH is only a thousand times larger than the size of an atom.
One would naively expect that such a small object penetrating our bodies would result in only a minor injury, confined to a finite cylindrical track microscopic in breadth. This would be the case for an energetic particle, like a cosmic ray, which passes through our body like a miniature projectile. But this expectation ignores the far-reaching influence of gravity. The attractive gravitational force created by a PBH of the above mass would shrink our entire body by several centimeters during its rapid transit. The pull would be impulsive and last 10 microseconds for the typical PBH speed of 160 miles per second in the dark matter halo of the Milky Way. The resulting pain would feel like a tiny vacuum cleaner with tremendous suction power was rapidly going through our bodies, shrinking our shells, bones, blood vessels and internal organs. The dramatic physical distortion would cause serious damage and cause instant death. How likely is it that we will experience such a fatal event in the course of our lives?
A reverse-of-the-envelope estimate is happy to take all worries away. If PBHs of the above mass form the dark matter, the chances of a PBH getting through our bodies during our lifetime are tiny, only a part in 1026. This corresponds to a small probability of the order of 10-16 for a single death in the total population of currently eight billion people on earth. The chance of death increases to 10–9 if the current population size lasts for another billion years, then it is expected that the expanding sun will boil away on all oceans on earth. And if we assume similar statistics for stars in other galaxies, then only up to a trillion people in the total observable volume of the universe could be killed by the passage of PBHs through their bodies. It is extremely safe to assume that none of us will be one of these people. The total number of deaths could be greater 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.
Still, it is possible that rare, invisible objects on the edge of the solar system, such as the hypothetical Planet Nine, are PBHs. In a recent work I wrote together with my student Amir Siraj, we showed that PBHs can be detected with the Vera C. Rubin Observatory in the entire solar system through the flares that they generate when they hit rocks from the Oort cloud to meet.
Obviously, the risks to life on Earth from other disasters like asteroid impacts are much greater than the dinosaurs learned firsthand. The above numbers imply that we shouldn’t lose sleep or increase our health insurance coverage because of concerns about invisible PBHs lurking in the halo of the Milky Way galaxy. In 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.