Observed variations in cosmic microwave background radiation are much lesser than what is required to create black holes. However, these observations are limited to large spatial scales. The possibility of rare, higher amplitude density enhancements on smaller scales due to new physics at high energies cannot be disregarded.
Dark matter, which constitutes most of the universe's matter, provides added motivation to consider the hypothetical possibility of primordial black holes creating it. Despite extensive searches, no elementary particles related to dark matter have been found in the sky or laboratory experiments.
Primordial black holes might be the source of dark matter, but their potential for this role depends on their mass. These black holes could have masses ranging from one billionth to one thousandth of the Moon's mass, equivalent to asteroids between one and one hundred kilometers.
The risk posed by asteroids of similar size is evident from the mass extinction event that occurred 66 million years ago due to an asteroid impact. To prevent future impacts, surveys are being conducted to detect hazardous objects larger than 140 meters.
However, primordial black holes, which do not reflect sunlight and emit extremely faint Hawking radiation, cannot be detected in this way. Despite their invisibility, the possibility of a primordial black hole posing a threat to life is minimal. If they do constitute dark matter, the probability of a fatal encounter with a human body is estimated to be extremely low. This represents an extraordinary encounter between the primordial and late universe, indicating the fascinating complexities of our cosmic existence.
Let explain.
For example, let's focus on the maximum allowed mass range, where dark matter is made up of primordial black holes (PBHs) with a mass one-thousandth that of the moon. Smaller PBHs might be more prevalent, but their impact is less potent. Such a primordial black hole's event horizon would be just 1,000 times larger than an atom's size.
At first glance, it may seem that a minuscule object traversing our bodies would only inflict a small, localized wound equivalent to a microscopic cylinder's width. That would hold true if an energy particle, like a cosmic ray, shot through our bodies like a tiny bullet.
However, this assumption overlooks gravity's far-reaching influence. A PBH with the aforementioned mass' gravitational pull would cause our entire bodies to contract by several centimeters during its swift transit. This gravitational pull would be impulsive, lasting a mere 10 microseconds for a typically moving PBH at 100 kilometers per second in the Milky Way's dark matter halo.
The resultant sensation would be akin to having a minuscule vacuum cleaner with immense suction power quickly move through our bodies, shrinking our bones, blood vessels, and internal organs. Such a dramatic bodily distortion would inflict severe damage and instantaneously result in death. But what are the chances of experiencing such a fatal event in our lifetime?
If PBHs of the discussed mass represent dark matter, the odds of a PBH passing through our bodies during our lifetime are infinitesimal, just one in 10^26. This corresponds to a small likelihood of about 10^-16 of a single death in the current global population of eight billion.
The likelihood of death increases to 10^-9 if the current population size persists for another billion years, at which point the Sun's expansion could cause the Earth's oceans to evaporate.
Assuming similar statistics for stars in other galaxies, only up to a trillion people in the universe's entire observable volume could be killed by a PBH passing through their bodies.
We can safely presume that none of us will be among these individuals. The total number of deaths could be higher in the multiverse if it contains many more similar volumes and if even more hazardous types of dark matter exist in some parts.
However, it's possible that infrequent and invisible objects at the solar system's edge, like the hypothetical Planet Nine, could be PBHs. In a recent paper I co-wrote with my student Amir Siraj, we proposed that PBHs could be detectable in the solar system by the flares they create when encountering rocks in the Oort cloud, using the Vera C. Rubin Observatory.
Clearly, the risks to Earth's life from other disasters, such as asteroid impacts, are significantly higher, as the dinosaurs discovered firsthand.
The numbers above suggest that we shouldn't worry or feel the need to increase our health insurance coverage due to concerns about invisible primordial black holes potentially hiding in the Milky Way halo.
In these times of rising threats from pandemics and climate change, this is a refreshingly positive message from Mother Nature that we should wholeheartedly welcome.
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