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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 hundred thousand 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.

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.

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