A significant increase in radiation intensity on the cosmic horizon scale might have caused certain tiny areas to act as closed universes in the early universe, sealing their fate in isolated black hole collapses.
The usual changes in the cosmic microwave background radiation that are measured have an initial amplitude that is a hundred thousand times lower than what is required to create black holes. These differences, however, can only be seen at vast geographic scales. Novel physics at high energy likely resulted in uncommon density boosts with substantially greater amplitudes on extremely small scales. Although available cosmic evidence only allows for this, the presence of dark matter adds to the impetus to investigate this speculative scenario.
The vast majority of stuff in the cosmos is dark, and despite searches for signs of related elementary particles in the sky or laboratory studies, none have been discovered thus far. Dark matter might be created by primordial black holes (PBHs). Various astronomical restrictions rule out PBHs as a dark matter if they have low or high masses, however, they can have masses ranging from a billionth to a thousandth of the mass of the moon, which is analogous to asteroids with sizes ranging from one to a hundred miles.
An asteroid in this size range slammed the Earth 66 million years ago, killing the dinosaurs as well as three-quarters of all life forms. This serves as a sobering reminder that even the sky may be dangerous. We might be able to avoid future asteroid collisions by looking for reflected sunlight from their surfaces as they approach Earth. The United States Congress entrusted NASA in 2005 with finding 90% of all dangerous objects larger than 140 meters, which is approximately a hundred times smaller than the Chicxulub impactor that killed the dinosaurs.
As a result, survey telescopes like Pan STARRS and the upcoming Vera C. Rubin Observatory have been built, which can meet two-thirds of the congressional objective. These surveys use the sun as a light source, illuminating the dark space around us. We would be able to divert deadly asteroids away from Earth if we had an early warning system. PBHs, on the other hand, do not reflect sunlight and hence cannot be detected in this manner before impact. They do shine dimly in Hawking radiation, but for masses more than a millionth of the mass of the moon, their luminance is less than a 0.1 watt small light bulb. Is this invisibility anything to be concerned about?
If PBHs in the permitted mass range make up the dark matter, one would ask if they constitute a threat to our life. A collision between a PBH and a human body would be an undetectable remnant from the first femtosecond after the big bang colliding with an intelligent organism—a pinnacle of intricate chemistry created 13.8 billion years later. Even though this is an exceptional confluence of the early and late universes, we would not want it upon ourselves.
Allow me to explain.
I’ll use the higher end of the allowable mass window as an example, where the dark matter is made up of PBHs a thousandth the mass of the moon. Smaller PBHs may be more prevalent, but their impact is less significant. A PBH with such a horizon is just a thousand times bigger than an atom in size.
One may erroneously believe that such a little thing going through our bodies would only cause tiny harm restricted to a miniscule cylindrical track. This would be the situation if an energetic particle, such as a cosmic ray, passed through our body like a small missile. However, this expectation ignores gravity’s long-term effects. During its short transit, the attractive gravitational attraction created by a PBH of the above-mentioned mass would decrease our entire body by several inches. For the usual PBH speed of 100 miles per second in the Milky Way’s dark matter halo, the pull would be impulsive, lasting 10 microseconds. The resultant discomfort would be as if a small vacuum cleaner with a powerful suction force sped through our body, shrinking our mussels, bones, blood vessels, and internal organs. The catastrophic body deformity would cause irreversible harm and death. How probable is it that we will face a tragic catastrophe like this in our lives?
Thankfully, a back-of-the-envelope estimate takes care of everything. If PBHs of the aforesaid mass make up dark matter, the chances of a PBH traveling through our body during our lifetime are extremely slim, at one part in 1026. This amounts to a low likelihood of order 10–16 for a single fatality in the world’s current population of eight billion people. If the present population number maintains for another billion years, the growing sun is anticipated to burn all of Earth’s waters, increasing the risk of one death to 10–9. If we assume comparable statistics for stars in other galaxies, then the passage of PBHs through their bodies might kill up to a trillion individuals in the whole observable expanse of the universe. It is exceedingly unlikely that any of us will be one of these individuals. If the multiverse contains many more volumes with comparable circumstances, and if even more hazardous varieties of dark matter exist in portions of it, the overall number of fatalities might be higher.
Nonetheless, rare, undetectable objects on the outside of the solar system, such as the theorized Planet Nine, are PBHs. In a recent study with my student Amir Siraj, we demonstrated that PBHs may be detected across the solar system by the flares they produce when they collide with objects from the Oort cloud using the Vera C. Rubin Observatory.
Other disasters, such as asteroid strikes, obviously pose considerably larger hazards to life on Earth, as the dinosaurs experienced firsthand. According to the aforementioned figures, we shouldn’t lose sleep or improve our medical insurance coverage because of fears about unseen PBHs hiding in the Milky Way’s halo. This is a wonderfully positive message from Mother Nature that we should enthusiastically embrace in these days of looming danger from pandemics and climate change.