Searching for small primordial black holes in planets, asteroids and here on Earth

TL;DR

This paper explores the possibility of detecting small primordial black holes within planets and asteroids by analyzing the structures they could create, such as hollow shells or tunnels, and discusses potential observational signatures on Earth.

Contribution

It provides a theoretical framework for identifying small primordial black holes through their effects on planetary and asteroid structures, including the formation of hollow shells and micro-tunnels.

Findings

Granite or iron can support hollow structures up to 0.1 Earth radii.

A black hole of 10^{22}g could leave detectable micro-tunnels in rocks.

Searching for tunnels in old rocks is a low-cost method with potential high payoff.

Abstract

Small primordial black holes could be captured by rocky planets or asteroids, consume their liquid cores from inside and leave hollow structures. We calculate the surface density and surface tension of a hollow structure around a black hole and compare them with the density and compressive strength of various materials that appear in nature to find the allowed parameter space. For example, granite or iron can support a hollow asteroid/planetoid/moon of the size of up to $0.1 R_\oplus$. Along the same lines, future civilizations might build spherical structures around black holes to harvest their energy. Using the strongest material that we currently know how to make (multiwall carbon nanotube), to withstand gravity of one solar mass black hole, the shell must be constructed at distances larger than $10^4 R_\odot$. Alternatively, a fast black hole can leave a narrow tunnel in a solid object while passing through it. For example, a $10^{22}$g black hole should leave a tunnel with a radius of $0.1$ micron, which is large enough to be seen by an optical microscope. We could look for such micro-tunnels here on Earth in very old rocks, or even glass or other solid structures in very old buildings. While our estimate gives a very small probability of finding such tunnels, looking for them does not require expensive equipment and long preparation, and the payoff might be significant.