Tiny Black Holes And Some Related Conjectures

The Schwarzschild radius is the size of the event horizon of a black hole and is a linear function of mass. Specifically, the Schwarzschild radius is calculated using the formula: R = 2GM/c² where G is the gravitational constant, M is the mass of the object, and c is the speed of light. (Spin and charge in a black hole tweak this value.)

Galaxies

The Schwarzschild radius of a typical galaxy is about 10^15 meters (i.e. about 10^12 km or 0.1 light years). In fact, however, this is about 10 times larger than the most massive theoretically possible black holes.

Supermassive Black Holes

The supermassive black hole at the center of the Milky Way has a Schwarzschild radius of around 12 million kilometers (i.e. 1.2 * 10^13 meters a.k.a. about 0.001 light years, which is about 4 light days) due to its mass of about 4.1 million solar masses. The supermassive black holes at the center of galaxies have densities comparable to that of water. The theoretical limit for the mass of a black hole with typical properties is only 5×10^10 M☉, but can reach 2.7×10^11 M☉ at maximal prograde spin (a = 1).

Stellar Black Holes

The Schwarzschild radius of the smallest possible stellar black hole is approximately 10^4 meters (i.e. about 10 km to 12 km), which means that the mass per event horizon volume (i.e. density) of a small stellar black hole is on the order of the same as an atomic nucleus (which is part of why the most dense macroscopic object that is not a black hole is called a neutron star). These are the highest density objects known in nature, and it may be that there is some theoretical maximum density of anything in this vicinity (which would render primordial black holes of sub-stellar mass theoretically impossible).

Black holes should not be able to form from the gravitational collapse of a star below this mass.

The Sun

The Schwarzschild radius of our Sun is 2.9 * 10^3 meters (i.e. 2.9 km).

Earth

The Schwarzschild radius of the Earth is 0.88 * 10^-2 meters (i.e. 0.88 cm).

Asteroids 

The Schwarzschild radius of a typical asteroid is approximately 10^-3 meters (i.e. it is about one millimeter). This density is about 10^21 times greater than an atomic nucleus or in the lightest possible stellar black hole. This is roughly the size of primordial dark matter candidates that have not been ruled out due to either evaporation due to Hawking radiation over the life of the universe so far, or due to astronomy efforts to detect micro-lensing.

Planck Length

The Planck length is 1.616255(18) × 10^−35 meters. 

All distances smaller than this may be ill-defined or limited by the discreteness of space-time if space-time is not continuous.

Top Quark

The Schwarzschild radius of a top quark is approximately 10^-52 meters. This radius is about 10^37 times smaller than the actual size of a proton or neutron, and the volume is about 10^111 times smaller than a proton or neutron. The density of a top quark mass black hole would be about 10^113 times greater than a proton or neutron.

The formula for the Compton wavelength, which is "λ = h/(mc)" where h is Planck's constant, m is the mass of the particle, and c is the speed of light. The Compton radius of a top quark is about 1.1 * 10^-18 meters. 

The mean lifetime of the top quark is slightly longer than that of the W boson by which it decays, which could also be a fundamental floor on the mean lifetime of a particle and as a result on the maximum mass of a fundamental particle, which would explain why there are only three generations of fundamental fermions.

Gauge Bosons

The Schwarzschild radius of a W boson, Z boson, or Higgs boson is approximately 10^-52 to 10^-53 meters.

Bottom Quark

The Schwarzschild radius of a bottom quark is about 10^-53 meters.

Tau Leptons

The Schwarzschild radius of a tau lepton is a little less than twice the Schwarzschild radius of a proton, and a little less than half the Schwarzschild radius of a bottom quark.

Atoms and Nucleons

A proton or neutron has a radius on the order of 10^-15 meters, while an atomic nucleus of the largest possible atom has a radius that is 6-7 times larger. An entire atom, including its associated electrons, has a radius on the order of 10^-10 meters. All atomic nuclei have densities roughly the same as the density of a proton or neutron.

The Schwarzschild radius of a proton or neutron is 2.4 * 10^-54 meters. 

Up and Down Quarks

The Schwarzschild radius of an up quark or down quark is about 10^-56 meters.

Electrons

The Schwarzschild radius of an electron is 1.35 x 10^-57 meters. The Compton radius of an electron (a.k.a. its Compton wavelength) is 2.43 * 10^-12 meters.

Neutrinos

Assuming the neutrinos have masses on the order of 0.6 meV to 60 meV, the Schwarzschild radius of an neutrino is about 10^-66 to 10^-64 meters. The Compton radius of a neutrino (a.k.a. its Compton wavelength) about 10^-6 to 10^-8 meters. A neutrino cross-section is around 10^-38 cm^2 (about 10^-42 meters squared). The Planck area is about 10^-66 cm^2 (about 10^-70 meters squared).

Primordial Black Holes And Renormalization Considerations

As a footnote, when it comes the primordial black hole formation, it also bears noting that the temperature of the universe was vastly higher, and the volume of the universe was vastly larger, in the time frame when this would have happened. 

This means that the Standard Model constants would run to their higher energy scale values, most notably, with the weakening of the Higgs field at high energy scales which reduces the rest masses of Standard Model particles. It isn't clear how that would impact general relativity considerations. 

Energy and not just mass gravitates, and pressure also impacts the E=mc^2 conversion, but energy is generally more diffuse in space than rest mass. Intuitively, while the smaller volume of the universe would tend to favor primordial black hole formation, the higher temperatures of the very early universe would tend to disfavor it, and it isn't clear how these factors would balance out. 

It could also be the case that clumps of matter that weren't initially sufficient to form a black hole due to the weak Higgs field could suddenly flip over into one as the Higgs field strengthened at lower temperatures.