How big is a Micro black hole?

Answer 1

The term microscopic black holes usually appears in context of the big bang, since they may have been created by conditions in the early universe. Since it implies microscopic, an easy answer would be that their size is "smaller than would be visible". One important fact about black holes to remember is that they could potentially have any radius - there is no lower limit to this, all the way down to approaching zero. This is because the radius of a black hole is a direct function of their mass. The Schwarzschild radius of a black hole is simply twice the gravitational constant times the mass, divided by the square of the speed of light. If an object has a radius smaller than its own Schwarzschild or gravitational radius, it is a black hole; if it is larger, then it is not, so in this sense their existence is defined by their density. The Earth has a gravitational radius that would make it about the size of a marble; the Sun's is about 3km.

The reason for the scant evidence of microcopic black holes might relate to the mechanisms by which black holes are created. The most common mechanism is believed to be associated with stellar evolution wherein, at the end of a star's life when it has exhausted its fuel and can no longer resist the inward pull of its own gravity, it will collapse; but the mass required to do this would not create a microscopic one. Whether the big bang created any microscopic black holes is still a subject of research; other microscopic ones may come into existence in association with collisions of extremely powerful cosmic rays.

One factor working against the existence of microscopic black holes is Hawking radiation, a mechanism by which they can lose mass and "evaporate". Although Hawking radiation is weak, the intensity of the radiation is inversely proportional to their size - so microscopic black holes are expected to be short-lived.