Q&A: Black Holes

Q:
I have read that the universe, at its inception, may have included billions of black holes that don't appear to exist currently. If that is the case, have some or all of these black holes ceased to exist at some point? Did they combine with others to form larger, consolidated black holes?

A:
The black holes that existed in the early universe haven’t ceased to exist. They have just gone into hibernation because as someone remarked, a black hole can eat, but it can’t hunt. In other words, if the black hole sucks up all the gas in its vicinity, the production of X-rays and other forms of light by matter swirling toward the black hole will cease. For a black hole containing the mass equivalent of a billion suns in a normal galaxy, its radius of attraction is about a thousand light years. Beyond that, the random motions of the stars and gas clouds would be enough to keep it out of the black hole’s gravitational grasp.

It is still possible to detect dormant black holes through their gravitational effects of the motions of nearby stars and gas clouds, or the deflection of light from a distant background star, but it is much more difficult.