Q&A: Black Holes
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?
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.