What is the importance of black holes to cosmology?
A. Indirectly, they tell us that our relativistic theory of gravity and space-time provided by Einstein’s general relativity is fundamentally correct, so that when we use these same equations to study cosmology we have some confidence that they may be correct. Directly, black holes tell us that the universe can hide much of its matter in a way that still contributes to the total mass of the universe, but may not contribute to the abundances of certain primordial elements such as hydrogen and helium. If enough black holes were produced soon after the Big Bang but before the first few minutes, this could have an impact on the relationship between how rapidly the universe is expanding and the origin of the primordial element abundances. But it is expected that most black holes formed long after the Big Bang by stellar evolution, and these black holes may contribute to the missing mass in the universe up to the maximum limit set by the primordial element abundances themselves. It is a bit
