The speed of gravity in general relativity

The question is discussed of what is the speed of gravity (at the fundamental non-perturbative level). The question is important, if nowhere else, in discussing the problem of information “lost” in black holes. It turns out that the duly defined “gravitational signal” may generally be causal, superluminal and “semi-superluminal.” In the class of globally hyperbolic spacetimes, the latter two variants coincide. And if some (often imposed, but not always satisfied) conditions hold, the signals may be only causal. In this sense the speed of gravity does not exceed the speed of light.