As we know many situations still fulfill physics law if time is reversed, such as particle collision. But how about black hole formation?
Suppose a star is turning to a black hole and starts to have event horizons. If now I can reverse all travel directions of all matter in the universe, can the 'black hole' turn back to a star again? If not, which part makes this situation asymmetry?
The entropy of a black hole is the maximum possible for a given volume of spacetime. Spontaneous un-formation of a black hole in this view would be like un-smashing an egg: theoretically permitted, but so improbable as to be ridiculous.
You are asking the dynamics of General Relativity without considering thermodynamics. Most processes are not reversible from the perspective of thermodynamics because of the second law.
However, if you persist, as a non-thermodynamical process, is the black hole formation reversible? Yes, the dynamics of General Relativity is reversible. You can simply reverse the time direction in the Black hole formation, or in the Kruskal coordinates. Then you will have a white hole explosion instead of black hole formation. But since we need to consider the initial condition for our universe, we don't believe such process indeed exists.
