I'm in gravity (cosmology, GWs, etc.) and starting my fourth year and only just recently took GR for the first time. I'm wondering what standard I should set for myself WRT GR. It feels like you'd need to be an expert in GR just to identify worthwhile problems to work on, and I'm expected to have a PhD in two years. When you get your PhD, are you expected to be an expert not just in your thesis topic, but your field too? Should I worry if I understand GWs but don't understand black holes so well, to pick an example?