On what subject(s) are you an authority?
Horticulture is the subject in which I have most knowledge. It is what I do for a living. I also spend a lot of time outside of work reading about plants, pottering around in my small patch of balcony garden and always trying to learn new things.
The great thing about horticulture is that there is always new things to learn and try, so no matter how expert I think I am, I can still learn more.