LAYERS OF THE ONION. “To understand the connection between complexity and black holes it is important to think of a black hole as an onion. The near‐horizon region of the black hole is a layered structure. Each layer has its own degrees of freedom. The CFT representation of the various layers differ in their degree of complexity.“ (CFT = Conformal field theory).
https://onlinelibrary.wiley.com/doi/full/10.1002/prop.201500092?casa_token=8OO3b1OY4e4AAAAA%3ARqriAgTwtxnvsxl09CqnNy79m7l4ye7ZrkZxcLtObB9kHsTwYgzBuIV2X-xXQ7ezoR6VQPfQLCS-vnI