"Open" and "closed" in the geometrical sense vs the thermodynamic sense Perhaps this is a silly question, but what is the relationship between the words "open" and "closed" in the geometrical sense (open, flat, closed universes) and in the thermodynamic sense (open and closed systems) in the context of General Relativity? Is there no connection at all? Is there such a thing as a closed thermodynamic system and an open geometry or visa versa? Must a closed geometry be a closed thermodynamic system and visa versa? I have a basic understanding of GR, and I understand a lot of the math behind it, so don't be afraid to give me a technical explaination. Thank you!