I recently was having a discussion about entropy and the concept of the universe being an open or closed system came up. Assuming that the universe is infinite, would it be considered to be an open or a closed system? A closed system being a system in the state of being isolated from the environment. If the universe is infinite, it is not isolated from anything and therefore an open system. But then you could also make the argument that there is no affect on it from an outside environment therefore it must be closed. Which is it?