Expanding universe and thermodynamics

[Paul Howard A]

That's because the thermodynamic budgetry you are used to is based on an underlying assumption of time translation invariance (which someone mentioned in a previous post). The expansion of the universe breaks time translation invariance; that means the thermodynamic budgetry you learned no longer holds in the form you learned it. There are ways of constructing a modified budgetry, but it still has limitations.

This article by Sean Carroll is worth reading in this connection:

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Excellent link, PeterDonis. Directly addresses the question.