Introduction

One of the most important laws in all of physics is the conservation of energy: that energy can change forms, but can never be created nor destroyed. At a fundamental level, however, that's only true because there's an underlying symmetry that the Universe obeys: time-translation symmetry, that it's the same from one moment to the next. Only, that's not true in the expanding Universe: the Universe is different from one moment to the next. As a result, energy is not conserved, with truly cosmic implications.