I think this is physics based(?), but I thought of this question during my summer chemistry course, so I wasn’t sure of the flair.
In: Physics
This is an excellent question with a surprisingly deep answer.
The short version is, the total energy in an expanding universe **is not constant**. For instance, when light from a distant galaxy is redshifted to lower energy wavelengths by traveling through the expanding universe, that energy doesn’t go anywhere, its just *gone*.
This can seem a little startling, Conservation of Energy is held up as one of the unbreakable laws of physics, but it actually comes from an even deeper relationship that most people have never heard of.
In the 1920s mathematician [Emmy Noether](https://en.wikipedia.org/wiki/Emmy_Noether) was interested in *why* the various conservation laws that scientists had discovered would be true. She managed to prove that any system–including the universe–that has a certain type of symmetry would have a resulting conserved quantity, and showed how to calculate one from the other.
In the case of energy, the symmetry is called “Time Translation Symmetry”, which just means that the laws of physics work the same no matter *when* you perform an experiment; today, last week, 20 million years in the future, whenever.
However, on *Cosmic* timescales of billions of years, this isn’t true. The universe used to be hot and dense, a soup of high energy particles blinking in and out of existence, and now its cold, dark, and mostly empty. You can experimentally determine *when* you are just by looking around. This means that Time Translation Symmetry is broken on very long timescales, as long as the age of the universe, and so on those same timescales energy isn’t exactly conserved.
This doesn’t show up in day to day life because our lives are too short for this to have any effect, but its real. Don’t get your hopes up for a perpetual motion machine or anything like that though.
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