Expansion leads to cooling. That’s what I was taught anyway. And so when I read that the expansion following the Big Bang caused the universe to cool, I first thought, “okay…” But then I thought some more. “Wait a minute! Expansion against what???”
The reason a system cools as it expands it that it pushes against something. For example, as a gas pushes a piston outward, the molecules recoil at a slower speed and cooling results. But there is no such piston at work in the universe. There is no boundary that the universe expands against. So why then did the universe cool?
As the universe expanded, space itself stretched and this in turn stretched the wavelengths of the photons, causing their energies to decrease and the universe to cool. The increasing wavelength of photons with increasing expansion of the universe follows from application of Einstein’s General Theory of Relativity. But this then begs another question: how does the resulting energy balance work? Where does the energy lost by the photons end up going?
I had to turn to an expert on this topic for final say, namely Dr. Katherine Holcomb, who co-authored Foundations of Modern Cosmology with John Hawley and is currently a Computational Research Consultant at University of Virginia. In 2019 Katherine very generously responded to my inquiry:
Where does the energy lost by the photons go? It seems to just disappear, but that is incompatible with energy conservation. However, we do not currently understand whether the law of conservation of energy applies to the universe as a whole or what a consistent definition of the total energy of the universe might be. Perhaps further research into the nature of space and time will explain this mystery.
And there you have it!
Thank you for reading my post about this interesting challenge of the conservation of energy. I into much greater detail about energy and its conservation in Chapter 5 of my book Block by Block – The Historical and Theoretical Foundations of Thermodynamics.
Identifying the Micro behind the Macro 