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[Physltest] [Phys-L] expansion of gases



My physics colleague brought a puzzling idea to me today regarding
cosmology. We all know that when a gas expands, it cools. By conservation
of energy, the expanding gas cools and its surroundings warm up.

But what happens when a gas expands in a vacuum? If it cools, is the total
energy still conserved? More specifically, what about the Big Bang? If the
Universe was very hot in the beginning and expanded (into a vacuum), then
it should cool. And it has -- to 2.7 K. But does this violate conservation
of energy? We were both baffled. Can anyone help us with this apparent
dilemma?
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