On the oddly pervasive notion of the conservation of energypic.twitter.com/j0iWw1sbTY
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The fact that energy is not conserved in an expanding universe is actually what makes the ground state (GS) special. In a fix cavity a system would go forth and back between an excited state and a GS + photon state. The redshift prevents this on universal scales.
Being wrong on the Internet: best way to learn 
Thanks to everyone who is clarifying this point!
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I thought that was supposed to be the quickest way. ^^ But to be fair, this crucial fact is never really discussed. In fact you can build a probabilistic expansion history from this alone because radiation and dust (i.e. atoms) have a different effect on expansion. 1/2
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take a universe with one sort of QM systems all in an excited state. they behave as dust. suppose they all decay together then the excitation E gets releases as radiation. A fraction of the energy content is radiation. depending on the time of decay you get a different expansion
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