[publiusr]

newtype_alpha wrote:

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Excitation is detectable for some types of matter in a diffuse state -- atomic hydrogen, for example.

Then too, there is this:

Negative temperature may also have implications for cosmology. Dark energy, thought to explain the accelerating expansion of the universe, exerts negative pressure, which suggests it might have negative temperature – Schneider is currently discussing the idea with cosmologists.


http://nextbigfuture.com/2013/01/neg...ature-for.html