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[Physltest] [Phys-L] Re: expansion of gases



Hi,
One ( probably crude and inperfect) way to look
at the energy conservation side of this is to sum
or integrate the radiative energy left over from
the big bang. The radiation has been greatly
redshifted, so the energy per unit volume has
decreased. But the volume has increased.

Thanks
Roger Haar



-----Original Message-----
From: Forum for Physics Educators on behalf of Anthony Lapinski
Sent: Mon 1/3/2005 3:30 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: 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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