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Old June 22nd 17, 02:54 AM posted to sci.physics.research,sci.astro.research
Steve Willner
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Posts: 1,172
Default entropy and gravitation

[[Mod. note -- This article was originally submitted on 2017-07-15
(about a week ago), but I mistakenly misfiled it and have only just
(re)discovered it. I apologise to the author and to readers for the
mixup & long delay.
-- jt]]

In article ,
Steven Carlip writes:
I don't know of anywhere this has been worked out, but I suspect
that if you found a measure of the amount of lumpiness in the
maximum entropy state you'd find that it varies smoothly with G.


More generally, I think one has to consider velocities as well as
positions. The initial state with random positions _but all zero
velocities_ (in co-moving coordinates) has low entropy. In
equilibrium, the velocities would follow a Maxwell distribution. With
gravity, you can raise the entropy of velocities at the expense of
introducing clumpiness, which lowers entropy of the positions. As
Martin (I think) indicated, the tradeoff between the two depends on
temperature. I'm not at all sure I have all the details right, but
this looks like at least one way to think about the problem.

For Martin in another message, the baryon census of the universe is a
subject of active research. Until recently, about half the baryons
were unaccounted for, but it now seems they are located in very hot
gas associated with galaxy clusters. The fraction of baryons in
stars increases with cosmic time but is only of order 10% now. One
recent paper, which apparently still finds baryons to be "missing" is
http://adsabs.harvard.edu/abs/2016A%26A...592A..12E
I have not researched this question in any detail, but the
Introduction of the above paper has lots of relevant references.

For another poster, 'entropy' is a defined physical quantity, not
some general synonym for disorder.

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