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**Gregor Scholten[_2_]** · June 7th 17, 08:08 AM posted to sci.physics.research,sci.astro.research

[I already tried to send this on Saturday, June 3. This is the second
attempt]

(Phillip Helbig (undress to reply))
wrote:

A smooth distribution corresponds to high entropy and a lumpy one to
low entropy if gravity is not involved. For example, air in a room
has high entropy, but all the oxygen in one part and all the nitrogen
in another part would correspond to low entropy.

If gravity is involved, however, things are reversed: a lumpy
distribution (e.g. everything in black holes) has a high entropy and a
smooth distribution (e.g. the early universe) has a low entropy.

Let's imagine the early universe---a smooth, low-entropy
distribution---and imagine gravity becoming weaker and weaker (by
changing the gravitational constant). Can we make G arbitrarily small
and the smooth distribution will still have low entropy? This seems
strange: an ARBITRARILY SMALL G makes a smooth distribution have a low
entropy.

The solution is that it is a matter of temperature. That a lumpy
distribution has higher entropy than a smooth distribution as soon as
gravity is involved is only true for low temperatures. For high
temperatures, the smooth distribution still has the higher entropy.
That's why the universe has to be cold enough before galaxies and stars
can form.

Imagine a van der Waals gas in a bottle: above the boiling point, the
gas phase with smooth distribution of the atoms is preferred, below the
boiling point, the liquid phase with lumpy distribution is preferred.
The value of the boiling point itself depends on the strength of the
attractive forces betweens the atoms. The stronger these forces are, the
higher is the boiling point.

So, for a small gravitational constant G, the universe has to be very
cold for lumpy distributions (galaxies, stars, planets) being preferred,
i.e. having higher entropy. For a higher value of G, the temperature can
be higher. In the limit G - 0, the critical temperature is running to T
= 0, too, making the behaviour being the same as in a universe without
gravity.

An example: in the 1980's, many physicists believed that neutrinos would
be a good candidate for Dark Matter. Since their masses are very small,
they would be "hot" Dark Matter, i.e. even for low temperatures around 3
K, their average velocities are very high, in the range of the speed of
light. Computer simulations showed then that such hot Dark Matter
couldn't yield the structures we observe in the universe. Hot Dark
Matter would allow for super-clusters and voids to form, but not for
galaxies or even stars. So, hot Dark Matter would be still above the
"boiling point", even at 3 K. That's why physicists search for "cold"
Dark Matter instead today.