meekerdb wrote:
On 11/9/2014 7:01 PM, Bruce Kellett wrote:
LizR wrote:
On 8 November 2014 16:53, John Clark <[email protected]
<mailto:[email protected]>> wrote:
On Thu, Nov 6, 2014 at 3:56 PM, meekerdb <[email protected]
<mailto:[email protected]>> wrote:
> I'd say that expansion of the universe is almost necessary,
not contingent.
I'd say that by about 1850 when people started to have a
understanding of what Entropy was physicists had all they needed to
have known that the universe must have started out in a very very
low entropy state, that is to say they could have predicted the Big
Bang in the early to mid 19th century; and they wouldn't have needed
to go near a telescope to do so. But unfortunately they didn't, it's
one of the great failures of nerve or imagination in the history of
science.
Another feature of the big bang / expanding universe is that it
continually raises the entropy ceiling (maxium entropy that can exist
in a given volume).
I think you should stop saying this. It is not true. You have not
defined what you mean by "maximum entropy" nor have you specified how
that maximum is calculated. If the maximum is defined as when all
available degrees of freedom are in thermal equilibrium, then the
universe has never been in such a state of maximum entropy,
How about defining the particle maximum as when all mass-energy is in
thermal equilibrium - leaving gravitational modes out. Isn't that how
inflation is used to explain the CMB uniformity?
Yes, but that is not the argument Liz is making.
and it probably will not be until all matter has collapsed into black
holes and these have decayed by Hawking radiation.
At any finite time, one useful concept of maximum entropy is to
consider the state in which all mass energy is in the form of black
holes. This has never happened either.
Presumably because inflation was much faster than the time for
gravitational collapse.
Of course. The interaction with gravitation is very weak and the
relevant time scales are much loger than those required for a plasma to
thermalize the particle degrees of freedom.
Neither of these maxima is in any way affected by the expansion of the
universe as a whole.
So you cannot get around the need to postulate a low entropy condition
at the BB.
I agree that there must have been a low entropy condition, but did it
have to be low relative to various constaints? Of all the ways for the
universe to be, being in a Planck-size volume is a very unlikely one
(which is what Penrose points out). But given that size I don't see why
it's conditional entropy could not be high.
The hot BB was never a universe of Planck size. The seed for inflation
might have been that small, but remember that at the end of inflation
the universe is at exactly zero degrees absolute -- it is totally
frozen. The hot BB arises with reheating, and that could potentially
been of almost infinite (or at least unbounded) size. It certainly was
not small on any scale.
Bruce
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