LizR wrote:
SO the AoT comes from the statistics of increasing entropy and is
quite disjoint from the expansion of the universe.
Bruce, I haven't got time to reply at length but one thing stands out.
You have said a few times that the AOT derives from the 2nd law /
increasing entropy. That is however just the definition of the
(thermodynamic) AOT. They're equivalent - you need something else from
which to /derive/ the 2nd law. That is, you have to answer the question
- why was the universe in a low entropy state in the past?
No, I don't have an explanation for the low entropy of the early
universe. One could play with anthropic explanations (if the entropy had
been maximal then there could not have bee an entropy gradient and we
couldn't exist.) This argument has some legs, but I must admit to
generally not being convinced by anthropic arguments. Such arguments can
explain anything, so they really explain nothing.
There are arguments for generation of low entropy starting states from a
pre-existing deSitter universe (Sean Carroll favours such arguments, I
think). But my problem with this is that the real problem arises during
reheating after inflation -- the original birth of the universe from
nothing, or tunnelling from some pre-existing universe, or whatever
becomes irrelevant by the time you get to reheating.
Re-heating is a great mystery in normal inflationary theory, and I do
not actually feel called on to explain absolutely everything. It is more
straightforward to find the flaws in other arguments.
Bruce
(I'll leave aside the radiation arrow and any others that might be
around the place. The thermodynamic one is enough to be going on with,
and I suspect they're all related anyway.)
I've come up with one suggestion which may or may not be correct (I
originally got it from P.C.W. Davies, although admittedly he only
applied it to the entropy of large scale systems, IIRC, not the BB
fireball) and I can think of at least a couple of others, although I
still think cosmic expansion is the most likely simply because it's
available everywhere at the start of the universe and certainly
generates the bound states which (I contend) act as useful sources of
negative entropy, allowing stars to run nuclear fusion and life to exist
- which already gives us some features of an entropy gradient.
Do you have a suggestion for an alternative mechanism that creates or
drives the entropy gradient? I'm just curious (but if you do, it might
enable me to get a better idea of where you're coming from on this).
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