thermodynamics and branes

[alex_j]

[I'm creating this thread to hold a discussion of some questions raised by alex_j in a thread where they were off topic. -- bcrowell]

Could someone elaborate on the relationship between the big bang and thermodynamics? (anthropic principle)

How does the state of entropy of a brane relate to that of the universe at the time of it's creation (assuming we exist within a brane)?

How "main-stream" is Boltzmann's universe and what are the general thoughts on fluctuations at high levels of entropy leading to lower levels?

How valid is the statement that the laws of entropy only apply to closed systems and what are the general views about branes being closed or open systems?


** questions arise out of:
1. Dr. Leonard Susskind, Stanford University
2. Dr. Sean Carroll, California Institute of Technology
3. Dr. Roger Penrose @ George Mason University

[Chalnoth]

Could someone elaborate on the relationship between the big bang and thermodynamics? (anthropic principle)
These are two extremely different topics, so I don't quite know why you're lumping them together here.

As for thermodynamics and the big bang, well, we know that early in our universe, the entropy of the universe was unbelievably low. Right now it is many, many orders of magnitude higher. It remains an open question as to precisely how that early, low-entropy state came about.

How "main-stream" is Boltzmann's universe and what are the general thoughts on fluctuations at high levels of entropy leading to lower levels?
The Boltzmann brain issue is basically a proof that you cannot simply consider a universe to be a state which periodically fluctuates out of equilibrium in a standard thermodynamic way. Any alternative theory for producing the low-entropy early universe is required to pass the Boltzmann brain test (that is, it must strongly favor real observers over Boltzmann brains). If it doesn't pass this test, it cannot describe our reality.

How valid is the statement that the laws of entropy only apply to closed systems and what are the general views about branes being closed or open systems?
The laws of entropy absolutely apply to open systems, you just have to take into account what goes on at the boundary.

[Tanelorn]

Chalnoth, I get confused by high, higher, very high, low, lower and extremely low levels of, well, anything really, especially entropy! eg. I often forget that heat death occurs when there is high entropy because that is also when the available energy density is low or zero.

Do you know if there may be a chart showing relative entropy vs time from 10-43 to 10+1500 years? perhaps also showing the energy density?

Thanks
Chris

[bcrowell]

Do you know if there may be a chart showing relative entropy vs time from 10-43 to 10+1500 years? perhaps also showing the energy density?

There have been various attempts to estimate the entropy of the observable universe, but this is highly speculative. It is not really established what should be the general definition of entropy in the context of cosmology. Here is a sample of this kind of work:
http://arxiv.org/abs/0801.1847