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Old Oct 30th 2018, 04:47 PM   #1
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Question How does Thermodynamics relate to Cosmology?

Hi, I get a lot of classical thermo e.g. concepts of pressure, temperature and entropy in a closed control volume, but I don't exactly get how it's applied to big picture things such as galaxies, nebula, black hole and the beginning of the universe.

I really can't conceptualise anything that doesn't fit in a room-sized closed control volume, such as in a piston cylinder. How do we formula theory and grand formulae on these enormous scales -

Can somebody shed some light?
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Old Oct 31st 2018, 05:21 AM   #2
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Thermodynamics can be applied to celestial entities just like anything else. So, heat transport across a medium by conduction, convection and radiation happens just like anywhere else. It's a fundamental concept, so you're going to see it everywhere.

A good example of thermodynamics applied in astrophysics is stellar interiors. Stars are great balls of hydrogen and helium gas (plus trace amounts of other stuff). Gravity and gas pressure (+ radiation pressure) compete with each other until equilibrium is established. The equilibrium pressure increase towards the centre of the star and, just like an ideal gas, the temperature increases with increasing pressure. In most stars, the temperatures are high enough to ignite fusion reactions, so there is additional heating in the centre of the star. A full treatment of the stellar conditions must therefore also include radiation pressure. Taking all these factors into account, thermodynamics concepts can be used, together with newton's laws of gravitation and some calculus, to derive models of stellar atmospheres. The results of these models can then be compared to surface phenomenon and observations can be used to verify the models.

Thermodynamics are also used to describe many other things in astronomy and astrophysics. It also gets super interesting when considering things like degenerate states of matter (white dwarfs and neutron stars), variable stars, novae, supernovae, thermal pulses and dredge-up, neutron-capture processes (e.g. s-process, r-process, i-process).

As for cosmology? Well, the thermodynamics starts to get exotic because of the inflation model. How to do you treat the energy content of something that represents the entire Universe at some early state? It'#s not a trivial question for sure! You might want to look at the lambda CDM model of Big Bang, the Friedman equation and go on from there.
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Last edited by benit13; Oct 31st 2018 at 05:33 AM.
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