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Gravity and Thermodynamics

  1. Jun 8, 2015 #1
    Gravity and Thermodynamics

    I was thinking about something today. Einstein's equations famously said that matter can neither be created nor destroyed, only converted from one form to another. Does this sort of equilibrium apply to temperature as well? One thing we can observe is that the universe is expanding and as so becoming increasingly colder by the light year, and that stars and black holes are forming creating immense temperatures. Is the formation of the stars and black holes consistent with the expansion of the universe and the over all temperature of matter? To say, as space gets greater and further apart, does matter become more dense in order to remain consistent with a constant that provides thermal equilibrium in a changing universe?
  2. jcsd
  3. Jun 8, 2015 #2


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    Due to the expansion of the universe, the temperature of the universe is going down. CMB temperature now is 2.725 deg. K. It started much higher.
  4. Jun 8, 2015 #3


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    I'm not sure where you got this from, but it isn't true. It's approximately true at low energies, and so is a component of classical thermodynamics. But it doesn't work at energies greater than the rest mass of the particles in question.

    As for matter becoming more dense, large gas clouds that are in the process of collapsing become more dense as they lose energy. They lose energy because their internal temperature is higher than the temperature of their surroundings. Because of the way gravity works, this loss of energy results in an increase in temperature (which causes the loss of energy to speed up). This is, fundamentally, why stars form (their collapse is stopped when the interior of the star begins nuclear fusion, so that the nuclear inferno at their cores balances the outgoing energy at their surface).

    For matter that doesn't radiate, such as dark matter, this process doesn't exist. And so dark matter never collapses significantly (there are other processes that cause a very, very slow collapse). So there is no noticeable tendency of dark matter to collapse once it is has formed a bound state. This is why we have yet to observe it here on Earth: the dark matter in our galaxy is spread so thinly that it's very difficult to observe. In fact, you can think of galaxies as gigantic blobs of dark matter spread very thinly, with a small little clump of stars, gas, and dust (the visible galaxy) sitting in the center of this gigantic blob.
  5. Jun 9, 2015 #4


    Staff: Mentor

    No, that's not what Einstein's equation says. It says that stress-energy can't be created or destroyed. But stress-energy is not the same as "matter".

    No, because "temperature" is related to energy density, but energy density is only one component of stress-energy. So there is no "law of conservation of temperature" in GR.
  6. Jun 12, 2015 #5
    "Is the formation of the stars and black holes consistent with the expansion of the universe
    There are no 'rules' being broken as the universe evolves. What's generally 'consistent is that at the end of the universe, entropy is at a maximum...information will be at a minimum. Thereafter it appears all processes stop. In the interim we generally move towards those final states.

    " as space gets greater and further apart, does matter become more dense.....


    From THE FABRIC OF THE COSMOS, Brian Greene, Entropy and Gravity, page 171-173:
    In an ordinary gas the configuration with the highest entropy is that in which particles are uniformly distributed. So one MIGHT expect the uniform early universe to have high entropy. Not so because of gravity.

    "ordinary" means low gravity. Normally a uniform gas distribution, say sitting on a table top here on earth, IS in it's highest state of entropy when equilibrium is reached BECAUSE GRAVITY IS NEGLIGIBLE.
    When gravity matters as it did in the high density early universe, clumpiness - not uniformity- is the norm.

    So shortly after the big bang uniformity actually means LOW entropy as gravity was huge...
    "In calculating entropy you need to tally up the contributions from all sources. For the initially diffuse gas cloud you find that the entropy decrease through the formation of orderly clumps is more than compensated by the heat generated as the gas compresses, and ultimately, by the enormous heat and light released when nuclear processes begin to take place...The overwhelming drive towards disorder does not mean that orderly structures like stars and planets...can't form...the entropy balance sheet is still in the black even though certain constitutents have become more ordered.."
  7. Jun 12, 2015 #6
    Lineweaver and Davis in discussing cosmological expansion, point out that when the CMBR originated it was about 3,000 degrees K; today that figure is under 3 degrees as already posted. That's because the universe has expanded and stretched the CMBR by a factor of about 1,000 during that time.

    "stars and black holes are forming creating immense temperatures.."

    Stars are 'hot'' [emit a lot of radiation]; Black holes are so 'cold' they emit practically none.
  8. Jun 12, 2015 #7
    Although I need to include a disclaimer that my views are singularly uninformed, this intrigues me because "stress-energy" .is sounding a lot like the "tension" that some of the stuff I read (rarely including any standard textbooks, but definitely in books by physicists who've been educated from them) describe as a sort of opposite to pressure, with pressure associated with the familiar attractive gravity, and tension (which Guth is fond of calling "negative pressure") associated with the repulsive gravity that is sometimes called "the driving force behind inflation" (although the views I've assimilated from the mentors on this forum incline me to believe that it IS inflation, or the expansion of space). Is its indestructibility the aspect of inflation that seems to require special fields (which usually seem kind of ad hoc) to end it globally?
  9. Jun 12, 2015 #8


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    Could you give some references about this? Preferably with some actual calculations!
  10. Jun 12, 2015 #9
    Seriously conservation of matter- energy is THE law of physics. It is true the mass- energy of the universe never changes. The system you described is a Dissipative system that spews the stars energy into the universe through radiating it from the surface. The total mass energy of the universe never changed. Theories that don't conserve matter- energy of the universe are thrown in the waste basket.
  11. Jun 12, 2015 #10


    Staff: Mentor

    No, local conservation of stress-energy is "THE law of physics". And this law is local; there is no meaningful notion of "the mass-energy of the universe" in a global sense.

    Sean Carroll has a good article about this:

  12. Jun 12, 2015 #11


    Staff: Mentor

    The "negative pressure" of dark energy is a form of stress-energy, yes. So is ordinary pressure, and energy density, and momentum density, and shear stress, etc. The stress-energy tensor, which appears on the RHS of the Einstein Field Equation, includes all of these things.
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