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Some questions about energy conservation

  1. Oct 7, 2010 #1
    Hello all.

    I was watching the prof. Susskind lectures on YouTube. I'm very grateful to Stanford and the professor for making that knowledge so easily available to people willing to learn.

    While I did understand almost everything said I still have some question of things that I did not understand or things that were not discussed.

    While discussing energy dominated universe the professor said that the expansion of space causes red shift to the photons, thus reducing their energy until the universe became matter dominated. My question is where that energy disappeared?

    While discussing vacuum(or dark energy) dominated universe he said that the energy density of vacuum does not change with time, thus expansion of the universe creates more energy. When asked where this energy comes from the professor pointed to the other side of the equation (the one not containing the dark energy density constant). I don't understand what he was talking about. The other side of the equation then has to be unlimited source of energy. How that is possible?
  2. jcsd
  3. Oct 7, 2010 #2


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    1. The short [and not entirely adequate] answer is redshifted photons are also time dilated. The total energy emitted by receeding objects in the universe is thus conserved. The deeper answer is - it depends [as lawyers are wont to say]. In GR, concepts such as energy conservancy are much squishier than in the classical, non-relativistic sense. Some scientists just shrug and say GR does not require conservation of energy, but, I think that is somewhat disengenuous. See: http://www.astronomycafe.net/cosm/expan.html

    2. The good professor was saying that, unlike photons, the dark energy density of spacetime is not diluted by expansion. Or simply, more space means more dark energy potential between objects. Dark energy is the antithesis of gravity. Gravity increases as distance decreases. Dark energy increases as distance increases. Dark energy, however, is even more pathetically weak than gravity [which is a good thing]. Its effect is only apparent over vast distances - many millions of light years.
  4. Oct 7, 2010 #3
    I see, there is hope for anti-gravity then. All wee need to do is to compress space. :rofl:
    Thanks for the answer.
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