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Does the expansion of the universe violate conservation of energy?

  1. Sep 4, 2008 #1
    Since the distance between galaxies increases with time does that mean that the matter in those galaxies gets potential gravitational energy? and if so where does this energy come from?
  2. jcsd
  3. Sep 4, 2008 #2


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    I don't think that happens in any simple way, Anzas, because in a uniform (homogeneous isotropic) universe there is negligible net pull on an isolated galaxy or cluster of galaxies.
    the pulls in different directions cancel out

    Were there a net pull on a galaxy that would probably be in a gravitationally bound structure, like a cluster. But the average increase in distance doesn't apply within clusters. Hubble law refers to average longrange distances and doesn't apply equally across the board. Within gravitationally bound systems average distances may not be increasing at all!

    So to a first approximation you can say that if the distance between two widely separated things A and B is increasing according to Hubble Law, then the increase in distance is not doing work at least in any obvious straightforward way, because there is no net force on A towards B.

    think of the balloon analog with pennies stuck on the balloon surface representing clusters of galaxies. each penny attracts every other along geodesics in the surface (but not acrosss the empty space inside the balloon which does not exist :smile:). each penny is attracted about equally in all directions (2D directions within the space). no net force on it in any one particular direction

    there's more to say, but I have to go. maybe someone else will elaborate.
  4. Sep 4, 2008 #3
    Hi Anzas,
    There are two components to the expansion: the original expansion thought to result from inflation, and the late times acceleration of the expansion thought to be caused by dark energy.

    Regarding the original expansion: In a Newtonian sense, as the universal expansion decelerates over time due to the cosmic gravitation, the galaxies' kinetic energy of expansion converts to potential energy of distance. No energy is gained or lost, it just changes form. This expansion conserves energy; it is modeled on the first law of thermodynamics for a closed system.

    Regarding the late times acceleration caused by dark energy: This is thought to be energy which is newly created as a result of the volume expansion of the universe. This energy is thought to most likely come from quantum fluctuations in empty space. As far as I know, standard theory does not assume that energy conservation applies to increase in this energy. We just don't know enough about what it is and how it works.

  5. Sep 5, 2008 #4
    I see, thanks for your answers.
  6. Sep 15, 2008 #5
    in my limited understanding and correct me if I am wrong but the energy of inflation / expansion has gravity, and gravity being a negative energy, they ballance out to a net zero?
  7. Sep 15, 2008 #6
    Hi TalonD,

    Dark Energy in its simplest theoretical form, the cosmological constant, is believed to possess both gravitational mass-energy and expansionary energy. The expansionary energy is in the form of negative pressure, or tension.

    The gravity and expansion energy of the cosmological constant do not entirely cancel out. Its expansion energy is 3x its 1x gravity; so after netting out the deceleration component of gravity, the net acceleration energy is 2x.

    The fact that there is net acceleration energy explains why the universe has stopped decelerating and begun reaccelerating since around the 7Gy mark in its history.

  8. Sep 20, 2008 #7
    I think one has to be careful talking about the conservation of energy over cosmological scales when the total energy of the universe is ill-defined.
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