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Accelerating Universe and the Loss of Mass

  1. Jun 1, 2005 #1
    I had a thought the other day that I would quite like feedback on:

    Could the universes accelerating expansion be fuelled by the amount of mass in the universe decreasing? Is it possible that a black hole could become so massive that it tears away from/gets pinched off spacetime, how much mass would we be talking about? How would a galaxy that lost its central black hole behave?
  2. jcsd
  3. Jun 1, 2005 #2


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    The expansion of space in an Friedmann universe filled only with matter and radiation (and no dark energy) does always decelerate. If you consider an universe filled only with matter and radiation and you imagine that some unknown mechanism is decreasing the amount of matter (or energy equivalent) in a comoving volume, then the effect will not be an acceleration of the expansion, but a decrease of the deceleration rate of expansion. You can think about it with the following analogy: if you send a rocket away from earth, it will start decelerating as soon as it has no fuel anymore. If you imagine a mechanism to decrease then the mass of earth, then the deceleration of the rocket will decrease, but it will never start accelerating away from earth.
    Last edited: Jun 1, 2005
  4. Jun 1, 2005 #3
    Ah, yes. Thanks.
  5. Jul 23, 2008 #4
    Interesting answer. However, in your analogy requires the rocket has run out of fuel. Lets tweak this analogy a little. Lets assume that you have two stars at some distance apart.

    lets look at what is going on.

    1. The stars have gravity that attracts them that generates a attractive force.
    2. The stars are outputting solar wind in all directions. however there is a buildup of matter between the stars where the solar wind is pushing againse each other causings a pressure differential that is greater between the stars than outside the stars (causing a repulsive force.
    3. The star is undergoing fusion which converts mass to energy. This does a few things.
    a. Lessens the gravitational force between the two stars
    b. Increases the effect that the pressure discussed in item 2 has on the object's acceleration.
    c. If the star is in motion to start it causes the star to accelerate in the direction of motion due to the conservation of momentum. (ie some of the E=mcc energy goes into the kenetic energy in order go conserve momentum.)

    So this goes back to the original question about the expanding universe. Is the release of energy from mass loss in combination with higher pressure inside the universe than outside add up to be grater than that of gravity?

    Another related tidit. One would also assume that as the mass of the Universe decreases due to fusion the velocity required to escape the universe (ie expand forever) would decrease as well.

    If I am in error in any of my assumptions or explainations I am not afraid of being wrong so please let me know.
  6. Jul 23, 2008 #5


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    What 'outside' of the universe do you have in mind? Energy loss, gravitational or otherwise, violates the laws of thermodynamics - which are still held in high regard by the scientific community.
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