Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Could it be possible that gravity is the reason why the universe is expanding?

  1. Jul 10, 2008 #1
    Could it be possible that gravity is the reason why the universe is expanding?

    When stars die and some create gravity waves, those waves are expanding at/around the speed of light. The waves then are traveling faster than a planets'/suns' gravity. Therefore the wave would pull in until it passed a planet/sun then it would end up dragging the objects as it passed. Though the gravity is weak; enough dying stars could possibly create the expanding the universe.

    Does this idea sound even plausible? If it does not, then why?
     
  2. jcsd
  3. Jul 10, 2008 #2

    Wallace

    User Avatar
    Science Advisor

    This would tear galaxies apart when the stars within them died. We observe that expansion does not occur on the scales of galaxies. In any case, there is no mystery as to why the Universe expands, it is akin to momentum, once the Universe begins to expand then just like a ball thrown it the air it continues to do so until the forces slowing in down (gravity) succeed in reversing the expansion, or if there is not enough material in the Universe then the expansion never slows to a stop, or dark energy (that accelerates the expansion) takes over and then continues to drive an increase in the rate of expansion forever.

    The mystery is why the expansion began in the first place, with inflation. However if we accept that for whatever reason that initial kick occurred, then the subsequent expansion after that impulse just follows from the conservation of momentum.
     
  4. Jul 11, 2008 #3
    Historically GRT (General Relativity Theory) was essentially complete, but had a cosmological constant. Then came Hubble and his redshifting of galaxies, intrepreted as expansion of universe. So the cosmological constant was dropped. Thus Hubble expansion is not part of GRT. In GRT one can have affine parameterization along a null geodesic, with a finite velocity of a photon. Yet the Hubble expansion (also a local description) can have superluminal stretching of the manifold (continuum i.e. inbetweeness). So pseudo-Riemannian spacetime manifold has a finite velocity for photon; while Hubble expansion of overall manifold, as well as for a local patch, can have a superluminal stretching. This always sounds somewhat strange (a koan), that the same manifold can have two such apparently disparate conceptual descriptions.
     
  5. Jul 11, 2008 #4
    GR with a cosmological constant is fully compatible with the Hubble expansion.
     
  6. Jul 11, 2008 #5

    Garth

    User Avatar
    Science Advisor
    Gold Member

    GR without a cosmological constant is also fully consistent/compatible with the Hubble expansion.

    Garth
     
  7. Jul 11, 2008 #6

    Wallace

    User Avatar
    Science Advisor

    Just not the particulars of the expansion observed in the Universe we happen to live in. In our Universe there is either a cosmological constant (or something like it) or GR is wrong.
     
  8. Jul 12, 2008 #7

    Garth

    User Avatar
    Science Advisor
    Gold Member

    The point I was making was that "GR without a cosmological constant is also fully consistent/compatible with the Hubble expansion."

    It is cosmic acceleration that requires a cosmological constant or some similar form of DE.

    I tend to answer assuming others have read the preceding posts.

    Garth
     
    Last edited: Jul 12, 2008
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Could it be possible that gravity is the reason why the universe is expanding?
Loading...