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Expanding Universe + Rotation - Lost it somewhere

  1. Jul 10, 2005 #1
    I am quite confused... I'll state that up front to make it simple. ^_^

    Here is my question, though:

    Assuming that the universe at some point started from a singularity, err... or even that, assuming that the universe is expanding at a rate that has varied over time... I assume that it's implied that is is, first off... I may be wrong, but if people generally believe the universe started small, is expanding and that at some point it will contract again, there has to be some acceleration going on somewhere.... Hubble's constant involves change in rate of speed of something over distance from us ... Okay, I'm lost...

    But, if the galaxy is picking up speed, accelerating in a certain direction, and the earth is rotating and revolving and all that... we're definately not in an inertial frame. How come at some points during the day/year/etc there isn't an increased force on us in one direction over the other due to the expansion of the universe?

    I'm guessing that the problem lies in my lack of complete understanding of general relativity, but ... that's my question. Where did I go wrong? :confused:
  2. jcsd
  3. Jul 10, 2005 #2
    there is NO direction vector in the expanding univerce
    by space streaching NOT MOVEMENT

    now our galaxy has proper motion but is not moving faster as space streaches
    as they are two very different things

    contraction ie the big crunch is dead
    there is NOT enuff MASS to cause a crunch
    so we will keep moving at the same speed
    as every thing else gets farther away
    other then our gravity bound local group
  4. Jul 10, 2005 #3
    Okay, so it's a twofold error...

    1) There is no center point from which the universe is expanding. So it's not like all the galaxies are moving away like fragments, each in a linear outward direction. Or at least there's no way to determine one with all the rotating and zooming around that's going on.

    2) It's an expansion at a constant velocity in whatever direction we happen to be going, so we remain in an inertial frame with regards to universal expansion. It's not like ... an explosion where things start from zero and suddenly accelerate outward...

    I think I've got it. :smile:
  5. Jul 11, 2005 #4


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    The Earth's rotation shows up in such effects as the Coriolis force, which is very important for weather on the planet, has significant effects on navigation, and is responsible for the precession of the Focault pendulum.

    See for instance (for the weather effects of the coriolis force).


    The effect of the expansion of the universe depends on what you believe the value of the deacceleration constant 'q'. With the universe currently believed to be accelerating in its expansion with a q value of -0.6, GR predicts that two points 1 kilometer apart will experience a tidal acceleration of

    3.12 × 10-33 m / s-2 for every km. This is totally unmeasurable. This number was calculated by Hellfire in this thread:


    which is rather technical. I'm not aware of any textbook that derives this result, but I get the same result as Hellfire does.
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