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Special relativity and Universe expansion.

  1. Nov 23, 2004 #1
    I have heard that the Universe is expanding, and the longer is the distance between two bodies, so the greater will be the speed of expansion. So, my question is: If two bodies were far enough, so tehy could reach c or even go faster? (i guess there is no limit for Universe expansion). I know I´m not a genius, and i havent discovered some paradox or somewhat, so, help me please and explain me what really happens. Thanks. :tongue2:
  2. jcsd
  3. Nov 23, 2004 #2
    Hi, bienvenido
    don't mix the concepts Special Relativity and Universe expansion, because Special Relativity is a theory that has as foreground a Minkowski space, that is a non-expanding space. The expansion can be modelled with General Relativity, that has Special Relativity as a special case. In GR, there are objects receding faster than c, because with Hubble Law
    where D is proper distance, H the hubble parameter, you can obtain recession velocities v greater than c. But there's no paradox because in the frame of reference of the observer there's no superluminal velocity observed. They are receding with spacetime, but their peculiar velocity inside spacetime remains subluminal
  4. Nov 24, 2004 #3
    I think there is some thing wrong with that.
    There are 3 versions of the expansion of the Universe according to Friedmann.1 of them states that we begin with Big BAng and finish with Big Crunch.Therefore, we definitely have the limit of the expansion(if the theory is right.)
  5. Nov 24, 2004 #4


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    Picture it this way: A 2d world on an expanding sphere.

    According to any person on that world, everything else is getting farther away. Someone who starts off moving at 5 kph relative away from some other person, and doesn't accelerate afterwards, will observer the relative speed as actually increasing! :surprised (at least until they went all the way around the sphere and were actually moving towards)
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