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Are they moving faster than light?

  1. Jul 5, 2008 #1
    I have heard and read that some galaxies are receding at speeds even faster than light's, is that true? If they are, then would this not contradict the very well accepted speed limit = 300,000 km/s?

    Note: the question asked by a non physicist/astronomer/cosmologist.
  2. jcsd
  3. Jul 5, 2008 #2
  4. Jul 6, 2008 #3


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    most galaxies that we can see are receding faster than light, at the present moment (in CMB restframe time). and they were receding faster than light at the time they emitted the light which is now reaching us. this is typical.
    true for anything with a redshift of 1.6 or more, if I remember right.

    it doesn't contradict Special Relativity, it just means that the distance to the galaxy (and its surrounding locale) is increasing by a certain percentage each year----and if the distance is large enough that annual percentage increase will be more than one lightyear-----so the distance is growing faster than c.

    that doesnt mean that the galaxy out there could catch up with and pass a photon of light!

    THAT would be a violation of Special Rel.
    figure. the photon out there in the same locale as the galaxy is ALSO receding along with everything else in the neighborhood.

    If the photon is aimed away from us and it has a race with the galaxy there is no way the galaxy can catch it. Both are receding faster than c along with the whole locale, the whole neighborhood. PLUS the photon is moving at c, relative to the galaxy.

    Special Rel is mainly useful as a local approximation. It applies to flat non-expanding spacetime. so over large scales, with a curved spacetime and expansion, the stuff about speed limit and addition of velocities doesn't apply in a straightforward fashion.

    Remember: it is a physics law that nothing can ever catch up with and pass a photon of light
    On the other hand, a whole locale with galaxies, photons of light, and the kitchen sink, can be receding at several times the speed c.

    You might enjoy Siobhan Morgan's cosmology calculator, which gives you recession speeds for objects at various redshifts. She's an astronomy professor at a University in Iowa, has a lot of her course materials online. Or did the last time I looked. by putting the right redshift in the calculator you can easily get recession speeds of 2, 3, four or more times the speed of light.
    this thread has some pointers about using the calculator
    new url given in post #4

    there's a technical point: the CMB defines a universal concept of rest (when the CMB sky is approximately the same temperature in all directions, no Doppler hot spot)
    and that provides an unambiguous meaning of simultaneity. a universal now. that all observers at CMB rest can agree on---at least to a good approximation.

    that's the time standard I'm referring to when I say the galaxy is receding NOW, or was receding THEN, at some speed.

    this universal standard time concept is built into the Hubble law, so when people use the Hubble parameter they are tacitly adopting universal time. The H parameter is the ratio of recession speed now, to distance now.

    it is actually pretty intuitive, so not something to worry about. gives cosmologists a convenient working timeframe for normal everyday calculation and discussion.
    Last edited: Jul 6, 2008
  5. Jul 6, 2008 #4
    Thanks marcus. Your info solved this riddle of speed limit to me.
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