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Superluminary travel at present

  1. Jan 25, 2008 #1
    Is there a part of today's observable universe that is travelling away from us at faster than light speed? (relatively speaking, of course) Is the known universe large enough for a cumulative expansion rate to exceed light speed?
  2. jcsd
  3. Jan 25, 2008 #2


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    Anything going faster than the speed of light relative to us (due to expansion) is no longer visible, since the light can't get here. Current theory has this happening.
  4. Jan 25, 2008 #3
    If you mean velocity through space the answer is no, due to the Lorentian transformation of special relativity. If you mean the general-relativitistic Hubble velocity of expanding space the answer is yes. The Hubble velocity of expanding space is not restrained by the Lorentian transformation of special relativity.
  5. Jan 25, 2008 #4
    In an expanding universe, a photon that is emitted from an atom toward an observer not only travels through its local space at the speed of light but the photon also rides on the Hubble flow velocity (relative to the source) of its local space. Therefore, barring absorption by intervening atoms the photon will eventually reach the observer even if the relative Hubble flow velocity of the source (i. e. velocity relative to the observer) is greater than the speed of light.
  6. Jan 26, 2008 #5
    Therefore, the text of my poem below would be an accurate assessment?


    The Universe is expanding,
    Faster than the limit of light,
    Beyond common understanding.

    Cosmology is demanding.
    Its study is by no means slight.
    The Universe is expanding.

    There are heroes quite outstanding
    Who apply their full mental might
    Beyond common understanding.

  7. Jan 26, 2008 #6
    The reason I ask is that I received a critique on the post below, telling me that I was inaccurate about the universe expanding faster than light. From your answers, I feel justified and that my interpretation is correct. Thank you. If you would like to check for yourself and comment, please go to
  8. Jan 26, 2008 #7


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    I liked reading the whole poem where you linked to. The villanelle form is a lovely tight form, I hope you write more. I think Dylan Thomas wrote a villanelle (Do not go gentle). The poem's name Superluminelle is also witty. I hope you come around here more and ask physics and cosmology questions.

    Most of the objects which are observable to us in the sense that their light is now reaching us were receding at speeds greater than c when they emitted the light that is now arriving.

    General Relativity pretty much forces superluminal expansion (by way of the Friedmann equation model derived from it---IOW standard cosmology.) The special relativity limit does not apply to the speeds that distances expand.

    This is a false statement. The Hubble parameter is still decreasing (even though a''(t) is positive due to Lambda) and therefore the Hubble radius is increasing. An object which is today just outside the Hubble radius CAN send us light today which will eventually reach us. Objects which are currently between 14 and 16 billion LY from us ARE receding at >c and nevertheless CAN send light today which will reach us.

    This effect was more remarkable in the past. For example the atoms which emitted the CMB light which we are now receiving were receding at over 60c (sixty times the speed of light) when they emitted the light that we are now getting.

    And they are now receding at about 3.3c. So clearly we can see some things which were receding superluminal when they sent the light, and which are now receding superluminal. That one example illustrates it. But actually that is typical. Most of the universe that we now observe, volumewise, was receding superluminal when the objects emitted the light we are now getting.
    Last edited: Jan 26, 2008
  9. Jan 30, 2008 #8
    So, the farther back we go to the fastest moving cosmos, the fastest we can see go 60c?
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