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B At what point would we stop seeing a star?

  1. Apr 28, 2016 #1
    The universe is expanding and at some point faster than light so once it reaches that point we wont see the light from those stars. I forget what this point is called.

    If someone happened to be watching a star as it crossed that point would we see it simply blink out or close to that or would it kind of fade away to blackness. It seems to me that there should be a precise point where the speed of light is reached and one mile an hour faster than that would stop us from seeing it.

    Of course since the star is huge the light from the leading edge of the star would be the first to go out then it would progress to the trailing edge so maybe it would not quite be a blink but close to it.

    Am I thinking right?

  2. jcsd
  3. Apr 28, 2016 #2

    George Jones

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    Care is needed with respect to the definition of "speed" in a cosmological setting.

  4. Apr 28, 2016 #3
    Since the speed of recession is approaching the speed of light the star would be red-shifting. The horizon of the Universe behave very much like a black hole event horizon, the star would appear frozen at the horizon and red shift out of existence.
  5. Apr 28, 2016 #4


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    This is not quite correct. The distance to the cosmological horizon D_Hor is larger than the Hubble radius R, where the recession rate equals c.
    They only approach each other in the far future, when H becomes constant. Distant stars were always moving through the Hubble radius and we will see them farther and farther away, just red-shifting out of detectability, but never "frozen" at the horizon.

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  6. May 14, 2016 #5


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    What do you mean by "light reflect through it"? And what matter are you referring to?
  7. May 15, 2016 #6
    Your second idea is right.
    It will be seen to red shift increasingly until it become indistinguishable from background noise.
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