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Question on gravitational waves and redshift in BH coalesce

  1. Feb 13, 2016 #1
    Dear Sirs

    My question relates to the recent observation of gravitational waves by LIGO.
    The paper PRL 116 "Observation of Gravitational Waves from a Binary Black Hole Merger -B.P Abbott et al"" depicts the chirp signal of the wave detected, where it is seen how both frequency and amplitude increase till coming to an end, where oscillations stop, indicating that both BHs coalesce into single one.

    Question: why do we see that end in the gravitational wave signal? in short, why we see the coalesce? I mean, I would have expected (surely wrong) that we, as external observers should have "seen"/detected the two BH, meanwhile approaching both event horizons to the final merge, suffering an increasing redhsift during the collapse that would have prevented us from detecting that one BH crossed the event horizon of the companion, and viceversa. Similar effect as if we, meanwhile remaining in orbit around a BH, see an object falling into the BH. We will see the light emitted by the object gradually and infinitely redshifted meanwhile approaching the event horizon.

    Should not the gravitational waves suffer from the same collapse redshift observed as in the electromagnetic waves?

    Where is my reasoning wrong?

    Thanks for your help.

  2. jcsd
  3. Feb 13, 2016 #2


    Staff: Mentor

    You are imagining one BH as a small test object falling into the other BH. That's not correct. This is a highly dynamic situation, and it doesn't work the same as a small test object falling into a static BH.

    The "coalescence" of the two BHs is not simply a merging of their horizons. Again, it's a highly dynamic process that produces strong fluctuations in spacetime curvature. Most of those fluctuations are outside the combined horizon of the two BHs, so they propagate outward; they are in fact the gravitational waves that we detect. (Some fluctuations are trapped inside the horizon, but of course we don't detect those.)
  4. Feb 13, 2016 #3
    Thanks PeterDonis.

    One more question. Shall the fluctuations propagating through the space-time be subject to the same redshift expansion as electromagnetic waves?

    I mean λ of GW detected= λ of GW emited * (1+z)

    Is it correct?

  5. Feb 13, 2016 #4


    Staff: Mentor

    They should be, yes. I believe that is taken into account in the calculations that were done for LIGO.
  6. Feb 14, 2016 #5
    Thanks a lot.
  7. Feb 14, 2016 #6
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