Question on gravitational waves and redshift in BH coalesce

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Discussion Overview

The discussion revolves around the observation of gravitational waves from a binary black hole merger, specifically addressing the nature of the signals detected by LIGO and the implications of redshift in this context. Participants explore the dynamics of black hole coalescence and the propagation of gravitational waves compared to electromagnetic waves.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why the gravitational wave signal shows a clear end during the coalescence of black holes, suggesting that redshift might prevent detection of the merger.
  • Another participant argues that the merging of black holes is a dynamic process that differs from a small object falling into a static black hole, emphasizing that fluctuations in spacetime curvature produce detectable gravitational waves.
  • A participant inquires whether gravitational waves are subject to redshift in the same manner as electromagnetic waves, proposing a relationship between the wavelengths detected and emitted.
  • Another participant agrees that gravitational waves should experience redshift and suggests that this is accounted for in LIGO's calculations.

Areas of Agreement / Disagreement

Participants express differing views on the nature of black hole coalescence and the implications for gravitational wave detection. While there is agreement on the redshift effect for gravitational waves, the understanding of the dynamics involved remains contested.

Contextual Notes

Participants have not fully resolved the implications of redshift on gravitational waves, nor have they reached consensus on the dynamics of black hole mergers compared to other scenarios involving black holes.

Javier Zapater
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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.

javier
 
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Javier Zapater said:
Where is my reasoning wrong?

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.)
 
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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?

Thanks
javier
 
Javier Zapater said:
Shall the fluctuations propagating through the space-time be subject to the same redshift expansion as electromagnetic waves?

They should be, yes. I believe that is taken into account in the calculations that were done for LIGO.
 
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Thanks a lot.
 

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