Question on gravitational waves and redshift in BH coalesce

In summary, the gravitational waves observed by LIGO indicate the merger of two black holes. The end of the signal indicates that the two black holes have merged into a single black hole. The gravitational waves are propagated through space-time and suffer from the same redshift expansion as electromagnetic waves.
  • #1
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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  • #2
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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  • #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?

Thanks
javier
 
  • #4
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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  • #5
Thanks a lot.
 

1. What are gravitational waves?

Gravitational waves are ripples in the fabric of spacetime that are produced by the acceleration of massive objects, such as black holes or neutron stars. These waves travel at the speed of light and can be detected by sensitive instruments on Earth.

2. How are gravitational waves produced in a black hole coalesce?

When two black holes merge or coalesce, they produce intense gravitational waves as they spiral towards each other. These waves are generated by the acceleration of the massive black holes and can be detected by gravitational wave detectors on Earth.

3. What is redshift and how does it relate to black hole coalesce?

Redshift is a phenomenon where the wavelength of light is stretched as it travels through space, causing it to appear more red. In the context of black hole coalesce, the gravitational waves produced by the merging black holes cause a redshift in the light and can be used to measure the distance and properties of the black holes.

4. How do scientists detect and study gravitational waves from black hole coalesce?

Scientists use specialized instruments called gravitational wave detectors, such as LIGO and Virgo, to detect and study gravitational waves from black hole coalesce. These detectors use lasers and mirrors to measure tiny distortions in spacetime caused by passing gravitational waves.

5. What have scientists learned from studying gravitational waves and redshift in black hole coalesce?

By studying gravitational waves and redshift in black hole coalesce, scientists have been able to confirm the existence of black holes, measure their properties, and gain a better understanding of the laws of gravity and the structure of spacetime. This research has also provided insights into the formation and evolution of galaxies and the origins of the universe.

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