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Author Topic: Does the cosmological expansion 'stretch' gravtational waves?  (Read 2264 times)

lean bean

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Hi all,
I understand  the cosmological expansion stretches light waves, does the same happen to gravitational waves ?


 

Offline imatfaal

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Good question!  I don't know but as it is a background expansion of space I would say yes. 

However!...Expansion happens noticeably only in areas in which there are no massive objects which are gravitationally bound.  That is to say you only really see expansion in the huge voids between superclusters where gravity is too weak to hold things together - in clusters and galaxies there is enough gravity to keep the systems bound together.   For a gravitational wave to be noticed you must have a fairly decent mass/energy set of objects rotating (or something like that) to create the wave AND a hefty mass in your detection apparatus (that's how the gravitational wave is noticed) - if you have two masses gravitationally bound then you won't notice expansion.  ie they might stretch - but how could you notice?
 

Offline yor_on

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Such a nice question :)

Does it? Assume it does, what about Ligo? Now turn it around, assume you are close to lights speed, uniformly moving, using Ligo to measure that gravitational wave, will it then be compressed?
 

lean bean

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AND a hefty mass in your detection apparatus (that's how the gravitational wave is noticed) - if you have two masses gravitationally bound then you won't notice expansion.  ie they might stretch - but how could you notice?
Are you sure there about the requirement of a ’hefty mass’?
 The LIGO detector relies on the difference in arm lengths caused by the gravitational wave, the result is a change in the interference pattern  produce by a single laser which is split to make two beams. A beam is sent up each arm and reflected back to make the interference pattern.



Does it? Assume it does, what about Ligo? Now turn it around, assume you are close to lights speed, uniformly moving, using Ligo to measure that gravitational wave, will it then be compressed?
Don’t really understand what your saying here, but since a detection relies on the stretch and then squeeze of a gravitational wave altering the LIGO arm lengths, then does it matter what speed your moving at? Since your not moving at light speed  the G wave will pass over you and LIGO and both you and LIGO will experience the G wave.
 

Offline yor_on

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Sure, exchange 'compress' for 'contract' and it may become clearer? It has to do with LorentzFitzGerald contractions and uniform motion close to light speed, that is if you presume, as I do, those effects actually being 'real' and measurable locally. This one is nice reading btw. http://news.discovery.com/space/quantum-entanglement-could-aid-ligo-in-hunt-for-gravitational-waves-111106.html
 

lean bean

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Sure, exchange 'compress' for 'contract' and it may become clearer? It has to do with LorentzFitzGerald contractions and uniform motion close to light speed, that is if you presume, as I do, those effects actually being 'real' and measurable locally.

If we assume the wave exists, that is, it’s a distortion of spacetime, then whether we or it is moving shouldn’t make a difference to its detection.
If we are moving and pass through this G wave, then we (LIGO) should detect it. Likewise if it is moving and passes through us, then we (LIGO) will detect it.

The LF contraction of length appears in in each frame, that is, from the G wave frame LIGO is contracted in one of its arms and not the other (because one arm is perpendicular to the other), and so alters the interference pattern and is detected.

From our/LIGO’s frame, the G wave should be contracted in its length but not in its perpendicular distortion. Since there is an unequal distortion the detection will be made.
 

Offline imatfaal

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AND a hefty mass in your detection apparatus (that's how the gravitational wave is noticed) - if you have two masses gravitationally bound then you won't notice expansion.  ie they might stretch - but how could you notice?
Are you sure there about the requirement of a ’hefty mass’?
 The LIGO detector relies on the difference in arm lengths caused by the gravitational wave, the result is a change in the interference pattern  produce by a single laser which is split to make two beams. A beam is sent up each arm and reflected back to make the interference pattern.

.../

What I meant (and I should have been clearer) was that any object that can be self contained in open space and detect gravitational waves will be massive - and thus, if close enough to be effected and in turn measure the spacetime distortion of a gravity wave will also be close enough to be accelerated towards the massive object(s) generating the gravity wave and be gravitationally bound 

BUT - a. Not sure if that is true now.  b. I think that is all immaterial - whether the detector is gravitationally bound will only affect if the distance between it and the g-wave source increases, not if the waves are stretched by the underlying space-time expansion.
 

Offline evan_au

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If a pair of black holes coalesces in a galaxy, they will radiate gravitational waves at the frequency of their final orbits (this frequency is itself affected by relativity).

If that galaxy is moving away from us as part of the general expansion of the universe, will the gravitational waves be doppler-shifted? I think yes.
If that galaxy has a very high red-shift, will there be additional relativistic effects? I think yes.

From a recent visit to a gravitational wave observatory using laser interferometry, I recall that their current sensitivity is not enough to cover neutron star/black hole mergers throughout our own galaxy, let alone distant galaxies.

http://www.gravity.uwa.edu.au/
 

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