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Now analogously, we could talk about GWs traveling in the empty voids of space, where the expansion of space is dominant.
There is a debate over whether gravitons exist or not, and whether gravitons are the quanta of GWs or not, by let's assume yes.
Just as EM waves wavelength get stretched as they travel in expanding space, GWs wavelength could get stretched.
We are actually able to measure the redshifting of EM waves from far away sources. We could be able to measure the redshifting of GWs from far away sources as we have already detected GWs.
Can expanding space stretch the wavelength of GWs?
When we detected the first GWs, did we measure the actual amount of redshift they went through as they traveled from the source?