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When galaxies move away from us (caused by the expanding spacetime) their light seems to show a redshift. But what is really needed for this?

What time lap is for example needed to see a difference in frequency, and how does this precisely work?

Is this also possible for gravitational waves, or are amplitudes the only solution to derive the redshift from?

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    $\begingroup$ Can you do some research on what "redshift" is en.wikipedia.org/wiki/Redshift There is no "time lap" involved in measuring a redshift, and the way your question is worded makes me wonder if you have done this kind of research. $\endgroup$
    – James K
    Oct 28 '17 at 20:54
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I don't really think I fully understand the first part of your question.

Redshift is measured by comparing the wavelengths of a redshifted pattern of absorption or emission lines with the wavelengths they would have in an object at rest. All lines are shifted by the same factor of $1+z$, where $z$ is the redshift.

Gravitational waves cannot be used to obtain a redshift because the intrinsic wavelength of the waves is not known. An inspiralling binary emits waves with a frequency equal to twice the orbital frequency. This in turn depends on the mass, but the observed frequency is also redshifted. There is thus a "degeneracy" between the mass and redshift.

What you can do with gravitational waves is estimate the physical distance to the source using the wave amplitude and how quickly the amplitude changes with time. If the host galaxy of the source can then be identified, then you can use the redshift of that galaxy to get an estimate of the Hubble parameter (e.g. see Abbott et al. 2017).

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  • $\begingroup$ Ok, so if the wavelength of hydrogen went from 122nm (rest frame/Sun) to 500nm (of far away galaxy) how can you derive the distance and the speed of it? Probably the Hubble constant will be used, but isn't the Hubble constant derived from redshift, so it looks like a circular reasoning? And how can the speed of change of an amplitude be of value to get the distance of the galaxy? $\endgroup$
    – Marijn
    Oct 29 '17 at 8:32
  • $\begingroup$ @marjinn The redshift (a cosmological redshift is not a "speed") is found from $(1+z) = 500/122$. Distance has to be measured in some other way to derive the Hubble parameter. If you think you already know the Hubble parameter then a redshift can be used to estimate a distance. Gravitational waves do not come from galaxies, they come from merging binary systems. If you can identify which galaxy the merger takes place in, then you can use the redshift of that galaxy and the distance to the gravitational wave source to independently derive the Hubble parameter. $\endgroup$
    – ProfRob
    Oct 29 '17 at 9:31
  • $\begingroup$ But is it true that, when the distance is measured in some other way (probably candle stars etc?) , that the redshift doesn't really has an extra value as when you know the distance than the speed of it can derived form the Hubble parameter? $\endgroup$
    – Marijn
    Oct 29 '17 at 9:44

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