# How is the redshift of starlight precisely measured?

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?

• 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. Oct 28 '17 at 20:54

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.
• @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. Oct 29 '17 at 9:31