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By observing how much the say hydrogen spectrum line was shifted we can tell whether the source is moving closer or further away, but then clearly cosmological redshift do not work this way so how can I tell them apart?

Qmechanic
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user6760
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    Cosmological redshift works just the same way. What makes you think it doesn't? – Guy Inchbald Jan 17 '21 at 13:41
  • @GuyInchbald: when the receding hydrogen atom emits a photon relative to the observer it is being redshifted, so far so good suppose now we know the space between the source and observer is expanding rapidly then what happens? Any diagram helps ;D – user6760 Jan 17 '21 at 13:50
  • Related recent post: https://physics.stackexchange.com/q/608195/2451 – Qmechanic Jan 17 '21 at 13:54
  • As the space stretches, it carries the wavelength of the light with it. We sometimes say the light gets "tired" over long distances. This stretch in wavelength is what redshift is (no matter what causes it). – Guy Inchbald Jan 17 '21 at 13:57
  • Actually, you can interpret cosmological redshift as a series of tiny Dopplershifts as you move from one Minkowski frame to the next. The expansion of space is really just a consequence of how we defined our coordinates, i.e. comoving with "stuff" (galaxies etc). An integral over those Dopplershifts will yield the same result as cosmological redshift. – theWrongAlice Jan 17 '21 at 14:55
  • I asked a similar question before and got some good answers that would mostly apply here as well: https://physics.stackexchange.com/questions/376343/ – safesphere Jan 17 '21 at 15:45
  • See https://physics.stackexchange.com/questions/186405/doppler-redshift-vs-cosmological-redshift and https://physics.stackexchange.com/questions/228492/redshift-of-distant-galaxies-why-not-a-doppler-effect – ProfRob Jan 18 '21 at 21:12

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I found this expression in my textbook, describing the luminosity of a light source:

$P = L (\frac{S}{S_tot})(\frac{\bar{h}\omega_0}{\bar{h}\omega_1})(\frac{\delta t_1}{\delta t_0})$

$\frac{S}{S_tot}$ describes the fraction of luminosity entering the telescope, $\frac{\bar{h}\omega_0}{\bar{h}\omega_1}$ ratio between the observed and emitted photon energies, $\frac{\delta t_1}{\delta t_0}$ time rate difference between two points in space.

So it seems that, theoretically, you have to make a distinction between of these effects. However, I do not know how you can observe the difference between the redshifts experimentally.

Side note: Is there some relationship between the redshift and the wavelength so that there could be a different effect if the photon (at different energies) enter a gravitational field so that the photon spectra is stretched due to the difference in energies of the photons that could be affected by the gravitational field. (Could the gravitational field exert a different force depending on the energy of the photon?)

Maj
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