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I'm from engineering background and not firmly familiar with cosmological equations and concepts. but i know what Redshift is and how it is related to the Doppler effect.

My question is how scientists know that the Redshift is related to the moving of the object and not related to the local effects such as the material of that object. how they know this Redshift is due to moving and not local events in that object like the nuclear reactions on the object or maybe it's because of the medium between observer(us) and the source. I will appreciate if the answer will be in simple words for non physicists.

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    $\begingroup$ They measure the shift of known narrow spectral lines. The only local effect that could cause this is gravitational time dilation, but gravity there is not strong enough to cause it. $\endgroup$ – safesphere Aug 6 '18 at 16:13
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The easiest way to argue for this is to note that large sky surveys such as sloan digital sky survey (SDSS) have surveyed large amounts of the deep sky and find a uniform relation between observed distance and observed redshift. This is consistent with Hubble's law which describes the universe as having a sort of background flow that uniformly depends on distance.

If the red shift were caused by material or local effects, I would not expect to see the same red shift in all patches of sky at the same distance but with different physical properties.

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  • $\begingroup$ It might be added that if you did see the same red shift in all patches of sky at the same distance, and if this red shift were due to local effects, you could conclude that the earth occupies a very special place in the Universe, which strikes many as highly implausible. $\endgroup$ – WillO Aug 6 '18 at 17:19
  • $\begingroup$ @fhorrobin @ WillO Even if we say that redshift is due to moving objects how you claim that the objects with equal distance from us having equal speed relative to us which causes equal doppler effect and finally equal redshift. it's only possible if we are in the center of the universe. if we are at the center then we can claim every object with equal distance may be expanding with equal speed. $\endgroup$ – yekanchi Aug 7 '18 at 6:48

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