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On episode 10 of the original Cosmos TV series, Carl Sagan mentions that some scientist don't agree that the redshift observed in distant galaxies is evidence of the big bang, stating that probably the doppler effect is not responsible for this effect. What other explanations are there?

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    $\begingroup$ The alternative was around the shift being an actual wavelength shift due to the age of those galaxies. Not sure that argument stacks up any more though (and it was always a bit difficult to argue) $\endgroup$ – Rory Alsop Mar 15 '14 at 16:50
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Tired light has been used as an explanation before, whereby light loses energy whilst travelling through space---a sort of drag effect.

I don't think anyone actually supports it nowadays, though.

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    $\begingroup$ Especially given the new gravity wave observational support for inflationary theory. $\endgroup$ – called2voyage Mar 20 '14 at 20:50
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In Cosmology, the main contribution to redshift would be cosmological redshift. Namely the redshift that occurs due to the Hubble flow. According to Hubble's law,

\begin{equation} v = H_0 d \end{equation} The further two objects are away from each other, the faster they are moving. Of course, all this is under the assumption of an expanding universe, such as the widely accepted $\Lambda CDM$ model.

The Hubble flow is in a sense a type of "speed", but this velocity can be faster than the speed of light. This is chiefly because the space between two points is expanding, causing the wavelength of the light to be stretched out.

There is also gravitational redshift, something that occurs due to gravity's effect on spacetime. Although it might be better to think that anything that can stretch the wavelength of light can cause redshift, suppose there exist some kind of god hand that allows one to just pull and subsequently stretch the light wave this would also cause redshift.

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