In fact, only in the first case (a nearby body moving away from the earth) is the redshift caused by the Doppler effect. But there's a subtle difference, which you sort of allude to.
its wavelength will be stretched, so the color of the light is more towards the red end of the spectrum. In both cases, the light emitted by one body and received by the other will be "redshifted" - i.e. Space itself is expanding over time and the doppler effect I described, although quite logical to me, is actually not applicable to this scenario. That's logical.īut I am told by Cosmologists that there is another reason for the redshift. Furthermore, if we see more redshift per unit distance at longer distances (as I believe we do), it simply indicates that the rate of expansion was greater further back in time than it is now, so the expansion of the universe is decelerating over time. If this is true, then the redshift we see represents the recession velocity of the emitting mass relative to Earth at the time of emission. And if the momentum of that photon was conserved during its journey through empty space, then it would also have that longer wavelength at its time of arrival on Earth. Since the momentum of a photon is inversely proportional to its wavelength, this would have resulted in a longer wavelength. When a photon was emitted from a galaxy that was receding relative to Earth at the time of emission due to expansion, the momentum it had relative to Earth at the time of emission would have been proportional to the relative (to Earth) momentum of the mass from which it was emitted at the time of emission.
This means that unbound structures have negative relative momentum due to universal expansion. I understand that the expansion of the universe causes unbound structures to move apart from each other.
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