In the case of the cosmological redshift, however, the emitting object is expanding along with the rest of the universe, and if the rate of expansion changes between the time the light is emitted and the time it is received, that will affect the received wavelength. After the light is emitted, it doesn't matter what happens to the emitting object - it won't affect the wavelength of the light that is received. Practically speaking, the difference between the two (Doppler redshift and cosmological redshift) is this: in the case of a Doppler shift, the only thing that matters is the relative velocity of the emitting object when the light is emitted compared to that of the receiving object when the light is received.

The classic example of the gravitational redshift has been observed on the earth if you shine a light up to a tower and measure its wavelength when it is received as compared to its wavelength when emitted, you find that the wavelength has increased, and this is due to the fact that the gravitational field of the earth is stronger the closer you get to its surface, causing time to pass slower - or, if you like, to be 'stretched' - near the surface and thereby affecting the frequency and hence the wavelength of the light.) (This is a special case of a more general phenomenon known as the 'gravitational redshift' which describes how gravity's effect on spacetime changes the wavelength of light moving through that spacetime. Any light moving through that space will also be stretched, and its wavelength will increase - i.e. According to general relativity, the expansion of the universe does not consist of objects actually moving away from each other - rather, the space between these objects stretches. In the case of distant objects where the expansion of the universe becomes an important factor, the redshift is referred to as the 'cosmological redshift' and it is due to an entirely different effect. It turns out that just like for sound waves, the wavelength of light emitted by an object that is moving away from you is longer when you measure it than it is when measured in the rest frame of the emitting object. The Doppler effect can happen for light waves too (though it can't be properly understood without knowing special relativity). You've experienced the Doppler effect if you've ever had a train go past you and heard the whistle go to a lower pitch (corresponding to a longer wavelength for the sound wave) as the train moves away. 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.

Cosmological redshift equation examples