Distant Galaxies: Cosmological vs. Velocity Redshift

Also, where did the big bang happen? Everywhere!—it was the origin of space as well as matter and energy.


Great explanation!


Isn’t gravitational redshift a thing in addition to the two mentioned in the video?

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Yes, gravitational redshift (the shifting downward in frequency of electromagnetic radiation climbing out of a gravity well) is a phenomenon predicted by general relativity and was one of the four “classical tests” of the theory proposed by Einstein in 1916. The effect is very small, with its magnitude given as the difference in the gravitational potential of the source and observer divided by the square of the speed of light. Einstein suggested it might be detected in the spectrum of a white dwarf star such as the companion of Sirius. A claim of discovery was made in 1925, but considered unreliable due to contamination of the spectrum with light from Sirius itself. It wasn’t until 1954, a year before Einstein’s death, that a reliable measurement was made of a different white dwarf.

Measuring the gravitational red shift of light from the Sun is difficult because convection of plasma in the Sun’s photosphere causes a Doppler shift with around the same magnitude as the gravitational red shift. It took until 1962 before a measurement was made that was considered reliable, and only in 2020 was a very accurate measurement made from sunlight reflected from the Moon, which acts to average out the convective Dopper shift over the entire Sun.

Other observations have measured the gravitational red shift in galaxy clusters, near the black hole at the centre of the Milky Way galaxy, and in distant quasars. The gravitational red shift was first demonstrated in a laboratory test by the Pound-Rebka experiment in 1959, and now can be measured by super-accurate atomic clocks.

The video did not mention gravitational red shift because the effect is so small it plays no part in cosmological models and their testing, where Doppler shift and cosmological red shift entirely dominate.