![]() It is common to think of the two redshifts as having nothing to do with each other. I try here to moderate my answer, but you might want to accept their answer instead. Thanks.Īfter considering comments, I realize that my first answer contained too strong statements about the relation between the two redshifts. ![]() Please correct me or tell me if I am right or wrong, I have spent a lot of time reading but still don't fully understand this. Even though the recession velocity is not due to a peculiar movement, it means that the source of light is moving away from the observer and hence the light should be redshifted and on top the light gets redshifted on its way through expanding space. So it should have a Doppler effect based on this recession velocity AND a cosmological (expansion) redshift should also take effect because the light gets stretched with the expansion of the universe. In fact though ( proper distance), it IS moving away from Earth with recession velocity V, caused by the expansion of the universe. This means that its position will stay (approximately) the same in comoving coordinates. Suppose a galaxy has no peculiar movement. I understand the concepts of both kinds of redshifts but have a hard time understanding this strict separation. He states that "they are produced by peculiar and not by recession velocities" I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift.
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