Hello PhysicsForum, There is something I don't get at the end of this course notes PDF file. In the last section, titled "Comoving distance and redshift", which I have copied below, we have a short derivation of the comoving distance and redshift relation. Almost all is well, the only thing that troubles me is : why is there no minus sign after da has been replaced by -a2dz ? I have searched the web and found almost identical derivations in other courses or publications, but I never read the explanation for why the minus sign drops. I have found what seems to be the source material for most of those derivations : this paper from 93 (see section 6.3, "The General Redshift-Distance Relation" on 3rd page). It is referenced quite often by others when this comoving distance and redshift relationship shows up. Maybe I am just missing some mathematical trick ? This is not super important of course, but it bugs me.