Good day all, Sorry if this has been posted a lot before but I've been fiddling with the formulas for the redshift and I came up with a question regarding the proportionality of it. I'm new at this so please bear with me. For the sake of argument, I'm talking about redshift in an expansion with a constant velocity. Here it goes: I was able to conclude that ΛObs / ΛEmit = D2 / D1 = z + 1 where D1 is the initial distance of a star when it emitted the light and D2 the distance of the star when the light is received. The D2 can be rewritten as v(D1/c) + D1 and plugging that in the formule gives me: (This formula can be dumbed down to v/c = z which is what z indeed is) Here's my problem. It seems that when plotting this relationship of v D1 and z, using v and D1 as the x variable (since they're related through the Hubble value) and z as the y, that z is proportional to v and D1 even when v reaches near lightspeed. This is obvious since the formula is basically x / c = y after simplifying. I have however read that z shouldn't be proportional to the velocity if it gets too high but my graph doesn't show this at all. What am I doing wrong here? What exact circumstances makes the redshift not proportional to the velocity and why?