That doesn't sound rigorous.

Adding a speed that is subjected to velocity composition (never faster than C) and another "speed" that is not subjected to velocity composition (expanding space) can only give wrong results IMHO.

 

If you make the attempt to add bananas & umbrellas, the result will certainly not be bananas, nor umbrellas: the result will be "something else" (objects?). Units don't match.

Nevertheless, the effects of different physical phenomena will have a net combined effect. For example, expansion and velocity would both cause Doppler effects, resulting in a single red/blue-shift value that can be calculated. The calculation is just a bit more complicated than simple addition.

 

The combination of things is not always modelled by an addition operation.

 

 

If you add bananas and umbrellas the result will certainly be bananas and umbrellas. You can also add space-time 4-vectors which include units of length and of time, which remain separate after the addition.

 

 

The combination of things is not always modelled by an addition operation.

 

Effectively, you are completely right. In relativity [math]v_{A+B} \neq v_A + v_B[/math] in the general case.

Edited November 8, 201213 yr by juanrga