Chapter 28, Problem 13CQ

All galaxies farther away than about $50\times {10}^6\text{ly}$ exhibit a red shift in their emitted light that is proportional to distance, with those father and farther away having progressively greater red shifts. What does this imply, assuming that the only source of red shift is relative motion? (Hint: At these large distances, it is space itself that is expanding, but the effect on light is the same.)