To answer both it is necessary to know two things: how far away certain galaxies are and how fast they are flying away from us (what is known as their recessional velocity).

为了回答这两个问题，首先必须知道两件事：某类星系离我们有多远，它们在以多快的速度远离我们而去（即现在所谓的退行速度）。