The cornerstone of the big bang theory is the Hubble diagram. It tells us that recessional velocities increase linearly with distance. In other words, the farther away a galaxy is, the faster it is receding. The points of the graph, however, do not lie in a perfectly straight line; there is some scatter. This is to be expected, since it is extremely difficult to determine the distance to galaxies accurately. As we saw earlier, Hubble used a crude “cosmic ladder.” Many improvements have been made since then, but there are still problems in determining distances accurately. Furthermore, we saw in an earlier chapter that peculiar velocities add to the problem. They are velocities that are caused by the mutual attraction between galaxies and clusters. Some of the scatter is no doubt due to them.