Step 5: distances beyond 1,000,000,000 l.y.

  For very far objects none of the above methods work. The reason is interesting: since we are looking at very distant objects their light has taken a very long time to reach us, so the light we get must have left the object a long time ago. Because of this the farther we look the earlier the images we get: looking far away is equivalent to looking back in time. When we look at the farthest obects we can see, what we get are images of their early stages of their development.

In addition, since the brightness drops as the square of the distance, these far objects must also be very bright. From this it follows that the most distant objects we see are necessarily very bright and very young.

In order to determine the distances with any degree of accuracy we need to know the brightness at a distance of 1 l.y., but here we hit a stone wall: the only objects we see are much older than the ones we are interested in, and we do not have a reliable theory of the way in which these things evolve, we have no way of calibrating our observations using any near-by objects.

It is here, in the observation of the universe at large, that the General Theory of Relativity must be used to measure distances. How this is done is described in the next section.