Just as the Hubble time provides a natural time scale for our universe,
the Hubble distance, c/H 0 = 4300 ± 400 Mpc, provides a natural distance
scale. Just as the age of the universe is roughly equal to H −1
0 in most Big
Bang models, with the exact value depending on the expansion history of the
universe, so the horizon distance (the greatest distance a photon can travel
during the age of the universe) is roughly equal to c/H 0 , with the exact value,
again, depending on the expansion history. (Later chapters will deal with
computing the exact values of the age and horizon size of our universe.)