2.3 Why Do We Care About Distances?
As a final note, it is worth considering why extragalactic distances warrant so much attention. Consider one of the most basic issues: the luminosity of an object. If the distance is not known to better than a factor of 2, then the power output could be in error by a factor of 4. This uncertainty undermines our claim to know one of the most basic properties of distant sources: their energy generation mechanisms.
Perhaps the most widely publicized aspect of the distance scale controversy is that the ages of the oldest stars (e.g., those in globular clusters) are inconsistent with an age of the universe deduced from a large value for H0. There are, of course, a variety of models for the universe in which this problem is resolved, but those are not necessarily the simplest models.
For the purposes of this paper, we emphasize that distances represent only one of the parameters in the definition of H0, the other being the recession velocity corrected for large scale motions. The uncertainty in any estimate of H0 must include the contributions from distances and velocities (which must also include the uncertainties in the large-scale motion corrections). In order to interpret an estimate of H0 in terms of an age parameter, we must also know the density of the universe (complete with uncertainties), and we must estimate the uncertainties in the computer models that predict stellar ages. Consequently, we feel that the age argument demands much more investigation before it can be used to constrain the observational results for H0. In any case, our primary concern in this review is the distances.
There is an often ignored aspect of the distance scale controversy. It seems fairly innocuous, but it adds a continuous element of distraction, annoyance, and cost to extragalactic astronomy. Our inability to ``scale the universe'' means that we must adopt a parametric treatment of distance through the variable h (= H0 / 100). This parameter now pervades the journals (and even appears in this review!) by introducing more ambiguity to an already difficult field. It is our sincere hope that the distance scale controversy can be resolved within this decade so that everyone can move on to solving other important astrophysical problems.