**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 *H*_{0}. 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 *H*_{0},
the other being
the recession velocity corrected for large scale motions. The
uncertainty in any estimate of *H*_{0} 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 *H*_{0} 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 *H*_{0}. 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* (=
*H*_{0} / 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.