  Recent astrophysical observations seem to indicate that the cosmological
constant is small but nonzero and positive. The old cosmological constant
problem asks why it is so small; we must now ask, in addition, why it is
nonzero (and is in the range found by recent observations), and why it is
positive. In this essay, we try to kill these three metaphorical birds with one
stone. That stone is the unimodular theory of gravity, which is the ordinary
theory of gravity, except for the way the cosmological constant arises in the
theory. We argue that the cosmological constant becomes dynamical, and
eventually, in terms of the cosmic scale factor $R(t)$, it takes the form
$\Lambda(t) = \Lambda(t_0)(R(t_0)/R(t))^2$, but not before the epoch
corresponding to the redshift parameter $z \sim 1$.
