A small but nonzero cosmological constant

Recent astrophysical observations seem to indicate that the cosmological co nstant is small but nonzero and positive. The old cosmological constant pro blem asks why it is so small; we must now ask, in addition, why it is nonze ro (and is in the range found by recent observations), and why it is positi ve. In this essay, we try to kill these three metaphorical birds with one s tone. 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 Lam bda (t) = Lambda (t(O))(R(t(O))/R(t))(2) but not before the epoch correspon ding to the redshift parameter z similar to 1.