A Testable Solution of the Cosmological Constant and Coincidence
  Problems
  We present a new solution to the cosmological constant (CC) and coincidence
problems in which the observed value of the CC, $\Lambda$, is linked to other
observable properties of the universe. This is achieved by promoting the CC
from a parameter which must to specified, to a field which can take many
possible values. The observed value of Lambda ~ 1/(9.3 Gyrs)^2$ (approximately
10^(-120) in Planck units) is determined by a new constraint equation which
follows from the application of a causally restricted variation principle. When
applied to our visible universe, the model makes a testable prediction for the
dimensionless spatial curvature of Omega_k0 = -0.0056 s_b/0.5; where s_b ~ 1/2
is a QCD parameter. Requiring that a classical history exist, our model
determines the probability of observing a given Lambda. The observed CC value,
which we successfully predict, is typical within our model even before the
effects of anthropic selection are included. When anthropic selection effects
are accounted for, we find that the observed coincidence between t_Lambda =
Lambda^(-1/2) and the age of the universe, t_U, is a typical occurrence in our
model. In contrast to multiverse explanations of the CC problems, our solution
is independent of the choice of a prior weighting of different $\Lambda$-values
and does not rely on anthropic selection effects. Our model includes no
unnatural small parameters and does not require the introduction of new
dynamical scalar fields or modifications to general relativity, and it can be
tested by astronomical observations in the near future.
