Nonlocal Cosmology II --- Cosmic acceleration without fine tuning or
  dark energy
  We present an improved version of our original cosmological model to explain
the current phase of cosmological acceleration without resorting to a
cosmological constant or any other mass scale. Like the original, this
phenomenological approach is based on an effective quantum gravitational
action, but now depends on the original nonlocal dimensionless scalar $X =
\square^{-1} R$ only through $Y = \square^{-1} g^{\mu\nu} X_{,\mu} X_{,\nu}$.
Both $X$ and $Y$ are quiescent during the radiation-dominated ($R=0$) era, both
only grow logarithmically during matter dominance, and neither affects the
propagation of gravitational radiation. However, while $X$ has the same sign
for gravitationally bound systems as for cosmology, we show that the sign of
$Y$ differs for the two cases: it is positive for cosmology and negative for
strongly gravitationally bound systems. We can therefore enforce the
$\Lambda$CDM expansion history by making a suitable choice of the nonlocal
distortion function $f(Y)$ for $Y > 0$, while ensuring that there is no change
in the heavily constrained solar system phenomenology simply by making $f$
vanish for $Y < 0$ without discontinuity. The required $f(Y>0)$ is determined
numerically to have a strikingly simple exponential form.
