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 = ^(−1) R only through Y = ^(−1) g^(μν) X,_μ X,_ν. 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 ΛCDM 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) is determined numerically to have a strikingly simple exponential form