  Motivated by the holographic principle, it has been suggested that the dark
energy density may be inversely proportional to the area of the event horizon
of the Universe. However, such a model would have a causality problem. In this
paper, we propose to replace the future event horizon area with the inverse of
the Ricci scalar curvature. We show that this model does not only avoid the
causality problem and is phenomenologically viable, but also naturally solves
the coincidence problem of dark energy. Our analysis of the evolution of
density perturbations show that the matter power spectra and CMB temperature
anisotropy is only slightly affected by such modification.
