  We discuss Cosmic Microwave Background constraints on the causal set theory
of quantum gravity, which has made testable predictions about the nature of
dark energy. We flesh out previously discussed heuristic constraints by showing
how the power spectrum of causal set dark energy fluctuations can be found from
the overlap volumes of past light cones of points in the universe. Using a
modified Boltzmann code we put constraints on the single parameter of the
theory that are somewhat stronger than previous ones. We conclude that causal
set theory cannot explain late-time acceleration without radical alterations to
General Relativity.
