Interacting dark energy in f(R) gravity

We analyze the interaction between matter and dark energy in f(R) gravity. We use the Palatini variational principle and the field equations in the Einstein conformal frame. For the 1/R-modified gravity, which is the simplest way of introducing current cosmic acceleration without a cosmological constant, the predicted value of the matter-dark energy interaction rate is relatively weak, on the order of 1% of the current value of the Hubble parameter. This prediction does not support the holographic models of dark energy with a constant coupling to matter. We also find that the mass of matter in the universe increased by a factor between 0.6% and 19% since the deceleration-to-acceleration transition, which provides another constraint on the 1/R-modified gravity