Self-tuning dark energy in brane world cosmology

In this letter, the self-tuning mechanism of dark energy is proposed by con sidering the cosmological matter in the brane world. In the first part of t his letter, the effective four-dimensional Friedmann equation in the presen ce of the bulk scalar field is derived. In our model, the bulk scalar field takes the role of the dark energy and its value is slowly varying in time. The claim is that, even if the enormous amount of vacuum energy exists on the brane, the present value of the dark energy is self-tuned to be consist ent with the current observations. This result will open the possibility to discuss the dark energy in the brane world in the context of the anthropic principle or quantum cosmology.