Observational constraints on self-accelerating cosmology

The Dvali-Gabadadze-Porrati (DGP) braneworld model provides a simple alternative to the standard lambda cold dark matter cosmology, with the same number of parameters. There is no dark energy---the late universe self-accelerates due to an infrared modification of gravity. We compute the joint constraints on the DGP model from supernovae, the cosmic microwave background shift parameter, and the baryon oscillation peak in the Sloan Digital Sky Survey luminous red galaxy sample. Flat DGP models are within the joint 2 sigma contour, but the lambda cold dark matter model provides a significantly better fit to the data. These tests are based on the background dynamics of the DGP model, and we comment on further tests that involve structure formation.