  The self-accelerating braneworld model (DGP) can be tested from measurements
of the expansion history of the universe and the formation of structure.
Current constraints on the expansion history from supernova luminosity
distances, the CMB, and the Hubble constant exclude the simplest flat DGP model
at about 3sigma. The best-fit open DGP model is, however, only a marginally
poorer fit to the data than flat LCDM. Its substantially different expansion
history raises structure formation challenges for the model. A dark-energy
model with the same expansion history would predict a highly significant
discrepancy with the baryon oscillation measurement due the high Hubble
constant required and a large enhancement of CMB anisotropies at the lowest
multipoles due to the ISW effect. For the DGP model to satisfy these
constraints new gravitational phenomena would have to appear at the non-linear
and cross-over scales respectively. A prediction of the DGP expansion history
in a region where the phenomenology is well understood is that high-redshift
galaxies should be substantially correlated with the CMB through the ISW
effect. This correlation should provide a sharp test of the DGP model in the
future.
