  The self-accelerating universe realizes the accelerated expansion of the
universe at late times by large-distance modification of general relativity
without a cosmological constant. The Dvali-Gabadadze-Porrati (DGP) braneworld
model provides an explicit example of the self-accelerating universe. Recently,
the DGP model becomes very popular to study the observational consequences of
the modified gravity models as an alternative to dark energy models in GR.
However, it has been shown that the self-accelerating universe in the DGP model
contains a ghost at the linearized level. The ghost carries negative energy
densities and it leads to the instability of the spacetime. In this article, we
review the origin of the ghost in the self-accelerating universe and explore
the physical implication of the existence of the ghost.
