Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ghosts in the self-accelerating universe   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.

14
The categories of this paper are: math-ph
3
The categories of this paper are: physics
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
1
The categories of this paper are: cs
37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
17
The categories of this paper are: q-fin
18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
4
The categories of this paper are: cond-mat