Ghost Dark Matter
  We revisit ghost dark matter, the possibility that ghost condensation may
serve as an alternative to dark matter. In particular, we investigate the
Friedmann-Robertson-Walker (FRW) background evolution and the large-scale
structure (LSS) in the $\Lambda$GDM universe, i.e. a late-time universe
dominated by a cosmological constant and ghost dark matter. The FRW background
of the $\Lambda$GDM universe is indistinguishable from that of the standard
$\Lambda$CDM universe if $M\gtrsim 1 {\rm eV}$, where $M$ is the scale of
spontaneous Lorentz breaking. From the LSS we find a stronger bound: $M\gtrsim
10 {\rm eV}$. For smaller $M$, ghost dark matter would have non-negligible
sound speed after the matter-radiation equality, and thus the matter power
spectrum would significantly differ from observation. These bounds are
compatible with the phenomenological upper bound $M\lesssim 100 {\rm GeV}$
known in the literature.
