Unification of Dark Matter - Dark Energy in Generalized Galileon
  Theories
  We present a unified description of the dark matter and the dark energy
sectors, in the framework of shift-symmetric generalized Galileon theories.
Considering a particular combination of terms in the Horndeski Lagrangian in
which we have not introduced a cosmological constant or a matter sector, we
obtain an effective unified cosmic fluid whose equation of state $w_U$ is zero
during the whole matter era, namely from redshifts $z\sim3000$ up to
$z\sim2-3$. Then at smaller redshifts it starts decreasing, passing the bound
$w_U=-1/3$, which marks the onset of acceleration, at around $z\sim0.5$. At
present times it acquires the value $w_U=-0.7$. Finally, it tends toward a
de-Sitter phase in the far future. This behaviour is in excellent agreement
with observations. Additionally, confrontation with Supernovae type Ia data
leads to a very efficient fit. Examining the model at the perturbative level,
we show that it is free from pathologies such as ghosts and Laplacian
instabilities, at both scalar and tensor sectors, at all times.
