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 ∼ 3000 up to z ∼ 2–3. Then at smaller redshifts it starts decreasing, passing the bound w_U = −1/3, which marks the onset of acceleration, at around z ∼ 0.5. 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.