A new Recipe for $\\Lambda$-CDM

It is well known that a canonical scalar field is able to describe either dark matter or dark energy but not both. We demonstrate that a non-canonical scalar field can describe both dark matter and dark energy within a unified setting. We consider the simplest extension of the canonical Lagrangian L ∝ x^{α} − V(φ) where α ≥ 1 and V is a sufficiently flat potential. In this case the kinetic term in the Lagrangian behaves just like a perfect fluid, whereas the potential term mimicks the cosmological constant. For very large values, α ≫ 1, the equation of state of the kinetic term drops to zero and the universe expands as ΛCDM. The velocity of sound in this model, and the associated gravitational clustering, is sensitive to the value of α. For very large values of α the clustering properties of our model resemble those of cold dark matter (CDM). But for smaller values of α, gravitational clustering on small scales is suppressed, and our model has properties resembling those of warm dark matter (WDM). Therefore our non-canonical model has an interesting new property: while the background universe expands like ΛCDM, its clustering properties can resemble those of either cold or warm dark matter.