$Lambda(t)$ cosmology induced by a slowly varying Elko field

In this work the exact Friedmann-Robertson-Walker equations for an Elko spinor field coupled to gravity in an Einstein-Cartan framework are presented. The torsion functions coupling the Elko field spin-connection to gravity can be exactly solved and the FRW equations for the system assume a relatively simple form. In the limit of a slowly varying Elko spinor field there is a relevant contribution to the field equations acting exactly as a time varying cosmological model $Lambda(t)=Lambda_*+3beta H^2$, where $Lambda_*$ and $beta$ are constants. Observational data using distance luminosity from magnitudes of supernovae constraint the parameters $Omega_m$ and $beta$, which leads to a lower limit to the Elko mass. Such model mimics, then, the effects of a dark energy fluid, here sourced by the Elko spinor field. The density perturbations in the linear regime were also studied in the pseudo-Newtonian formalism.