$\Lambda(t)$ 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 $\Lambda(t)=\Lambda_*+3\beta H^2$, 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.
