Unified dark sectors in scalar-torsion theories of gravity
  We present a unified description of the matter and dark energy epochs, using
a class of scalar-torsion theories. We provide a Hamiltonian description, and
by applying Noether's theorem and by requiring the field equations to admit
linear-in-momentum conservation laws we obtain two specific classes of
scalar-field potentials. We extract analytic solutions and we perform a
detailed dynamical analysis. We show that the system possesses critical points
that correspond to scaling solutions in which the effective, total
equation-of-state parameter is close to zero and points in which it is equal to
the cosmological constant value $-1$. Therefore, during evolution, the Universe
remains for sufficiently long times at the epoch corresponding to dust-matter
domination, while at later times it enters the accelerated epoch and it
eventually results in the de Sitter phase. Finally, in contrast to other
unified scenarios, such as Chaplygin gas-based models as well as
Horndeski-based constructions, the present scenario is free from instabilities
and pathologies at the perturbative level.
