Fermion condensate from torsion in the reheating era after inflation
  The inclusion of Dirac fermions in Einstein-Cartan gravity leads to a
four-fermion interaction mediated by non-propagating torsion, which can allow
for the formation of a Bardeen-Cooper-Schrieffer condensate. By considering a
simplified model in 2+1 spacetime dimensions, we show that even without an
excess of fermions over antifermions, the nonthermal distribution arising from
preheating after inflation can give rise to a fermion condensate generated by
torsion. We derive the effective Lagrangian for the spacetime-dependent pair
field describing the condensate in the extreme cases of nonrelativistic and
massless fermions, and show that it satisfies the Gross-Pitaevski equation for
a gapless, propagating mode.
