IR Divergence in Inflationary Tensor Perturbations from Fermion Loops
  We estimate fermion loop corrections to the two-point correlation function of primordial tensor perturbations in a slow-roll inflationary background. We particularly compute an explicit term of one-loop correction from a massless fermion, and then extend to the complete Interaction Hamiltonian. After that, we study one-loop corrections contributed by a massive fermion to primordial tensor fluctuations. The loop correction arisen from a massless fermion field contains logarithms and thus may constrain the validity of perturbation theory in inflationary cosmology, but the situation could be relaxed once the fermion's mass is taken into account. Another one-loop diagram for a massive fermion which involves one vertex is constrained by a UV cutoff as expected by quantum field theory. Our result shows that loop corrections of a fermion field have the same sign as those of a scalar field, and thus implies that the inclusion of fermion loop corrections may not help to alleviate the issue of IR divergence in inflationary cosmology. 
Kaixi Feng, Yi-Fu Cai, Yun-Song Piao
hep-th astro-ph.CO gr-qc