Beginning of Universe through large field hybrid inflation
  Recent detection of $B$-mode polarization induced from tensor perturbations
by the BICEP2 experiment implies so-called large field inflation, where an
inflaton field takes super-Planckian expectation value during inflation, at a
high energy scale. We show however, if another inflation follows hybrid
inflation, the hybrid inflation can generate a large tensor perturbation with
not super-Plankian but Planckian field value. This scenario would relax the
tension between BICEP2 and Planck concerning the tensor-to-scalar ratio,
because a negative large running can also be obtained for a certain number of
e-fold of the hybrid inflation. A natural interpretation of a large
gravitational wave mode with or without the scalar spectral running might be
multiple inflation in the early Universe.
