Gravitational waves from smooth hybrid new infation
  We calculate the production of the gravitational waves from a double inflation model with lattice simulations. Between the two inflationary stages, gravitational waves with a characteristic frequency are produced by fluctuations of the scalar fields enhanced through parametric resonance. The wavelength of the produced gravitational waves gets extra redshift during the second inflationary stage and it can be in the observable range for the direct gravitational wave detectors. It is found that there is a possibility for the produced gravitational waves to be detected in the planned experiments. 
Masahiro Kawasaki, Ken'ichi Saikawa and Naoyuki Takeda
astro-ph.CO gr-qc hep-ph