Grand Unification and Subcritical Hybrid Inflation
  We consider hybrid inflation for small couplings of the inflaton to matter
such that the critical value of the inflaton field exceeds the Planck mass. It
has recently been shown that inflation then continues at subcritical inflaton
field values where quantum fluctuations generate an effective inflaton mass.
The effective inflaton potential interpolates between a quadratic potential at
small field values and a plateau at large field values. An analysis of the
allowed parameter space leads to predictions for the scalar spectral index n_s
and the tensor-to-scalar ratio r similar to those of natural inflation. Using
the range for n_s and r favoured by the Planck data, we find that the energy
scale of the plateau is constrained to the interval (1.6 - 2.4) * 10^16 GeV
which includes the energy scale of gauge coupling unification in the
supersymmetric standard model. The tensor-to-scalar ratio is predicted to have
the lower bound r > 0.049 for 60 e-folds before the end of inflation.
