Shift Symmetry and Higgs Inflation in Supergravity with Observable
  Gravitational Waves
  We demonstrate how to realize within supergravity a novel chaotic-type
inflationary scenario driven by the radial parts of a conjugate pair of Higgs
superfields causing the spontaneous breaking of a grand unified gauge symmetry
at a scale assuming the value of the supersymmetric grand unification scale.
The superpotential is uniquely determined at the renormalizable level by the
gauge symmetry and a continuous R symmetry. We select two types of Kahler
potentials, which respect these symmetries as well as an approximate shift
symmetry. In particular, they include in a logarithm a dominant shift-symmetric
term proportional to a parameter c- together with a small term violating this
symmetry and characterized by a parameter c+. In both cases, imposing a lower
bound on c-, inflation can be attained with subplanckian values of the original
inflaton, while the corresponding effective theory respects perturbative
unitarity for r+-=c+/c-<1. These inflationary models do not lead to
overproduction of cosmic defects, are largely independent of the one-loop
radiative corrections and accommodate, for natural values of r+-, observable
gravitational waves consistently with all the current observational data. The
inflaton mass is mostly confined in the range (3.7-8.1)x10^10 GeV.
