Consistent inflationary cosmology from quadratic gravity with dynamical
  torsion
  The idea of gauge theories of gravity predicts that there should exist not
only the massless graviton but also massive particles carrying the
gravitational force. We study the cosmology in a quadratic gravity with
dynamical torsion where gravity may be interpreted as a gauge force associated
with the Poincar\'{e} group. In addition to the massless spin-2 graviton, the
model contains four non-ghost massive particle species: a couple of spin-0, a
spin-1 and a spin-2. Supposing the restoration of the local Weyl invariance in
the UV limit and the parity invariance, we find the most general minisuperspace
action describing a homogeneous and isotropic universe with a flat spatial
geometry. We then transform the minisuperspace action to a quasi-Einstein frame
in which the field space is a hyperboloid and the field potential is a
combination of those of a Starobinsky-like inflation and a natural inflation.
Remarkably, thanks to the multi-field dynamics, the Starobinsky-like
inflationary trajectory can be realized even if the initial condition is away
from the top of the Starobinsky-like potential. We also study linear tensor
perturbations and find qualitatively different features than the Starobinsky
inflation, spontaneous parity violation and mixing of the massless and massive
spin-2 modes, which might reveal the underlying nature of gravity through
inflationary observables.
