Inflation in a viscous fluid model
  We explore a fluid description of the inflationary universe. In particular,
we investigate a fluid model in which the equation of state (EoS) for a fluid
includes bulk viscosity. We find that the three observables of inflationary
cosmology: the spectral index of the curvature perturbations, the
tensor-to-scalar ratio of the density perturbations, and the running of the
spectral index, can be consistent with the recent Planck results. We also
reconstruct the explicit EoS for a fluid from the spectral index of the
curvature perturbations compatible with the Planck analysis. In the
reconstructed models of a fluid, the tensor-to-scalar ratio of the density
perturbations can satisfy the constraints obtained from the Planck satellite.
The running of the spectral index can explain the Planck data. In addition, it
is demonstrated that in the reconstructed models of a fluid, the graceful exit
from inflation can be realized. Moreover, we show that the singular inflation
can occur in a fluid model. Furthermore, we show that a fluid description of
inflation can be equivalent to the description of inflation in terms of scalar
field theories.
