Axion Landscape and Natural Inflation
  Multiple axions form a landscape in the presence of various shift symmetry
breaking terms. Eternal inflation populates the axion landscape, continuously
creating new universes by bubble nucleation. Slow-roll inflation takes place
after the tunneling event, if a very flat direction with a super-Planckian
decay constant arises due to the alignment mechanism. We study the vacuum
structure as well as possible inflationary dynamics in the axion landscape
scenario, and find that the inflaton dynamics is given by either natural or
multi-natural inflation. In the limit of large decay constant, it is
approximated by the quadratic chaotic inflation, which however is disfavored if
there is a pressure toward shorter duration of inflation. Therefore, if the
spectral index and the tensor-to-scalar ratio turn out to be different from the
quadratic chaotic inflation, there might be observable traces of the bubble
nucleation. Also, the existence of small modulations to the inflaton potential
is a common feature in the axion landscape, which generates a sizable and
almost constant running of the scalar spectral index over CMB scales.
Non-Gaussianity of equilateral type can also be generated if some of the axions
are coupled to massless gauge fields.
