Domain Wall Formation from Level Crossing in the Axiverse
  We point out that domain wall formation is a more common phenomenon in the
Axiverse than previously thought. Level crossing could take place if there is a
mixing between axions, and if some of the axions acquire a non-zero mass
through non-perturbative effects as the corresponding gauge interactions become
strong. The axion potential changes significantly during the level crossing,
which affects the axion dynamics in various ways. We find that, if there is a
mild hierarchy in the decay constants, the axion starts to run along the valley
of the potential, passing through many crests and troughs, until it gets
trapped in one of the minima; the {\it axion roulette}. The axion dynamics
exhibits a chaotic behavior during the oscillations, and which minimum the
axion is finally stabilized is highly sensitive to the initial misalignment
angle. Therefore, the axion roulette is considered to be accompanied by domain
wall formation. The cosmological domain wall problem can be avoided by
introducing a small bias between the vacua. We discuss cosmological
implications of the domain wall annihilation for baryogenesis and future
gravitational wave experiments.
