Quantum fluctuations in planar domain-wall space-times: A possible
  origin of primordial preferred direction
  We study the gravitational effects of a planar domain wall on quantum
fluctuations of a massless scalar field during inflation. By obtaining an exact
solution of the scalar field equation in de Sitter space, we show that the
gravitational effects of the domain wall break the rotational invariance of the
primordial power spectrum without affecting the translational invariance. The
strength of rotational violation is determined by one dimensionless parameter
$\beta$, which is a function of two physical parameters, the domain wall
surface tension $\sigma$ and cosmological constant $\Lambda$. In the limit of
small $\beta$, the leading effect of rotational violation of the primordial
power spectrum is scale-invariant.
