A separate universe view of the asymmetric sky
  We provide a unified description of the hemispherical asymmetry in the cosmic
microwave background generated by the mechanism proposed by Erickcek,
Kamionkowski, and Carroll, using a delta N formalism that consistently accounts
for the asymmetry-generating mode throughout. We derive a general form for the
power spectrum which explicitly exhibits the broken translational invariance.
This can be directly compared to cosmic microwave background observables,
including the observed quadrupole and fNL values, automatically incorporating
the Grishchuk--Zel'dovich effect. Our calculation unifies and extends previous
calculations in the literature, in particular giving the full dependence of
observables on the phase of our location in the super-horizon mode that
generates the asymmetry. We demonstrate how the apparently different results
obtained by previous authors arise as different limiting cases. We confirm the
existence of non-linear contributions to the microwave background quadrupole
from the super-horizon mode identified by Erickcek et al. and further explored
by Kanno et al., and show that those contributions are always significant in
parameter regimes capable of explaining the observed asymmetry. We indicate
example parameter values capable of explaining the observed power asymmetry
without violating other observational bounds.
