  Physical models for the hemispherical power asymmetry in the cosmic microwave
background (CMB) reported by the Planck Collaboration must satisfy CMB
constraints to the homogeneity of the Universe and quasar constraints to power
asymmetries. We survey a variety of models for the power asymmetry and show
that consistent models include a modulated scale-dependent isocurvature
contribution to the matter power spectrum or a modulation of the reionization
optical depth, gravitational-wave amplitude, or scalar spectral index. We
propose further tests to distinguish between the different scenarios.
