  Measurements of CMB temperature fluctuations by the Wilkinson Microwave
Anisotropy Probe (WMAP) indicate that the fluctuation amplitude in one half of
the sky differs from the amplitude in the other half. We show that such an
asymmetry cannot be generated during single-field slow-roll inflation without
violating constraints to the homogeneity of the Universe. In contrast, a
multi-field inflationary theory, the curvaton model, can produce this power
asymmetry without violating the homogeneity constraint. The mechanism requires
the introduction of a large-amplitude superhorizon perturbation to the curvaton
field, possibly a pre-inflationary remnant or a superhorizon curvaton-web
structure. The model makes several predictions, including non-Gaussianity and
modifications to the inflationary consistency relation, that will be tested
with forthcoming CMB experiments.
