  Higgs inflation is a simple and elegant model in which early-universe
inflation is driven by the Higgs sector of the Standard Model. The Higgs sector
can support early-universe inflation if it has a large nonminimal coupling to
the Ricci spacetime curvature scalar. At energies relevant to such an
inflationary epoch, the Goldstone modes of the Higgs sector remain in the
spectrum in renormalizable gauges, and hence their effects should be included
in the model's dynamics. We analyze the multifield dynamics of Higgs inflation
and find that the multifield effects damp out rapidly after the onset of
inflation, because of the gauge symmetry among the scalar fields in this model.
Predictions from Higgs inflation for observable quantities, such as the
spectral index of the power spectrum of primordial perturbations, therefore
revert to their familiar single-field form, in excellent agreement with recent
measurements. The methods we develop here may be applied to any multifield
model with nonminimal couplings in which the ${\cal N}$ fields obey an $SO
({\cal N})$ symmetry in field space.
