Magnon Inflation: Slow Roll with Steep Potentials
  We find multi-scalar effective field theories (EFTs) that can achieve a slow
inflationary roll despite having a scalar potential that does not satisfy the
usual slow-roll condition (d V)^2 << V^2/Mp^2. They evade the usual slow-roll
conditions on $V$ because their kinetic energies are dominated by
single-derivative terms rather than the usual two-derivative terms. Single
derivatives dominate during slow roll and so do not require a breakdown of the
usual derivative expansion that underpins calculational control in much of
cosmology. The presence of such terms requires some sort of UV Lorentz-symmetry
breaking during inflation (besides the usual cosmological breaking).
Chromo-natural inflation provides an example of a UV theory that can generate
the multi-field single-derivative terms we consider, and we argue that the EFT
we find indeed captures the slow-roll conditions for the background evolution
for Chromo-natural inflation. We also show that our EFT can be understood as a
multi-field generalization of the single-field Cuscuton models. The multi-field
case introduces a new feature, however: the scalar kinetic terms define a
target-space 2-form, F_{ab}, whose antisymmetry gives new ways for slow roll to
be achieved.
