  We present the first quantum field theory model of inflation that is
renormalizable in the matter sector, with a super-Hubble inflaton mass and
sub-Planckian field excursions, which is thus technically natural and
consistent with a high-energy completion within a theory of quantum gravity.
This is done in the framework of warm inflation, where we show, for the first
time, that strong dissipation can fully sustain a slow-roll trajectory with
slow-roll parameters larger than unity in a way that is both theoretically and
observationally consistent. The inflaton field corresponds to the relative
phase between two complex scalar fields that collectively break a U(1) gauge
symmetry, and dissipates its energy into scalar degrees of freedom in the warm
cosmic heat bath. A discrete interchange symmetry protects the inflaton mass
from large thermal corrections. We further show that the dissipation
coefficient decreases with temperature in certain parametric regimes, which
prevents a large growth of thermal inflaton fluctuations. We find, in
particular, a very good agreement with the Planck legacy data for a simple
quadratic inflaton potential, predicting a low tensor-to-scalar ratio
$r\lesssim 10^{-5}$.
