  We report low-energy inelastic neutron scattering data of the paramagnetic
(PM) to hidden-order (HO) phase transition at $T_0=17.5\,{\rm K}$ in
URu$_2$Si$_2$. While confirming previous results for the HO and PM phases, our
data reveal a pronounced wavevector dependence of low-energy excitations across
the phase transition. To analyze the energy scans we employ a damped harmonic
oscillator model containing a fit parameter $1/\Gamma$ which is expected to
diverge at a second-order phase transition. Counter to expectations the
excitations at $\vec{Q}_1=(1.44,0,0)$ show an abrupt step-like suppression of
$1/\Gamma$ below $T_0$, whereas excitations at $\vec{Q}_0=(1,0,0)$, associated
with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement
and a pronounced peak of $1/\Gamma$ at $T_0$. Therefore, at the critical HO
temperature $T_0$, LMAF fluctuations become nearly critical as well. This is
the behavior expected of a super-vector order parameter with nearly degenerate
components for the HO and LMAF leading to nearly isotropic fluctuations in the
combined order-parameter space.
