  Dynamic renormalization group (RG) of fluctuating viscoelastic equations is
investigated to clarify the cause for numerically reported disappearance of
anomalous heat conduction (recovery of Fourier's law) in low-dimensional
momentum-conserving systems. RG flow is obtained explicitly for simplified two
model cases: a one-dimensional continuous medium under low pressure and
incompressible viscoelastic medium of arbitrary dimensions. Analyses of these
clarify that the inviscid fixed point of contributing the anomalous heat
conduction becomes unstable under the RG flow of nonzero elastic-wave speeds.
The dynamic RG analysis further predicts a universal scaling of describing the
crossover between the growth and saturation of observed heat conductivity,
which is confirmed through the numerical experiments of Fermi-Pasta-Ulam
$\beta$ (FPU-$\beta$) lattices.
