  Despite qualitative differences in their underlying physics, both hard and
soft glassy materials exhibit almost identical linear rheological behaviors. We
show that these nearly universal properties emerge naturally in a
shear-transformation-zone (STZ) theory of amorphous plasticity, extended to
include a broad distribution of internal thermal-activation barriers. The
principal features of this barrier distribution are predicted by
nonequilibrium, effective-temperature thermodynamics. Our theoretical loss
modulus $G"(\omega)$ has a peak at the $\alpha$ relaxation rate, and a power
law decay of the form $\omega^{-\zeta}$ for higher frequencies, in quantitative
agreement with experimental data.
