  The recent discovery of ultra-long wavelength gravitational waves through the
advent of pulsar timing arrays (PTA) has opened up new avenues for fundamental
science. Here we show that every PTA source will be diffractively lensed by
potentially hundreds of galactic disks transverse to its line of sight, leading
to modest modulations in the strain, $\Delta h / h \sim 10^{-3} \lambda^{-1}_{1
\rm pc.}$, due to wave lensing effects. The induced interference, or
scintillation, pattern will be resolvable by coherent PTAs and may be
leveraged, alongside fore-ground redshift information, to make precise
measurements of cosmic expansion. If future PTA experiments can achieve enough
signal-to-noise to detect these small modulations, hundreds of
redshift-distance pairs may be inferred from the diffractive lensing of an
individual PTA source.
