Measuring Cosmic Expansion with Diffractive Gravitational Scintillation of Nanohertz Gravitational Waves

The recent discovery of ultralong 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, Δh/h∼10^{-3}λ_{1  pc}^{-1}, due to wave lensing effects. The induced interference, or scintillation, pattern will be resolvable by coherent PTAs and may be leveraged, alongside foreground 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.