Probing Gravitational Slip with Strongly Lensed Fast Radio Bursts
  The rapid accumulation of observed Fast Radio Bursts (FRBs) originating from
cosmological distances makes it likely that some will be strongly lensed by
intervening matter along the line of sight. Detection of lensed FRB repeaters,
which account for a noteworthy fraction of the total population, will allow not
only an accurate measurement of the lensing time delay, but also follow-up
high-resolution observations to pinpoint the location of the lensed images.
Recent works proposed to use such strongly-lensed FRBs to derive constraints on
the current expansion rate $ H_{0} $ as well as on cosmic curvature. Here we
study the prospects for placing constraints on departures from general
relativity via such systems. Using an ensemble of simulated events, we focus on
the gravitational slip parameter $\gamma_{\rm PN}$ in screened modified gravity
models and show that FRB time-delay measurements can yield constraints as tight
as $ \left| \gamma_{\rm PN}-1\right| \lesssim 0.04\times(\Lambda/100\rm
kpc)\times[N/10]^{-1/2} $ at $1\sigma$ with $10$ detections.
