Observing Cosmological Processes in Real Time with Repeating Fast Radio Bursts

It is noted that the duration of a fast radio burst (FRB), about ${10}^{-3}$ s, is a smaller fraction of the time delay between multiple images of a source gravitationally lensed by a galaxy or galaxy cluster than the human lifetime is to the age of the universe. Thus repeating, strongly lensed FRBs may offer an unprecedented opportunity for observing cosmological evolution in “real time”. The possibility is discussed of observing cosmic expansion, transverse proper motion, mass accretion and perhaps growth of density perturbations, as a function of redshift.