Peculiar velocities in dynamic spacetimes

We investigate the asymptotic behavior of peculiar velocities in certain physically significant time-dependent gravitational fields. Previous studies of the motion of free test particles have focused on the collapse scenario, according to which a double-jet pattern with Lorentz factor gamma -> infinity develops asymptotically along the direction of complete gravitational collapse. In the present work, we identify a second wave scenario, in which a single-jet pattern with Lorentz factor gamma -> infinity develops asymptotically along the direction of wave propagation. The possibility of a connection between the two scenarios for the formation of cosmic jets is critically examined.