Gravitational wave memory and geodesic congruence response in generalised Ellis-Bronnikov wormholes

Abstract

We investigate gravitational wave memory in generalised Ellis-Bronnikov wormhole spacetimes by studying the response of neighbouring timelike geodesics and geodesic congruences to a localized test gravitational wave pulse. Using proper time as the evolution parameter, we identify pulse-induced displacement and velocity memory after subtracting the background geodesic evolution. We further investigate the response of timelike congruences through the expansion and shear using the Raychaudhuri equations. We find that the memory response depends systematically on the wormhole throat radius, the steepness parameter and the location of the pulse relative to the throat. In particular, a pulse at the throat produces zero net integrated expansion but a finite shear response, while a pulse away from the throat generates an asymmetric congruence response. These results show that wormhole geometry can imprint characteristic signatures on both detector-based and congruence-based memory observables.

14 pages, 16 figures, some explanations are improved, figures modified, references added