Gravity Seminars

The gravitational memory effect and its electromagnetic (EM) analog are potential probes in the strong gravity regime. In the literature, this effect is derived for static observers at asymptotic infinity. While this is a physically consistent approach, it restricts us to deriving memory for an asymptotically flat spacetime. In this talk, I will discuss how we evaluate EM and gravitational memory with respect to a comoving observer (defined by timelike 4-velocity u_{a}). We split 4-D the spacetime into 1+1+2 parts, consisting of one timelike, one preferred spacelike, and one 2-D hypersurface. It allows us to obtain EM memory in an arbitrary curved background spacetime and gravitational memory in a class of spacetime with spherical symmetry often referred to as Locally Rotationally Symmetric(LRS) type II in covariant formalism. The master equation corresponding to the acceleration of the comoving observer in the 2D surface provides a physical understanding of the EM memory. The leading order contribution to the EM memory comes from the total energy density of the EM field, while the subleading contributions contain the spacetime geometry and the other components of the energy-momentum tensor of the EM field. In the case of gravitational memory, we show it is manifested in the integral of the shear tensor of the 2-D hypersurface. We obtain the master equation of gravitational memory in LRS-II spacetime and finally, we show that the memory exhibits distinct signatures between accelerated and decelerated universes, potentially enabling the identification of the transition redshift from a matter-dominated to a dark-energy-dominated universe. In the end(if time permits), I will briefly discuss the generation of detectable GW from the interaction of very high energized EM wave and stellar-mass Schwarzschild black holes in the Milky Way galaxy.