Kundt geometries and memory effects in the Brans–Dicke theory of gravity

Memory effects are studied in the simplest scalar–tensor theory, the Brans–Dicke (BD) theory. To this end, we introduce, in BD theory, novel Kundt spacetimes (without and with gyratonic terms), which serve as backgrounds for the ensuing analysis on memory. The BD parameter $$\omega $$ ω and the scalar field ($$\phi $$ ϕ ) profile, expectedly, distinguishes between different solutions. Choosing specific localised forms for the free metric functions $$H'(u)$$ H ′ ( u ) (related to the wave profile) and J(u) (the gyraton) we obtain displacement memory effects using both geodesics and geodesic deviation. An interesting and easy-to-understand exactly solvable case arises when $$\omega =-2$$ ω = - 2 (with J(u) absent) which we discuss in detail. For other $$\omega $$ ω (in the presence of J or without), numerically obtained geodesics lead to results on displacement memory which appear to match qualitatively with those found from a deviation analysis. Thus, the issue of how memory effects in BD theory may arise and also differ from their GR counterparts, is now partially addressed, at least theoretically, within the context of this new class of Kundt geometries.

Doubly torqued vectors and a classification of doubly twisted and Kundt spacetimes

The simple structure of doubly torqued vectors allows for a natural characterization of doubly twisted down to warped spacetimes, as well as Kundt spacetimes down to PP waves. For the first ones the vectors are timelike, for the others they are null. We also discuss some properties, and their connection to hypersurface orthogonal conformal Killing vectors, and null Killing vectors.

KILLING VECTORS IN HIGHER-DIMENSIONAL SPACETIMES WITH CONSTANT SCALAR CURVATURE INVARIANTS

We study the existence of a non-spacelike isometry, ζ, in higher-dimensional Kundt spacetimes with constant scalar curvature invariants (CSI). We present the particular forms for the null or timelike Killing vectors and a set of constraints for the metric functions in each case. Within the class of N-dimensional CSI Kundt spacetimes, admitting a non-spacelike isometry, we determine which of these can admit a covariantly constant null vector that also satisfy ζ[a;b] = 0.