12 research outputs found
Universal spacetimes in four dimensions
Universal spacetimes are exact solutions to all higher-order theories of gravity. We study these spacetimes in four dimensions and provide necessary and sufficient conditions for universality for all Petrov types except of type II. We show that all universal spacetimes in four dimensions are algebraically special and Kundt. Petrov type D universal spacetimes are necessarily direct products of two 2-spaces of constant and equal curvature. Furthermore, type II universal spacetimes necessarily possess a null recurrent direction and they admit the above type D direct product metrics as a limit. Such spacetimes represent gravitational waves propagating on these backgrounds. Type III universal spacetimes are also investigated. We determine necessary and sufficient conditions for universality and present an explicit example of a type III universal Kundt non-recurrent metric.publishedVersio
Black rings with a small electric charge: gyromagnetic ratios and algebraic alignment
We study electromagnetic test fields in the background of vacuum black rings
using Killing vectors as vector potentials. We consider both spacetimes with a
rotating S^1 and with a rotating S^2 and we demonstrate, in particular, that
the gyromagnetic ratio of slightly charged black rings takes the value g=3
(this will in fact apply to a wider class of spacetimes). We also observe that
a S^2-rotating black ring immersed in an external "aligned" magnetic field
completely expels the magnetic flux in the extremal limit. Finally, we discuss
the mutual alignment of principal null directions of the Maxwell 2-form and of
the Weyl tensor, and the algebraic type of exact charged black rings. In
contrast to spherical black holes, charged rings display new distinctive
features and provide us with an explicit example of algebraically general (type
G) spacetimes in higher dimensions. Appendix A contains some global results on
black rings with a rotating 2-sphere. Appendix C shows that g=D-2 in any D>=4
dimensions for test electromagnetic fields generated by a time translation.Comment: 22 pages, 3 figures. v2: new appendix C finds the gyromagnetic ratio
g=D-2 in any dimensions, two new references. To appear in JHE