Abstract
It is well-known that blackhole and cosmological horizons in equilibrium situations are well-modeled by non expanding horizons (NEHs) [1–3]. In the first part of the paper we introduce multipole moments to characterize their geometry, removing the restriction to axisymmetric situations made in the existing literature [4]. We then show that the symmetry group mathfrak{G} of NEHs is a 1-dimensional extension of the BMS group mathfrak{B} . These symmetries are used in a companion paper [5] to define charges and fluxes on NEHs, as well as perturbed NEHs. They have physically attractive properties. Finally, it is generally not appreciated that mathcal{I} ±of asymptotically flat space-times are NEHs in the conformally completed space-time. Forthcoming papers will (i) show that mathcal{I} ± have a small additional structure that reduces mathfrak{G} to the BMS group mathfrak{B} , and the BMS charges and fluxes can be recovered from the NEH framework; and, (ii) develop gravitational wave tomography for the late stage of compact binary coalescences: reading-off the dynamics of perturbed NEHs in the strong field regime (via evolution of their multipoles), from the waveform at mathcal{I} +.
Highlights
We show that the symmetry group G of non expanding horizons (NEHs) is a 1-dimensional extension of the BMS group B
While the notion of NEHs is very general, they are endowed with sufficiently rich structure for obtaining results on black hole and cosmological horizons that are of direct physical interest
We introduce the definition of multipoles — a set of numbers extracted from the NEH geometry
Summary
This section is divided into three parts. In the first, we recall salient features of the geometry of NEHs. In the second part we introduce necessary conceptual framework and in the third we define multipoles and discuss their salient properties
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