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1. Adding Gravitational Memory to Waveform Catalogs using BMS Balance Laws

   Mitman, K.; Iozzo, D.; Khera, N.; Boyle, M.; De Lorenzo, T.; Deppe, N.; Kidder, L. E.; Moxon, J.; Pfeiffer, H. J.; Scheel, M. A.; et al (January 2021, Physical review)

   null (Ed.)

   Accurate models of gravitational waves from merging binary black holes are crucial for detectors to measure events and extract new science. One important feature that is currently missing from the Simulating eXtreme Spacetimes (SXS) Collaboration’s catalog of waveforms for merging black holes, and other waveform catalogs, is the gravitational memory effect: a persistent, physical change to spacetime that is induced by the passage of transient radiation. We find, however, that by exploiting the Bondi-van der Burg-Metzner-Sachs (BMS) balance laws, which come from the extended BMS transformations, we can correct the strain waveforms in the SXS catalog to include the missing displacement memory. Our results show that these corrected waveforms satisfy the BMS balance laws to a much higher degree of accuracy. Furthermore, we find that these corrected strain waveforms coincide especially well with the waveforms obtained from Cauchy-characteristic extraction (CCE) that already exhibit memory effects. These corrected strain waveforms also evade the transient junk effects that are currently present in CCE waveforms. Last, we make our code for computing these contributions to the BMS balance laws and memory publicly available as a part of the python package sxs, thus enabling anyone to evaluate the expected memory effects and violation of the BMS balance laws. 

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2. Conservation of asymptotic charges from past to future null infinity: Lorentz charges in general relativity

   https://doi.org/10.1007/JHEP08(2022)029  

   Prabhu, Kartik; Shehzad, Ibrahim (August 2022, Journal of High Energy Physics)

   A bstract We show that the asymptotic charges associated with Lorentz symmetries on past and future null infinity match in the limit to spatial infinity in a class of asymptotically-flat spacetimes. These are spacetimes that obey the Ashtekar-Hansen definition of asymptotic flatness at null and spatial infinity and satisfy an additional set of conditions which we lay out explicitly. Combined with earlier results on the matching of supertranslation charges, this shows that all Bondi-Metzner-Sachs (BMS) charges on past and future null infinity match in the limit to spatial infinity in this class of spacetimes, proving a relationship that was conjectured by Strominger. Assuming additional suitable conditions are satisfied at timelike infinities, this proves that the flux of all BMS charges is conserved in any classical gravitational scattering process in these spacetimes. 

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3. An asymptotic framework for gravitational scattering

   https://doi.org/10.1088/1361-6382/acf5c1  

   Compère, Geoffrey; Gralla, Samuel E.; Wei, Hongji (September 2023, Classical and Quantum Gravity)

   Abstract Asymptotically flat spacetimes have been studied in five separate regions: future/past timelike infinity $i^{\pm }$, future/past null infinity, and spatial infinityi⁰. We formulate assumptions and definitions such that the five infinities share a single Bondi–Metzner–Sachs (BMS) group of asymptotic symmetries and associated charges. We show how individual ingoing/outgoing massive bodies may be ascribed initial/final BMS charges and derive global conservation laws stating that the change in total charge is balanced by the corresponding radiative flux. This framework provides a foundation for the study of asymptotically flat spacetimes containing ingoing and outgoing massive bodies, i.e. for generalized gravitational scattering. Among the new implications are rigorous definitions for quantities like initial/final spin, scattering angle, and impact parameter in multi-body spacetimes, without the use of any preferred background structure. 

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4. Asymptotic symmetries in Bondi gauge and the sub-subleading soft graviton theorem

   https://doi.org/10.1088/1361-6382/ad0215  

   Horn, Bart (November 2023, Classical and Quantum Gravity)

   We investigate asymptotic symmetries which preserve the Bondi gauge conditions but do not preserve the asymptotic falloff conditions for the metric near the null boundary, and their connection to soft graviton theorems for scattering amplitudes. These include generalized superrotation symmetries parameterized by a smooth vector field $$Y^{A}$$ obeying $$D_{A}Y^{A} = 0$$, for which we show that the associated conserved charge can be derived by applying the Noether procedure to the Einstein–Katz action. We also discuss the connection between asymptotic symmetries and the conserved charge associated with the sub-subleading soft theorem, and we find that in Bondi gauge this charge is generated by the combination of a diffeomorphism together with an extra transformation of the metric. 

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5. Higher memory effects in numerical simulations of binary black hole mergers

   https://doi.org/10.1088/1361-6382/ad5d46  

   Grant, Alexander_M; Mitman, Keefe (July 2024, Classical and Quantum Gravity)

   Abstract Gravitational memory effects are predictions of general relativity that are characterized by an observable effect that persists after the passage of gravitational waves. In recent years, they have garnered particular interest, both due to their connection to asymptotic symmetries and soft theorems and because their observation would serve as a unique test of the nonlinear nature of general relativity. Apart from the more commonly known displacement and spin memories, however, there are other memory effects predicted by Einstein’s equations that are associated with more subleading terms in the asymptotic expansion of the Bondi-Sachs metric. In this paper, we write explicit expressions for these higher memory effects in terms of their charge and flux contributions. Further, by using a numerical relativity simulation of a binary black hole merger, we compute the magnitude and morphology of these terms and compare them to those of the displacement and spin memory. We find that, although these terms are interesting from a theoretical perspective, due to their small magnitude they will be particularly challenging to observe with current and future detectors. 

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