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1. 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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2. Outlook for detecting the gravitational-wave displacement and spin memory effects with current and future gravitational-wave detectors

   https://doi.org/10.1103/PhysRevD.107.064056  

   Grant, Alexander M.; Nichols, David A. (March 2023, Physical Review D)

   Gravitational-wave memory effects arise from nonoscillatory components of gravitational-wave signals, and they are predictions of general relativity in the nonlinear regime that have close connections to the asymptotic properties of isolated gravitating systems. There are many types of memory effects that have been studied in the literature. In this paper we focus on the “displacement” and “spin” memories, which are expected to be the largest of these effects from sources such as the binary black hole mergers which have already been detected by LIGO and Virgo. The displacement memory is a change in the relative separation of two initially comoving observers due to a burst of gravitational waves, whereas the spin memory is a portion of the change in relative separation of observers with initial relative velocity. As both of these effects are small, LIGO, Virgo, and KAGRA can only detect memory effects from individual events that are much louder (and thus rarer) than those that have been detected so far. By combining data from multiple events, however, these effects could be detected in a population of binary mergers. In this paper, we present new forecasts for how long current and future detectors will need to operate in order to measure these effects from populations of binary black hole systems that are consistent with the populations inferred from the detections from LIGO and Virgo’s first three observing runs. We find that a second-generation detector network of LIGO, Virgo, and KAGRA operating at the O4 (“design”) sensitivity for 1.5 years and then operating at the O5 (“plus”) sensitivity for an additional 1.5 years can detect the displacement memory. For Cosmic Explorer, we find that displacement memory could be detected for individual loud events, and that the spin memory could be detected in a population after 5 years of observation time. 

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3. 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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4. Neutrino Flavor Transformation in Neutron Star Mergers

   https://doi.org/10.1103/h2q7-kn3v  

   Qiu, Yi; Radice, David; Richers, Sherwood; Bhattacharyya, Maitraya (August 2025, Physical Review Letters)

   We present the first numerical relativity simulations including neutrino flavor transformations that could result from flavor instabilities, quantum many-body effects, or potential beyond standard model physics in neutron star mergers. We find that neutrino flavor transformations impact the composition and structure of the remnant, potentially leaving an imprint on the post-merger gravitational-wave signal. They also have a significant impact on the composition and nucleosynthesis yields of the ejecta. 

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5. Study of the intermediate mass ratio black hole binary merger up to 1000:1 with numerical relativity

   https://doi.org/10.1088/1361-6382/acc7ef  

   Lousto, Carlos O; Healy, James (April 2023, Classical and Quantum Gravity)

   Abstract We explicitly demonstrate that current numerical relativity techniques are able to accurately evolve black hole binaries with mass ratios of the order of 1000:1. This proof of principle is relevant for future third generation gravitational wave detectors and space mission LISA, as by purely numerical methods we would be able to accurately compute gravitational waves from the last stages of black hole mergers, as directly predicted by general relativity. We perform a sequence of simulations in the intermediate to small mass ratio regime, m 1 p / m 2 p = 1 / 7 , 1 / 16 , 1 / 32 , 1 / 64 , 1 / 128 , 1 / 256 , 1 / 512 , 1 / 1024 , with the small hole starting from rest at a proper distance D ≈ 13 M . We compare these headon full numerical evolutions with the corresponding semianalytic point particle perturbative results finding an impressive agreement for the total gravitational radiated energy and linear momentum as well as for the waveform spectra. We display numerical convergence of the results and identify the minimal numerical resolutions required to accurately solve for these very low amplitude gravitational waves. This work represents a first step towards the considerable challenge of applying numerical-relativity waveforms to interpreting gravitational-wave observations by LISA and next-generation ground-based gravitational-wave detectors. 

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