Resonant locking between binary systems induced by gravitational waves
Physical Review D American Physical Society (APS) 113:2 (2026) 023040
Abstract:
The interaction of gravitational waves (GWs) with matter is thought to be typically negligible in the Universe. We identify an exception in the case of resonant interactions, where GWs emitted by a background binary system, such as an inspiraling supermassive black hole (SMBH) binary, cause a resonant response in a stellar-mass foreground binary and the frequencies of the two systems become, and remain, synchronized. We point out that this previously unexplored dynamical phenomenon is not only possible, but can lead to binary systems becoming resonantly locked in the host galaxy of merging SMBHs of mass , each of which has a significantly reduced merger time. We predict binary systems have been locked in the Universe鈥檚 history. Resonant locking could be detected through anomalous inspiral of binary systems.Angular-momentum pairs in spherical systems: applications to the Galactic centre
Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2026) stag039
Abstract:
Abstract Consider a system of point masses in a spherical potential. In such systems objects execute planar orbits covering two-dimensional rings or annuli, represented by the angular-momentum vectors, which slowly reorient due to the persistent weak gravitational interaction between different rings. This process, called vector resonant relaxation, is much faster than other processes which change the size/shape of the rings. The interaction is strongest between objects with closely aligned angular-momentum vectors. In this paper, we show that nearly parallel angular-momentum vectors may form stable bound pairs in angular-momentum space. We examine the stability of such pairs against an external massive perturber, and determine the critical separation analogous to the Hill radius or tidal radius in the three-body problem, where the angular-momentum pairs are marginally disrupted, as a function of the perturber鈥檚 mass, the orbital inclination, and the radial distance. Angular-momentum pairs or multiples closer than the critical inclination will remain bound and evolve together in angular-momentum-direction space under any external influence, such as anisotropic density fluctuations, or massive perturbers. This study has applications in various astrophysical contexts, including galactic nuclei, in particular the Milky Way鈥檚 Galactic centre, globular clusters, or planetary systems. In nuclear star clusters with a central super-massive black hole, we apply this criterion to the disc of young, massive stars, and show that clusters in angular-momentum space may be used to constrain the presence of intermediate-mass black holes or the mass of the nearby gaseous torus.An Introduction to General Relativity and Cosmology: Theory, Observations, and Applications
, 2026
Abstract:
General relativity has entered a new phase of its development as technical advances have led to the direct detection of gravitational radiation from the merging of single pairs of stellar-sized black holes. The exquisite sensitivity of pulsar signal timing measurements has also been exploited to reveal the presence of a background of gravitational waves, most likely arising from the mergers of supermassive black holes thought to be present at the center of most galaxies. This book demonstrates how general relativity is central to understanding these and other observations while explaining the role of relativity in modern cosmology. An Introduction to General Relativity and Cosmology is an essential entr茅e to the subject, combining full discussions of underlying principles with detailed derivations as well as an accessible treatment of the mathematical foundations of singularity theorems and more advanced topics. Combines rigor with a conversational, highly pedagogical approach Emphasizes connections with other areas of physics to sharpen intuition while emphasizing general relativity鈥檚 unique contributions to modern theoretical physics Presents a novel derivation of the energy flux of gravitational waves Provides detailed and explicit derivations of important results, including binary star evolution by gravitational radiation losses Features a detailed treatment of the Hellings-Downs formula, key to understanding pulsar timing array results for gravitational radiation Provides a reexamination of the equivalence principle for relativistic particles Presents new results on the mathematical solutions of the innermost orbits of the gaseous disks that surround black holes Includes a wealth of exercises Solutions manual (available only to instructors)
Reconstructions of electron-temperature profiles from EUROfusion pedestal database using turbulence models and machine learning
Journal of Plasma Physics. 2025;91(6):E155.
Abstract:
This study makes use of plasma-profile data from the EUROfusion pedestal database (Frassinetti et al. 2020 Nucl. Fusion vol. 61, p. 016001), focusing on the electron-temperature and electron-density profiles in the edge region of H-mode ELMy JET ITER-Like-Wall (ILW) pulses. We make systematic predictions of the electron-temperature pedestal, taking engineering parameters of the plasma pulses and the density profiles as inputs. We first present a machine-learning (ML) algorithm which, given more inputs than theory-based modelling, is able to reconstruct unseen temperature profiles within 20 % of the experimental values. We find a hierarchy of the most consequential engineering parameters for such predictions. This result confirms the conceptual possibility of accurate data-driven prediction. Next, taking a simple theoretical approach that assumes a definite local relationship between the electron-density (饾憛/饾惪饾憶饾憭) and electron-temperature (饾憛/饾惪饾憞饾憭) gradients, we find that a range of power-law scalings 饾憛/饾惪饾憞饾憭 =饾惔鈦(饾憛/饾惪饾憶饾憭)饾浖 with 饾浖 鈮0.4 correctly capture the behaviour of the electron-temperature in the steep-gradient region. Fitting 饾惔 and 饾浖 independently for each pedestal reveals a clear one-to-one correlation, suggesting an underlying constraint in pedestal physics. The measured 饾渹饾憭 =饾惪饾憶饾憭/饾惪饾憞饾憭 values across the pedestal exhibit a wide distribution, significantly exceeding the slab-ETG linear stability threshold, implying either a non-linear threshold shift or a measurably supercritical saturated turbulent state. Finally, we fit parameters for scalings that relate the turbulent heat flux to the gradients 饾憛/饾惪饾憞饾憭 and 饾憛/饾惪饾憶饾憭, similarly to models extracted from gyrokinetic simulations. The inclusion of more experimental parameters is necessary for such models to match the accuracy of our ML results.
Finding Local Parallel Electric Fields in Magnetotail Reconnection Using a Two-Spacecraft Method
(2025)