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1. No-boundary state for Klein space

   https://doi.org/10.1088/1751-8121/adc882  

   Melton, Walker; Sharma, Atul; Strominger, Andrew; Wang, Tianli (April 2025, Journal of Physics A: Mathematical and Theoretical)

   Abstract Analytic continuation from (3, 1) signature Minkowski to (2, 2) signature Klein space has emerged as a useful tool for the understanding of scattering amplitudes and flat space holography. Under this continuation, past and future null infinity merge into a single boundary ( $J$ ) which is the product of a null line with a (1, 1) signature torus. The Minkowskian $S$ -matrix continues to a Kleinian $S$ -vector which in turn may be represented by a Poincaré-invariant vacuum state $|C⟩$ in the Hilbert space built on $J$ . $|C⟩$ contains all information about $S$ in a novel, repackaged form. We give an explicit construction of $|C⟩$ in a Lorentz/conformal basis for a free massless scalar. $J$ separates into two halves $J_{\pm }$ which are the asymptotic null boundaries of the regions timelike and spacelike separated from the origin. $|C⟩$ is shown to be a maximally entangled state in the product of the $J_{\pm }$ Hilbert spaces. 

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2. The linearization of the boundary rigidity problem for MP-systems and generic local boundary rigidity

   https://doi.org/10.1088/1361-6420/ad8d77  

   Muñoz-Thon, Sebastián (November 2024, Inverse Problems)

   Abstract We consider an $\mathrm{MP}$ -system, that is, a compact Riemannian manifold with boundary, endowed with a magnetic field and a potential. On simple $\mathrm{MP}$ -systems, we study the $\mathrm{MP}$ -ray transform in order to obtain new boundary rigidity results for $\mathrm{MP}$ -systems. We show that there is an explicit relation between the $\mathrm{MP}$ -ray transform and the magnetic one, which allows us to apply results from Dairbekovet al(2007Adv. Math.216535–609) to our case. Regarding rigidity, we show that there exists a generic set $G^m$ of simple $\mathrm{MP}$ -systems, which is open and dense, such that any two $\mathrm{MP}$ -systems close to an element in it and having the same boundary action function, must bek-gauge equivalent. 

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3. GR-Athena++ simulations of spinning binary black hole mergers

   https://doi.org/10.1088/1361-6382/ae4da4  

   Shukla, Estuti; Rashti, Alireza; Gamba, Rossella; Radice, David; Chandra, Koustav (March 2026, Classical and Quantum Gravity)

   Abstract We present the second release of theGR-Athena++waveform catalog, comprising four new quasi-circular, non-precessing, spinning binary black hole simulations. These simulations are performed at high resolutions and represent a step toward generating high-fidelity gravitational waveforms that can eventually meet the accuracy requirements of upcoming next-generation detectors, including LISA, cosmic explorer, and Einstein Telescope. Gravitational waves are extracted at future null infinity ( $I^{+}$ ) using both Cauchy characteristic extraction and finite-radius extraction. For each simulation, we provide strain data across multiple resolutions and analyze waveform accuracy via convergence studies and self-mismatch analyses. The absolute phase and relative amplitude differences reach their largest values near the merger, while the smallest errors are of order $O\left({10}^{-2}\right)$ and $O\left({10}^{-3}\right)$ , respectively. A self-mismatch analysis of the dominant (2, 2) mode yields mismatches between $O\left({10}^{-5}\right)$ and $O\left({10}^{-7}\right)$ for a total binary mass of ${10}^6{M}_{\odot }$ over the frequency range $f\in [0.002,0.1]\mathrm{Hz}$ using LISA’s noise curve. All waveforms are publicly available viaScholarSphere. 

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4. On the generalization of the Kruskal–Szekeres coordinates: a global conformal charting of the Reissner–Nordström spacetime

   https://doi.org/10.1088/1361-6382/ad4dff  

   Aly, Fawzi; Stojkovic, Dejan (May 2024, Classical and Quantum Gravity)

   Abstract The Kruskal–Szekeres coordinate construction for the Schwarzschild spacetime could be interpreted simply as a squeezing of thet-line into a single point, at the event horizon $r=2M$ . Starting from this perspective, we extend the Kruskal charting to spacetimes with two horizons, in particular the Reissner–Nordström manifold, $M_{\text{RN}}$ . We develop a new method to construct Kruskal-like coordinates through casting the metric in new null coordinates, and find two algebraically distinct ways to chart $M_{\text{RN}}$ , referred to as classes: type-I and type-II within this work. We pedagogically illustrate our method by crafting two compact, conformal, and global coordinate systems labeled ${\mathrm{GK}}_{\text{I}}$ and ${\mathrm{GK}}_{\text{II}}$ as an example for each class respectively, and plot the corresponding Penrose diagrams. In both coordinates, the metric differentiability can be promoted to $C^{\mathrm{\infty }}$ in a straightforward way. Finally, the conformal metric factor can be written explicitly in terms of thetandrfunctions for both types of charts. We also argued that the chart recently reported in Soltani (2023 arXiv:2307.11026) could be viewed as another example for the type-II classification, similar to ${\mathrm{GK}}_{\text{II}}$ . 

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5. Density Fluctuation–Mach Number Scaling in Compressible, High Plasma Beta Turbulence: In Situ Space Observations and High-Reynolds Number Simulations

   https://doi.org/10.3847/2041-8213/adbe3b  

   Bandyopadhyay, Riddhi; Beattie, James R; Bhattacharjee, Amitava (March 2025, The Astrophysical Journal Letters)

   Abstract Understanding the nature of compressible fluctuations in a broad range of turbulent plasmas, from the intracluster medium to the solar wind, has been an active field of research in the past decades. Theoretical frameworks for weakly compressible magnetohydrodynamical turbulence in an inhomogeneous background magnetic field predict a linear scaling of the normalized mass density fluctuation (δρ/ρ₀), as a function of the turbulent Mach number ( ${\mathcal{M}}_t$ ), $\delta \rho /{\rho }_0\propto {\mathcal{M}}_t$ . However, so far, the scaling relation has been tested only using moderate to low plasma beta (β ≲ 1) solar wind observational data, where the compressibility is weakδρ/ρ₀ ∼ 0.1. Here, we combine NASA’s Magnetospheric Multiscale Mission data in Earth’s magnetosheath, whereβ ∼ 10 is high, andβ ∼ 1/8 highly compressible magnetohydrodynamic turbulence simulations at unprecedented resolutions. Both show that $\delta \rho /{\rho }_0\propto {\mathcal{M}}_t$ holds across a broad range ofδρ/ρ₀, ${\mathcal{M}}_t$ , andβ, demonstrating that $\delta \rho /{\rho }_0\propto {\mathcal{M}}_t$ is a robust compressible turbulence relation, going beyond the asymptotics of the weakly compressible theory. We discuss the findings in the context of understanding the nature of strongly compressible turbulent fluctuations and the driving parameter in astrophysical and space plasmas. 

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