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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. Scattering rigidity for standard stationary manifolds via timelike geodesics

   https://doi.org/10.1088/1361-6544/ae1ac0  

   Muñoz-Thon, Sebastián (November 2025, Nonlinearity)

   Abstract We study the scattering rigidity problem for standard stationary manifolds using timelike geodesics with fixed momentum. Taking advantage of the symmetry of these manifolds, we apply Hamiltonian reduction to show that this problem is related to scattering rigidity for $\mathrm{MP}$ -systems, a problem that has been studied previously. This connection yields several new rigidity results (up to some gauge) for this type of Lorentzian manifold. 

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3. 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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4. 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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5. Tip of the Iceberg: Overmassive Black Holes at 4 < z < 7 Found by JWST Are Not Inconsistent with the Local $M_{\mathrm{BH}}-M_{\star }$ Relation

   https://doi.org/10.3847/1538-4357/ada603  

   Li, Junyao; Silverman, John D; Shen, Yue; Volonteri, Marta; Jahnke, Knud; Zhuang, Ming-Yang; Scoggins, Matthew T; Ding, Xuheng; Harikane, Yuichi; Onoue, Masafusa; et al (February 2025, The Astrophysical Journal)

   Abstract JWST is revealing a remarkable new population of high-redshift (z ≳ 4), low-luminosity active galactic nuclei in deep surveys and detecting the host galaxy's stellar light in the most luminous and massive quasars atz ∼ 6 for the first time. Recent findings claim that supermassive black holes (SMBHs) in these systems are significantly more massive than predicted by the local black hole (BH) mass–stellar mass ( ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ ) relation and that this is not due to sample selection effects. Through detailed statistical modeling, we demonstrate that the coupled effects of selection biases (i.e., finite detection limit and requirements for detecting broad lines) and measurement uncertainties can largely explain the reported offset and flattening in the observed ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ relation toward the upper envelope of the local relation, even for those at ${\mathcal{M}}_{\mathrm{BH}}<10^8{M}_{\odot }$ . We further investigate the possible evolution of the ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ relation atz ≳ 4 with careful treatment of observational biases and consideration of the degeneracy between intrinsic evolution and dispersion in this relation. The bias-corrected intrinsic ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ relation in the low-mass regime ( ${\mathcal{M}}_{\star }\lesssim 10^{10}{M}_{\odot }$ ) suggests a large population of low-mass BHs ( ${\mathcal{M}}_{\mathrm{BH}}\lesssim 10^5{M}_{\odot }$ ), possibly originating from lighter seeds, may remain undetected or unidentified. These results underscore the importance of forward modeling observational biases to better understand BH seeding and SMBH–galaxy coevolution mechanisms in the early universe, even with the deepest JWST surveys. 

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