arXiv subjects

Alexandru Lupsasca

Publications and source records attributed to Alexandru Lupsasca.

At least 19 recordsLinked to original sources

arXiv2026

Single-Minus Graviton Amplitudes from Matrix Theory

The $n$-point single-minus graviton amplitudes are nonzero in a special kinematic region that preserves some supersymmetry when embedded in supergravity. We compute these amplitudes within the BFSS matrix theory by relating them to an index problem considered by A. Sen. More precisely, the single-minus amplitudes are obtained from an index that counts BPS states in the Coulomb branch of four-dimensional ${\cal N}=4$ super Yang-Mills. We explicitly check this relation in a specific kinematic region by computing the index using BPS wall-crossing formulae and matching it to the gravity result. We further employ BPS wall-crossing to show that the index satisfies a tower of $w_{1+\infty}$ soft theorems in any kinematic regime, again matching gravity. Finally, we show that for certain kinematics, the single-minus amplitudes can be realized by scattering a graviton off a plane wave.

hep-th↗

arXiv2026

Black Hole Polarimetry: Universal Polarization of Synchrotron Radiation at the Horizon

Polarized images of a black hole encode the direction of electromagnetic energy flow near its event horizon. Measuring polarization from near-horizon emission can help determine whether this energy flow is powered by the accreting plasma or the black hole spin. Here we consider the linear polarization of synchrotron radiation emitted from the base of horizon-threading field lines in a time-stationary, axisymmetric, and degenerate Kerr magnetosphere with nonzero poloidal current. We show that the observed polarization pattern displays universal behavior: it is completely determined by the black hole spin and observer inclination and is independent of the magnetic field geometry. We derive a simple analytic formula for this spin-dependent horizon polarization pattern. We find that this predicted pattern is also approached in time-averaged images from General Relativistic Magnetohydrodynamic simulations. Future observations with Very-Long-Baseline Interferometry at microarcsecond resolution could detect the trend of polarization toward the unique horizon value in M87*. Such observations may enable new measurements of black hole spin and provide evidence that magnetic field lines thread the horizon, a necessary condition for spin-energy extraction via the Blandford--Znajek process.

astro-ph.HE↗

arXiv2026

Chaos and fractals of the black hole photon ring

The photon ring of a Kerr black hole decomposes into a self-similar hierarchy of subrings. Here, we show that this self-similar structure persists in phase space. Moreover, near the photon shell of bound photon orbits, dynamics are controlled by a Lyapunov exponent $\gamma$, whose role we highlight by computing the first-return map for light rays close to an unstably bound orbit. Despite an exponential $e^\gamma$ sensitivity to initial conditions, nearly bound rays do not exhibit chaotic behavior. However, as the background spacetime is deformed away from the Kerr geometry, chaos sets in, with its first onset most visible near strongly resonant bound orbits in the photon shell. We display two animations: one illustrating the emergence of chaos near the photon shell, which results in a fractal phase-space structure, and another exhibiting how this chaotic, fractal, self-similar structure is encoded in the first-return map.

gr-qc↗

arXiv2026

Black Hole Vision: An Interactive iOS Application for Visualizing Black Holes

The Black Hole Explorer (BHEX) is a proposed mission to launch a sub-millimeter radio telescope into Earth orbit that will take the sharpest images in the history of astronomy and reveal novel horizon-scale features of supermassive black holes. Black Hole Vision is an open-source application, freely available on the iOS App Store, that produces lensed images which highlight the key features expected to appear in the black hole images BHEX will capture. The app combines video feeds from the front- and rear-facing iPhone cameras and uses the black hole lensing equations to synthesize an onscreen image displaying the user's surroundings as if they were gravitationally lensed by a black hole within the cameras' field of view. Here, we describe how light rays are lensed by non-rotating (Schwarzschild) and rotating (Kerr) black holes, and we list the equations needed for computing black-hole-lensed images. We also describe their specific implementation within Black Hole Vision.

gr-qc↗

arXiv2026

Single-minus graviton tree amplitudes are nonzero

Single-minus tree-level $n$-graviton scattering amplitudes are revisited. Often presumed to vanish, they are shown here to be nonvanishing for certain "half-collinear" configurations existing in Klein space or for complexified momenta. A Berends-Giele recursion relation for these amplitudes is derived and solved in a form involving a sum over trees. In a restricted kinematic decay region, this solution simplifies significantly to an $(n{-}2)$-fold product of soft factors. It is further shown in this region that, combined with suitable analyticity assumptions, the $n$-graviton amplitude is generated by a recursive $\mathcal{L}w_{1+\infty}$ Ward identity with the three-graviton amplitude as a seed.

hep-th↗

arXiv2026

Single-minus gluon tree amplitudes are nonzero

Single-minus tree-level $n$-gluon scattering amplitudes are reconsidered. Often presumed to vanish, they are shown here to be nonvanishing for certain "half-collinear" configurations existing in Klein space or for complexified momenta. We derive a piecewise-constant closed-form expression for the decay of a single minus-helicity gluon into $n-1$ plus-helicity gluons as a function of their momenta. This formula nontrivially satisfies multiple consistency conditions including Weinberg's soft theorem.

hep-th↗

arXiv2026

Resonances in binary extreme mass ratio inspirals

Stellar-mass binaries evolving in the vicinity of supermassive black holes (SMBHs) may be common in the universe, either in active galactic nuclei or in other astrophysical environments. Here, we study in detail the resonant excitation of SMBH modes driven by a nearby stellar-mass binary. The resulting resonant energy fluxes vary with the orbital location and frequency of the binary, exhibiting a rich and complex structure. In particular, we find that the total energy flux radiated to infinity is maximized at a gravitational-wave frequency that is close to, but not exactly equal to, the real part of the corresponding quasinormal-mode frequency. Moreover, as the binary is moved farther away from the SMBH, this offset from the mode frequency becomes increasingly pronounced. In addition, for suitable orientations, the binary can effectively ``feed'' the light ring of the SMBH, selectively exciting particular oscillation modes. For rotating (Kerr) black holes, the mode spectrum is significantly more intricate; however, individual modes are also less strongly damped, leading to an enhanced -- but more difficult to interpret -- resonant response.

gr-qc↗

arXiv2025

Generalized Couch--Torrence inversions

The Laplace equation on Euclidean flat space admits a discrete radial inversion symmetry. In 1983, Couch and Torrence (CT) found -- surprisingly -- that the massless wave equation continues to display this symmetry on the background of an extremal (and asymptotically flat) black hole, where the inversion interchanges horizon and infinity while preserving the singularity structure of the separated radial mode equation. We revisit this CT inversion symmetry and investigate its possible extensions beyond the extremal (Reissner-Nordstr\"om or Kerr) setting in which it was originally identified. Using the example of the static lukewarm de Sitter black hole, we show that neither the exchange of horizon with infinity, nor the preservation of radial singularities, are essential features needed for a CT inversion to exist. Instead, we interpret CT transformations through their action on photon spheres, providing a unified viewpoint that extends to the (phase-space-dependent) CT inversions of the extremal Kerr-Newman geometry. For scalar fields on that spacetime, we find a simple relation between the fixed point of a CT inversion and the coefficient of superradiant scattering. Finally, we exhibit a hidden CT inversion symmetry that arises in the static limit of the Kerr Laplacian for all spins. Together, these results suggest that CT symmetry may admit a broader generalization than previously understood.

gr-qc↗

arXiv2025

Early science acceleration experiments with GPT-5

AI models like GPT-5 are an increasingly valuable tool for scientists, but many remain unaware of the capabilities of frontier AI. We present a collection of short case studies in which GPT-5 produced new, concrete steps in ongoing research across mathematics, physics, astronomy, computer science, biology, and materials science. In these examples, the authors highlight how AI accelerated their work, and where it fell short; where expert time was saved, and where human input was still key. We document the interactions of the human authors with GPT-5, as guiding examples of fruitful collaboration with AI. Of note, this paper includes four new results in mathematics (carefully verified by the human authors), underscoring how GPT-5 can help human mathematicians settle previously unsolved problems. These contributions are modest in scope but profound in implication, given the rate at which frontier AI is progressing.

cs.CL↗

arXiv2025

Gravitational Waves on Kerr Black Holes II: Metric Reconstruction with Cosmological Constant

In this second paper of our series started with \cite{Berens2024}, we investigate linearized gravitational perturbations of a rotating Kerr black hole in a non-asymptotically flat spacetime with (anti-)de Sitter boundary conditions. Here, we explicitly write down the metric components (in both ingoing and outgoing radiation gauge) of the perturbations that correspond to a given mode of either Weyl scalar. We provide formulas involving Hertz potentials (intermediate quantities with a holographic interpretation) as well as some that involve only the separated radial and angular modes. We expect these analytic results to prove useful in numerical studies of black hole perturbation theory in the context of the holographic correspondence.

gr-qc↗

arXiv2025

Interferometric inference of black hole spin from photon ring size and brightness

The $n=1$ photon ring is a full image of the astrophysical source around a black hole, produced by photons that execute $n\approx1$ half-orbit around the event horizon on their way to an observer. The Black Hole Explorer (BHEX) is a proposed extension of the Event Horizon Telescope to space that will target the $n=1$ photon rings of the supermassive black holes M87${}^\ast$ and Sgr\,A${}^\ast$. In this paper, we introduce a new interferometric observable that will be directly measurable on BHEX baselines and which admits a clear image-domain interpretation in terms of the photon ring brightness profile. Across a wide range of semi-analytic equatorial emission models, we find that the azimuthal intensity profile of the ring can change depending on the astrophysics of the source, but its width $w_b$ is weakly sensitive to these details -- much like the ring shape, which has previously been identified as a probe of the spacetime geometry. Our survey suggests that interferometric measurements of the photon ring diameter and $w_b$ can place constraints (to $\lesssim\!20\%$) on the spin and inclination of a black hole with a known mass-to-distance ratio, such as Sgr\,A${}^\ast$. State-of-the-art numerical simulations support this finding, paving the way to a precise photon-ring-based spin measurement for Sgr\,A${}^\ast$ with BHEX.

astro-ph.HE↗

arXiv2025

Black Hole Polarimetry II: The Connection Between Spin and Polarization

We study synchrotron polarization in spatially resolved horizon-scale images, such as those produced by the Event Horizon Telescope (EHT). In both general relativistic magnetohydrodynamic (GRMHD) simulations as well as simplified models of the black hole magnetosphere, the polarization angle, quantified by the complex observable arg(beta_2), depends strongly and systematically on the black hole spin. This relationship arises from the coupling between spin and the structure of the magnetic field in the emission region, and it can be computed analytically in the force-free limit. To explore this connection further, we develop a semi-analytic inflow framework that solves the time stationary axisymmetric equations of GRMHD in the black hole's equatorial plane; this model can interpolate between the force-free and inertial regimes by varying the magnetization of the inflow. Our model demonstrates how finite inertia modifies the structure of the electromagnetic field and can be used to quantitatively predict the observed polarization pattern. By comparing reduced models, GRMHD simulations, and analytic limits, we show that the observed synchrotron polarization can serve as a robust diagnostic of spin under assumptions about Faraday rotation and the emission geometry. Applied to EHT data, the model disfavors high-spin configurations for both M87* and Sgr A*, highlighting the potential of polarimetric imaging as a probe of both black hole spin and near-horizon plasma physics.

astro-ph.HE↗

arXiv2025

Photon rings in a holographic toy model

Light circling around an astrophysical black hole can spend a long time skirting its unstably bound photon orbits before escaping to infinity. To a distant observer, this orbiting light would appear as a bright ring encircling the image of the black hole. Though not yet resolved by radio-interferometric observations from the ground, this ``photon ring'' will be the target of future space-based black hole observations. Motivated by this experimental prospect, studies have sought to elucidate the theoretical connections between the photon ring -- an observable, classical effect -- and the putative holographic description of black holes in quantum gravity. General relativity predicts that the detailed structure of the photon ring encodes the high-frequency (eikonal) spectrum of quasinormal modes (QNMs) emitted by a perturbed black hole as it rings down, and also that the photon ring displays an emergent conformal symmetry that acts upon this spectrum. In holography, the classical QNM frequencies are expected to map to Ruelle resonances of the dual quantum theory. In this paper, we explore these connections in a lower-dimensional toy model based on Warped AdS$_3$ black holes that shares many features with the (3+1)-dimensional Kerr background -- including a photon ring at finite radius -- while still providing analytic control of the QNM frequencies.

hep-th↗

arXiv2025

Why there is no Love in black holes

This paper presents a new conformal symmetry of stationary, axisymmetric Kerr perturbations. This symmetry is exact but non-geometric (or "hidden"), and each of its generators has an associated infinite family of eigenstate solutions. Tidal perturbations of a black hole form an irreducible highest-weight representation of this conformal group, while the tidal response fields live in a different such representation. This implies that black holes have no tidal deformability, or vanishing Love numbers.

gr-qc↗

arXiv2024

Assessing the impact of instrument noise and astrophysical fluctuations on measurements of the first black hole photon ring

Currently envisioned extensions of the Event Horizon Telescope to space will soon target the black hole photon ring: a narrow ring-shaped imprint of a black hole's strong gravity produced in its images by highly bent photon trajectories. In principle, the shape of the photon ring encodes information about the geometry of the underlying black hole spacetime. In practice, however, whether or not this information can be extracted from the ring shape depends on several factors, ranging from the astrophysical details of the emitting source (such as the magnitude of its plasma fluctuations) to the specific configuration of the interferometric array (such as the separation between its telescopes, or the level of noise in its instruments). Here, we employ a phenomenological model to assess the impact of astrophysical fluctuations and instrument noise on the inferred shape of the photon ring. Our systematic study of several astrophysical profiles suggests that this shape can be measured even in the presence of instrument noise across a wide range of baselines. The measurement accuracy and precision appear relatively insensitive to the noise level, up to a sharp threshold beyond which any measurement becomes incredibly challenging (at least without recourse to more sophisticated data analysis methods). Encouragingly, we find that only a few snapshot images are generally needed to overcome the impact of astrophysical fluctuations and correctly infer the ring diameter. Inference becomes more challenging when analyzing the visibility amplitude in a baseline window that is not entirely dominated by a single photon ring. Nevertheless, in most cases, it is still possible to fit a ring shape with the correct fractional asymmetry. These results provide excellent prospects for future precision measurements of black hole spin and fundamental astrophysics via black hole imaging.

gr-qc↗

arXiv2024

The Black Hole Explorer: Using the Photon Ring to Visualize Spacetime Around the Black Hole

The Black Hole Explorer (BHEX), an orbiting, multi-band, millimeter radio-telescope, in hybrid combination with millimeter terrestrial radio-telescopes, is designed to discover and measure the thin photon ring around the supermassive black holes M87* and Sgr A*. As background to the BHEX instruments, this paper explores various aspects of the photon ring, focusing on the intricate flow of light around a spinning black hole, and tracking, through visual simulations, photons as they course along geodesics. Ultimately, the aim of these visualizations is to advance the foundational aims of the BHEX instrument, and through this experiment to articulate spacetime geometry via the photon ring.

gr-qc↗

arXiv2024

The Black Hole Explorer: Photon Ring Science, Detection and Shape Measurement

General relativity predicts that black hole images ought to display a bright, thin (and as-of-yet-unresolved) ring. This "photon ring" is produced by photons that explore the strong gravity of the black hole, flowing along trajectories that experience extreme light bending within a few Schwarzschild radii of the horizon before escaping. The shape of the photon ring is largely insensitive to the precise details of the emission from the astronomical source surrounding the black hole and therefore provides a direct probe of the Kerr geometry and its parameters. The Black Hole Explorer (BHEX) is a proposed space-based experiment targeting the supermassive black holes M87* and Sgr A* with radio-interferometric observations at frequencies of 100 GHz through 300 GHz and from an orbital distance of ~30,000 km. This design will enable measurements of the photon rings around both M87* and Sgr A*, confirming the Kerr nature of these sources and delivering sharp estimates of their masses and spins.

gr-qc↗

arXiv2024

The Black Hole Explorer: Motivation and Vision

We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow "photon ring" that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole's spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole's spin. In addition to studying the properties of the nearby supermassive black holes M87* and Sgr A*, BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade.

astro-ph.IM↗