Event Horizon Research Articles

Deep in the knotted black hole

We consider the transition rate of a freely falling Unruh-DeWitt detector, coupled linearly to a massless scalar quantum field prepared in the Hartle-Hawking-Israel state, as a probe of the interior of a black hole. Specifically, we consider the transition rate of a detector in the spinless Bañados-Teitelboim-Zanelli (BTZ) black hole as it freely falls toward and across the horizon and compare it to the corresponding situation for an RP2 geon. Both the BTZ black hole and its geon counterpart are quotients of AdS3 spacetime that are identical exterior to the horizon but have different interior topologies. We find outside the horizon that the rates are qualitatively similar, but with the amplitude in the geon spacetime larger than in the BTZ case. Once the detector crosses the horizon, there are notable distinctions characterized by different discontinuities in the temporal derivative of the response rate. These discontinuities can appear outside the horizon if the detector is switched on at a sufficiently early time, within the past white hole horizon. In general, the detector can act as an “early warning system” that both spots the black hole horizon and discerns its interior topology. Published by the American Physical Society 2025

Bridging Scales: Coupling the Galactic Nucleus to the Larger Cosmic Environment

Abstract Coupling black hole (BH) feeding and feedback involves interactions across vast spatial and temporal scales that are computationally challenging to model. Tracking gas inflows and outflows from kiloparsec scales to the event horizon for non-spinning BHs in the presence of strong magnetic fields, H. Cho et al. report strong suppression of accretion on horizon scales and low (2%) feedback efficiency. In this letter, we explore the impact of these findings for the supermassive BHs M87* and Sgr A*, using high-resolution, non-cosmological, magnetohydrodynamic simulations with the FIRE-2 model. Without feedback, we find rapid BH growth due to “cooling flows,” with 2% feedback efficiency, while accretion is suppressed, the rates still remain higher than constraints from Event Horizon Telescope (EHT) data for M87* and Sgr A*. To match the EHT observations of M87*, an efficiency greater than 15% is required, suggesting the need to include enhanced feedback from BH spin. Similarly, a feedback efficiency of >15% is needed for Sgr A* to match the observationally estimated star formation rate of ≲2M ⊙ yr−1. Even with 100% feedback efficiency, the simulation-predicted Sgr A* accretion rate remains higher than EHT-inferred levels on average, while only episodically matching it, suggesting that Sgr A* is currently in a temporary quiescent phase. Bridging accretion and feedback across scales, we conclude that higher feedback efficiencies, possibly due to nonzero BH spin, are necessary to suppress “cooling flows” and match both the observed accretion and star formation rates in M87* and Sgr A*.

How Theory-laden are Observations of Black Holes?

Abstract We evaluate the roles general relativistic assumptions play in simulations used in recent observations of black holes including LIGO-Virgo and the Event Horizon Telescope. In both experiments simulations play an ampliative role, enabling the extraction of more information from the data than would be possible otherwise. This comes at a cost of theory-ladenness. We discuss the issue of inferential circularity, which arises in some applications; classify some of the epistemic strategies used to reduce the extent of theory-ladenness; and discuss ways in which these strategies are model independent.

Signatures of Black Hole Spin and Plasma Acceleration in Jet Polarimetry

Abstract We study the polarization of black hole jets on scales of 10−103 GM/c 2 and show that large spatial swings in the polarization occur at three characteristic distances from the black hole: the radius where the counter-jet dims, the radius where the magnetic field becomes azimuthally dominated (the light cylinder), and the radius where the plasma reaches its terminal Lorentz factor. To demonstrate the existence of these swings, we derive a correspondence between axisymmetric magnetohydrodynamic outflows and their force-free limits, which allows us to analytically compute the plasma kinematics and magnetic field structure of collimated, general relativistic jets. We then use this method to ray trace polarized images of black hole jets with a wide range of physical parameters, focusing on roughly face-on jets like that of M87. We show that the location of the polarization swings is strongly tied to the location of the light cylinder and thus to the black hole’s spin, illustrating a new method of measuring spin from polarized images of the jet. This signature of black hole spin should be observable by future interferometric arrays like the (Next Generation) Event Horizon Telescope, which will be able to resolve the polarized emission of the jet down to the near-horizon region at high dynamic range.

Black holes in the expanding Universe

Abstract The McVittie metric does not describe a physical black hole in an expanding Universe because the curvature scalar and pressure at its event horizon are infinite. We show that extending this metric to an inhomogeneous scale factor, which depends on both the time and radial coordinate, removes those infinities by imposing at the horizon the constancy of the Hubble parameter and a particular constraint on the gradient of the scale factor. We consider a special case of this metric, and show that the Hubble parameters at the event horizons of all centrally symmetric black holes are equal to the same constant H hor = ( Λ / 3 ) 1 / 2 . Because of this equality and the equivalence to the Kottler metric near the horizon, black holes do not grow with the Universe expansion.

Multiwavelength Observations of a Jet Launch in Real Time from the Post-changing-look Active Galaxy 1ES 1927+654

Abstract We present results from a high-cadence multiwavelength observational campaign of the enigmatic changing-look active galactic nucleus 1ES 1927+654 from 2022 May to 2024 April, coincident with an unprecedented radio flare (an increase in flux by a factor of ∼60 over a few months) and the emergence of a spatially resolved jet at 0.1–0.3 pc scales. Companion work has also detected a recurrent quasi-periodic oscillation (QPO) in the 2–10 keV band with an increasing frequency (1–2 mHz) over the same period. During this time, the soft X-rays (0.3–2 keV) monotonically increased by a factor of ∼8, while the UV emission remained nearly steady with <30% variation and the 2–10 keV flux showed variation by a factor ≲2. The weak variation of the 2–10 keV X-ray emission and the stability of the UV emission suggest that the magnetic energy density and accretion rate are relatively unchanged and that the jet could be launched owing to a reconfiguration of the magnetic field (toroidal to poloidal) close to the black hole. Advecting poloidal flux onto the event horizon would trigger the Blandford–Znajek mechanism, leading to the onset of the jet. The concurrent softening of the coronal slope (from Γ = 2.70 ± 0.04 to Γ = 3.27 ± 0.04), the appearance of a QPO, and the low coronal temperature ( k T e = 8 − 3 + 8 keV ) during the radio outburst suggest that the poloidal field reconfiguration can significantly impact coronal properties and thus influence jet dynamics. These extraordinary findings in real time are crucial for coronal and jet plasma studies, particularly as our results are independent of coronal geometry.

Thermal Radiation of Accretion Disk Around ChargedHayward Black Hole

Abstract This study explores the dynamics of charged Hayward black holes, focusing on the effects of electric charge and the length factor on accretion disk characteristics. Our results show that increasing both parameters reduces the size of the event horizon and ISCO radius, with electric charge exerting a more pronounced influence. Additionally, the length factor and electric charge can effectively replicate the spin of a Kerr black hole. Both parameters also affect the electromagnetic radiation emitted from the accretion disk, increasing the flux, temperature, and radiative efficiency. The peak radiation occurs in the soft X-ray band, with higher values of electric charge and length factor enhancing disk luminosity and shifting the peak to higher frequencies. These findings can offer valuable insights into the accretion processes around black holes and their observable signatures, particularly in X-ray astronomy.

Massive particle surfaces, partial umbilicity, and circular orbits

The generalization of photon spheres by considering the trajectories of massive particles leads to the definition of massive particle surfaces (MPS). These surfaces, built with the trajectories of massive particles, have a partial umbilicity property. Using the geodesic and Gaussian curvature of the Jacobi metric (a Riemannian metric), we derive a general condition for the existence of a massive particle surface defined for an asymptotically flat spacetime metric. Our results can be applied to the worldlines of charged massive particle surfaces. We provide a simple characterization for timelike and null trajectories using a Riemannian geometric approach. We are able to recover the results for the existence of light rings (LR’s) and timelike circular orbits (TCO’s). We show how an event horizon gets characterized using the curvatures of a Riemannian metric. We discuss several examples, where we derive conditions for the existence of photon sphere and a massive particle surface. We calculate the radius of the photon sphere and the radius of the innermost stable circular orbits (ISCO). Published by the American Physical Society 2025

Analytic expressions for grey-body factors of the general parametrized spherically symmetric black holes

Abstract In light of the recently discovered connection between grey-body factors and quasinormal ringing, we derive analytic expressions for the grey-body factors of generic parametrized spherically symmetric and asymptotically flat black holes. These expressions are presented as expansions in terms of the inverse multipole number and the coefficients of the parametrization. The obtained analytic formulas serve as good approximations whenever the deviation from the Schwarzschild geometry is not very large. We demonstrate that the primary parameter determining the grey-body factors is the deviation of the event horizon radius from its Schwarzschild value, while the higher-order coefficients of the parametrization, which govern the near-horizon geometry, are much less significant. This finding is consistent with recent observations that grey-body factors are considerably more stable against small deformations of the near-horizon geometry than quasinormal modes.

Gravitational waves and Black Hole perturbations in acoustic analogues

Phonons in Bose–Einstein condensates propagate as massless scalar particles on top of an emergent acoustic metric. This hydrodynamics/gravity analogy can be exploited to realize acoustic black holes, featuring an event horizon that traps phonons. The authors show that by an appropriate external potential, gravitational wave-like perturbations of the acoustic metric can be produced. Such perturbations can be used to excite an acoustic black hole, which should then relax by phonon emission.

Political Spirits at the Dawn and Dusk: Exploring History Through Literature

The introduction of democracy in 1951 brought the enjoyment of fundamental rights for the citizens and the spirited election environment. However, the spirits could not last longer as an encroachment on democracy was forwarded by King in 1960, which altered ecstasy into resistance and rebellion. The research explores the political spirits prevalent at the beginning of the first decade of infant democracy and the rise of agitation against the despotic regime at the end of the decade, which covers the period of 2007 BS to 2017 BS through Govinda Bahadur Malla's "Chunab" and B. P. Koirala's "Ek Raat." The data on political aspects have been purposefully selected and situated against the historical document. Since this study employs the exploratory qualitative approach, primary data from the short stories are approached from the lens of new historicism, the theory of power and resistance, and the concept of state apparatus. However, literary data provide a broader horizon of historical events. The themes of election and rebellion are contrasted to discover the contrary spirits of the same decade. The results indicate that the dawn of the infant democracy is full of enthusiasm with the spirits caused by the fervor of election. In contrast, the dusk of democracy is full of despair and agitation, giving rise to resistance and rebellion. The results indicate that approaching history and historical documents through the lens of literature set simultaneously can broaden the horizon of understanding of historical events.

On the mechanism of black hole energy reduction in the Blandford-Znajek process

Abstract The Blandford-Znajek (BZ) process is steady electromagnetic energy release from rotating black holes (BHs) along magnetic field lines threading them and widely believed to drive relativistic jets. This process is successfully demonstrated in general relativistic magnetohydrodynamic (MHD) simulations with the coordinate system regular on the event horizon, in which the outward Poynting flux on the horizon is considered to reduce BH energy. Meanwhile, alternative pictures for the BH energy reduction that invoke infall of negative energy objects were also discussed, although all of the proposed definitions of the negative energy and/or its infall velocity were ambiguous. We revisit the mechanism of BH energy reduction in the BZ process under the ideal MHD condition by utilizing the coordinate system singular on the horizon, in which the falling membrane of past accreted matter should exist above the horizon. We find that the Poynting flux is produced at the boundary between the falling membrane and the magnetically-dominated inflow, and the front of the inflow creates the negative electromagnetic energy, which reduces the rotational energy of spacetime. We also clarify that the poloidal electric current does not form a closed circuit within the magnetically-dominated flow. Previous interpretations of the BZ process and possibilities of violation of ideal MHD condition and BH charging are also discussed.

Role of the irreducible mass in repetitive Penrose energy extraction processes in a Kerr black hole

The concept of the irreducible mass (Mirr) has led to the mass-energy (M) formula of a Kerr black hole (BH), in turn leading to its surface area S=16πMirr2. This also allowed the coeval identification of the reversible and irreversible transformations, soon followed by the concepts of and energy. This new conceptual framework avoids inconsistencies recently evidenced in a repetitive Penrose process. We consider repetitive decays in the ergosphere of an initially extreme Kerr BH and show the processes are highly irreversible. For each decay, the particle that the BH captures causes an increase of the irreducible mass (so the BH horizon), much larger than the extracted energy. The energy extraction process stops when the BH reaches a positive spin lower limit set by the process boundary conditions. Thus, the reaching of a final nonrotating Schwarzschild BH state through this accretion process is impossible. We have assessed such processes for selected decay radii and incoming particle with rest mass 1% of the BH initial mass M0. For r=1.2M and 1.9M, the sequence stops after 8 and 34 decays, respectively, at a spin 0.991 and 0.857, the energy extracted has been only 1.16%, and 0.42%, the extractable energy is reduced by 17% and 56%, and the irreducible mass increases by 5% and 22%, all values in units of M0. These results show the highly nonlinear change of the BH parameters, dictated by the BH mass-energy formula, and that the BH rotational energy is mainly converted into irreducible mass. Thus, evaluating the irreducible mass increase in any energy extraction processes in the Kerr BH ergosphere is mandatory. Published by the American Physical Society 2025

Traces of quantum fuzziness on the black hole shadow and particle deflection in the multi-fractional theory of gravity

Abstract In this paper, we investigate the properties of black holes within the framework of multi-fractional theories of gravity, focusing on the effects of q-derivatives and weighted derivatives. These modifications, which introduce scale-dependent spacetime geometries, alter black hole solutions in intriguing ways. Within these frameworks, we analyze two key observable phenomena - black hole shadows and particle deflection angle in the weak field limit - using both analytical techniques and observational data from the Event Horizon Telescope (EHT) for M87* and Sgr A*. The study from $q$-derivative formalism reveals that the multi-scale length $\ell_*$ influences the size of the black hole shadow in two ways, and modifies the weak deflection angle. Constraints on $\ell_*$ are derived from the EHT observations, showing significant deviations from standard Schwarzschild black hole predictions, which range from $10^{9}$ to $10^{10}$ orders of magnitude. Additionally, the weak deflection angle is computed using the non-asymptotic generalization of the Gauss-Bonnet theorem, revealing the effects of finite-distance and multi-scale parameters. Using the Sun for Solar System test, the constraints for $\ell_*$ range from $10^{8}$ to $10^{9}$ orders of magnitude. Results from the weighted derivative formalism generates a dS/AdS-like behavior, where smaller deviations are found in the strong field regime than in the weak field regime. The results suggest that while these effects are subtle, they provide a potential observational signature of quantum gravity effects. The findings presented here contribute to the broader effort of testing alternative theories of gravity through black hole observations, offering a new perspective on the quantum structure of spacetime at cosmological and astrophysical scales.

Shadow Cast by the Kerr MOG Black Hole under the Influence of Plasma and Constraints from EHT Observation

Abstract The study of black hole (BH) shadows provide crucial insights into the nature of strong gravitational effects and the intricate structure of the spacetime surrounding BHs. In this paper, we explore the shadow of Kerr MOG BH within a plasma environment, investigating how much the presence of plasma influences the characteristics of the observed shadow compared to those in vacuum conditions. Our analysis reveals that the shadow characteristics of M87* and Sgr A* are more compatible with event horizon telescope (EHT) observational data in nonhomogeneous plasma spacetime compared to homogeneous distributions. For small metric deformation parameter $\alpha$, the shadow aligns within $2\sigma$ uncertainty for homogeneous plasma and within $1\sigma$ for nonhomogeneous plasma. Next, we determine the energy emission rate for the Kerr MOG BH and analyze the influence of parameters $\alpha$, $k_o$, $k_\theta$, and $k_r$ on particle emissions in the BH vicinity. We further analyze the deflection angle in the presence of homogeneous and nonhomogeneous plasma profiles. The findings indicate notable differences from the vacuum scenario, underscoring the importance of accounting for plasma effects in studying light propagation around compact objects.

A Survey of General Relativistic Magnetohydrodynamic Models for Black Hole Accretion Systems

Abstract General relativistic magnetohydrodynamics (GRMHD) simulations are an indispensable tool in studying accretion onto compact objects. The Event Horizon Telescope (EHT) frequently uses libraries of ideal GRMHD simulations to interpret polarimetric, event-horizon-scale observations of supermassive black holes at the centers of galaxies. In this work, we present a library of 10 nonradiative, ideal GRMHD simulations that were utilized by the EHT Collaboration in their analysis of Sagittarius A*. The parameter survey explores both low (SANE) and high (MAD) magnetization states across five black hole spins a * = −15/16, −1/2, 0, +1/2, +15/16 where each simulation was run out to 30,000 GM/c −3. We find the angular momentum and energy flux in SANE simulations closely matches the thin-disk value, with minor deviations in prograde models due to fluid forces. This leads to spin equilibrium around a * ∼ 0.94, consistent with previous studies. We study the flow of conserved quantities in our simulations and find mass, angular momentum, and energy transport in SANE accretion flows to be primarily inward and fluid dominated. MAD models produce powerful jets with outflow efficiency >1 for a * = + 0.94, leading to black hole spin-down in prograde cases. We observe outward directed energy and angular momentum fluxes on the horizon, as expected for the Blandford–Znajek mechanism. MAD accretion flows are sub-Keplerian and exhibit greater variability than their SANE counterpart. They are also hotter than SANE disks within r ≲ 10 GM/c −2. This study is accompanied by a public release of simulation data at http://thz.astro.illinois.edu/.

The existence and distribution of photon spheres near spherically symmetric black holes: a geometric analysis

Photon sphere has attracted significant attention since the capture of black hole shadow images by Event Horizon Telescope. Recently, a number of studies have highlighted that the number of photon spheres and their distributions near black holes are strongly constrained by black hole properties. Specifically, for black holes with event horizons and proper asymptotic behaviors, the number of stable and unstable photon spheres satisfies the relation nstable-nunstable=-1. In this study, we provide a new proof on this relation using a geometric analysis, which is carried out using intrinsic curvatures in the optical geometry of black hole spacetimes. Firstly, we demonstrate the existence of photon spheres near black holes assuming most general asymptotic behaviors (asymptotically flat black holes, asymptotically de-Sitter and anti-de-Sitter black holes). Subsequently, we prove that the stable and unstable photon spheres near black holes must be one-to-one alternatively separated from each other, such that each unstable photon sphere is sandwiched between two stable photon spheres (and each stable photon sphere is sandwiched between two unstable photon spheres). Our analysis in this study is applicable to any spherically symmetric black hole spacetimes.

Nonstatic Reissner–Nordstrϕm metric in the perturbative f(R) theory: embedding in the background of the FLRW cosmology, uniqueness of solutions, the TOV equation

This article introduces a nonstatic Reissner–Nordstrϕm metric, a metric that does not emit electromagnetic waves but can emit gravitational waves. We first use the GR theory to study a charged spherically symmetric gravitational source (CSSGS), the obtained results are further improved in comparison with the previous studies. In particular, this article considers that the field is not necessarily static. The metric tensors gμν are considered both outside and inside the gravitational source (the results show that in the first case gμν are time independent, in the latter case they are time dependent). The gravitational acceleration and the event horizon of a charged black hole are investigated. The results prove that the gravitational field is always attractive. We then use the perturbative f(R) theory to consider CSSGS. The obtained results not only correct the solution of Einstein’s equation in magnitude (this will describe astronomical and cosmological quantities more accurately than Einstein’s equation), but also reveal new effects. Outside the gravitational source, the metric tensors can depend on time, this makes it possible for a spherically symmetric gravitational source to emit gravitational waves (Einstein’s equation cannot give this effect). However, a spherically symmetric field still does not emit electromagnetic waves. Next, we present a new method for embedding the spherically symmetric metrics of a star (or a black hole) in the background of the FLRW cosmological. Finally, we discuss the uniqueness of the solutions of the f(R) theory. The perturbative TOV equation is also found.

Spherically symmetric and static black bounces with multiple horizons, throats, and anti-throats in four dimensions

Abstract Black bounce (BB) spacetimes usually arise from the Simpson–Visser regularization method. This type of metric presents a wormhole throat inside an event horizon. In this paper, we presented new classes of BB spacetime solutions, which have multiple horizons, throats, and anti-throats. These solutions are variants of black holes and wormholes, based on modifications of the Schwarzschild and Simpson–Visser metrics. The metric function allows for multiple horizons and throats, and the asymptotic behavior recovers the Schwarzschild solution. The article considers a scalar field coupled to nonlinear electrodynamics, generating solutions with a partially phantom scalar field and a magnetic monopole. The energy conditions can be satisfied or violated depending on the region of spacetime, analyzed through an anisotropic fluid. The regularity of the spacetime is ensured by the analysis of the Kretschmann scalar.

Survey of radiative, two-temperature magnetically arrested simulations of the black hole M87* I: Turbulent electron heating

Abstract We present a set of eleven two-temperature, radiative, general relativistic magnetohydrodynamic (2TGRRMHD) simulations of the black hole M87* in the magnetically arrested (MAD) state, surveying different values of the black hole spin a*. Our 3D simulations self-consistently evolve the temperatures of separate electron and ion populations under the effects of adiabatic compression/expansion, viscous heating, Coulomb coupling, and synchrotron, bremsstrahlung, and inverse Compton radiation. We adopt a sub-grid heating prescription from gyrokinetic simulations of plasma turbulence. Our simulations have accretion rates $\dot{M}=(0.5-1.5)\times 10^{-6}\dot{M}_{\rm Edd}$ and radiative efficiencies εrad = 3 − 35 %. We compare our simulations to a fiducial set of otherwise identical single-fluid GRMHD simulations and find no significant changes in the outflow efficiency or black hole spindown parameter. Our simulations produce an effective adiabatic index for the two-temperature plasma of Γgas ≈ 1.55, larger than the Γgas = 13/9 value often adopted in single-fluid GRMHD simulations. We find moderate ion-to-electron temperature ratios in the 230 GHz emitting region of R = Ti/Te ≈ 5. While total intensity 230 GHz images from our simulations are consistent with Event Horizon Telescope (EHT) results, our images have significantly more beam-scale linear polarization (〈|m|〉 ≈ 30 %) than is observed in EHT images of M87* (〈|m|〉 < 10 %). We find a trend of the average linear polarization pitch angle ∠β2 with black hole spin consistent with what is seen in single-fluid GRMHD simulations, and we provide a simple fitting function for ∠β2(a*) motivated by the wind-up of magnetic field lines by black hole spin in the Blandford-Znajek mechanism.