Shadows of Wormholes: Accretion, Photon Rings, and Flickering ✦
Within the framework of GR, magnetohydrodynamic simulations of accretion onto a rotating Kerr wormhole with a throat parameter ℓ = 2.5 M were performed. Both 2D and 3D cases were considered. The accretion flow is a magnetized geometrically thick torus at one mouth; the opposite mouth is initially empty. Spin significantly affects the dynamics of matter on both sides of the wormhole due to inertial frame dragging. Based on the MHD data, ray tracing at 230 GHz (RAPTOR code) was carried out and the image structure near the horizon was studied via high-order photon trajectories. Radiation from the immediate vicinity of the throat may dominate, creating a variable signal with clear quasi-periodic modulation on light curves. These results pave the way for confirming or refuting the existence of wormholes with horizon-scale observations.
Context
Direct images of the 'shadows' of supermassive compact objects, obtained by the Event Horizon Telescope for M87 and Sagittarius A*, have opened a new era in testing theories of gravity. But what if these objects are not classical Kerr black holes? Alternative solutions to Einstein's equations, such as traversable wormholes, can mimic black holes yet leave observable traces. Studying how accreting plasma behaves in such curved spacetimes is key to deciphering the nature of supermassive central bodies in active galactic nuclei.
Methods
To find out, the scientists performed three-dimensional simulations within the framework of general relativistic magnetohydrodynamics (GRMHD) for the Kerr-like black-bounce metric, which describes a wormhole with a throat parameter ℓ = 2.5 M and spin a from 0 to 0.9. The accretion flow was initialized as a magnetized accretion torus at one mouth, while the opposite mouth was empty. Then, using the RAPTOR code, they carried out radiative transfer and constructed images that can be compared with 230 GHz observations.
Results
The simulations showed that mass accumulates near the wormhole throat due to a saddle point in the effective potential: the matter flow through mouth A (inflow) systematically exceeds the flow through mouth B (outflow). Although the global accretion structure resembles a black hole, spin noticeably influences the magnetization: at high a, the toroidal magnetic field is amplified by frame-dragging – a manifestation of spacetime curvature – which leads to a more extended and strongly magnetized outflow region. In shadow images, the behavior of higher-order photon rings was key. For moderate spin (a=0.3), the critical curve consists solely of the 'Kerr' branch, while for rapid rotation (a=0.9), an additional 'throat' branch appears, distorting the left side of the shadow – this is in good agreement with analytical calculations. Moreover, the light curves reveal quasi-periodic oscillations with a period of about ~100 GM/c³, which are linked to vertical epicyclic motions of plasma in the potential well near the throat. It is worth noting that the term 'wormhole' was coined by John Archibald Wheeler in 1957.
Implications
These results mean that by combining the analysis of fine photon ring structure with timing variability characteristics, astronomers can obtain decisive evidence for or against wormholes. Even if the overall accretion picture resembles a black hole, the throat leaves an 'imprint' in the form of bright matter accumulation at the center and a distinctive shadow geometry, governed by gravity.
Future development
Future studies should include simulations with variable throat parameters and higher resolution using numerical methods, as well as consider the magnetically arrested disk (MAD) regime, where magnetic fields are much stronger. This could alter outflow dynamics and the observable properties of wormholes.
Impact
The work directly impacts the interpretation of data from the Event Horizon Telescope and future submillimeter observations, as well as fundamental physics, as it necessitates the study of exotic plasma and spacetime geometry.
Next steps
Next steps include polarized radiative transfer and comparison with actual EHT observations, as well as searching for similar signatures in other candidates, such as the active nucleus of M87.
Key open problems
The study connects to unsolved problems in quantum gravity and the nature of singularities: wormholes offer an alternative without an event horizon, but they require violation of standard energy conditions by matter, which has not yet been confirmed in particle physics. As Wheeler noted, such objects challenge our understanding of spacetime structure.
🎯 The idea of a wormhole as a 'bridge' between two universes was first proposed by Ludwig Flamm in 1916, just a year after Einstein published his general theory of relativity. The actual term 'wormhole' came later, in 1957, coined by John Archibald Wheeler.
🎬 In the movie 'Interstellar', a wormhole is depicted as a sphere that allows instantaneous travel to another galaxy. Although the film's consultants used real Einstein equations, modern simulations show that the throat of such a wormhole would collect hot plasma, and its shadow could tell us about the geometry of spacetime.
Key numbers
- Throat parameter ℓ: 2.5 M
- Spin a: 0, 0.3, 0.9
- Period of quasi-periodic oscillations: ~100 GM/c³ (≈4 min for Sagittarius A*)
- Mass of Sagittarius A*: 4.14×10⁶ M☉
- Observation frequency: 230 GHz