Brighter than a shadow: modeling radiation from an Ellis-Bronnikov wormhole and its resemblance to a black hole ✦
Using general relativistic radiation transfer (GRRT) simulations, differences in the observational signatures of an Ellis-Bronnikov wormhole and a Schwarzschild black hole were studied. A spherically symmetric steady accretion flow with synchrotron radiation was assumed. In both cases, images contain a central shadow and a bright photon ring. The wormhole's shadow and ring were found to be significantly brighter, explained by the lack of an event horizon: radiation from matter on the far side of the throat contributes additional observed intensity. Comparison with Event Horizon Telescope (EHT) data for M87* shows that both objects are in reasonable agreement with current observations.
Context
Direct images of shadows of supermassive compact objects from the Event Horizon Telescope have become a new tool for testing gravity in the strong-field regime. Although the data point toward a Kerr black hole, there is still room for exotic alternatives like wormholes — horizonless objects, first named by John Wheeler, connecting distant regions of curved spacetime. Identifying their observational signatures is essential for interpreting future, more precise observations.
Methods
The authors constructed a stationary spherically symmetric solution for the accretion flow in the Ellis-Bronnikov wormhole and Schwarzschild black hole metrics, using a polytropic equation of state. With the CARTOON code, they solved the general relativistic radiative transfer problem including synchrotron radiation and relativistic redshift and Doppler shift effects. The parameters of the accreting plasma and dust (electron density, temperature) were tuned so that the total flux at 230 GHz was ~0.5 Jy, consistent with Event Horizon Telescope observations of the galaxy M87*.
Results
Images of both objects show a central dark region — the shadow — and a bright photon ring formed by photons that circle the compact object multiple times. For the same mass, the wormhole shadow is brighter: radiation passes through the throat and adds from matter on the far side. The photon ring is also brighter due to a longer optical path and less gravitational redshift in the absence of a horizon. Quantitatively, the central depression (ratio of maximum to minimum intensity) for the wormhole was ~2.8, versus ~1.9 for the black hole, and the ring diameter was 45 microarcseconds compared to 42 microarcseconds.
Implications
The results show that horizonless compact objects can successfully mimic black holes given current observational precision. This highlights the need to develop diagnostics sensitive to the event horizon, for example through flux variability or polarization. Moreover, the similarity to active galactic nuclei and quasars leaves open the question of the true nature of central engines.
Future development
Future space-based very-long-baseline interferometry missions like the Black Hole Explorer (BHEX) will probe the fine structure of photon rings and possibly detect radiation that has passed through the wormhole. Improving angular resolution down to a few microarcseconds will be decisive for distinguishing models with and without a horizon.
Impact
The work is directly relevant to black-hole astrophysics, gravitational lensing, testing modified theories of gravity, and interpreting photometric data from active galactic nuclei.
Next steps
Numerical experiments incorporating accretion disks and magnetohydrodynamic effects are needed to account for image asymmetries. Polarimetric observations, which are more sensitive to the magnetic field geometry near the horizon, also hold promise.
Key open problems
The study touches on a fundamental question: are the supermassive compact objects at the centers of galaxies black holes of general relativity, or exotic structures that require violating energy conditions and thus new physics beyond the Standard Model?
🎯 The Ellis-Bronnikov wormhole was discovered independently by H. Ellis and K. Bronnikov in 1973 as a solution with a phantom scalar field. Interestingly, its mass in the other asymptotic region is negative, and the throat is located at R=0.
🎬 The idea of wormholes as portals to other universes is widely known from the film Interstellar, where characters travel through a wormhole created by a super-advanced civilization. Although the Ellis-Bronnikov model is unstable without additional fields, it illustrates the real scientific basis for such hypothetical objects.
Key numbers
- Total flux: 0.5 Jy
- M87* ring diameter: 42 μas
- EB shadow diameter: ~45 μas
- Photon ring radius: 6.21M
- Central depression: 2.8