Many compact gravitational objects in the cosmos such as black holes, naked singularities, and wormholes, can only be detected by their shadows’ signatures. Distinguishing their different natures through their shadows is a difficult task because many times their shadows are very similar. Therefore, we can’t rely exclusively on this information to discern unambiguously the specific spacetime geometries from the objects.
For instance, the radio images obtained from the Event Horizon Telescope to directly observe the accretion disks of the supermassive black holes in the galactic centers, are hard to interpret since the information about their gravitational field is coupled non-linearly to the magneto-hydrodynamics of the system.
The Role of Polarization in Identifying Compact Objects
The main feature analyzed in such images, is their intensity profile. An interesting approach to constrain further the space time geometry, is to consider the polarization of the electromagnetic radiation emanating from the accretion disk as well. The polarization gives information about the structure of the magnetic field in the strong-gravity region, and therefore it serves to probe the interaction of the local magnetic field with the spacetime geometry.
A recent survey of the observable polarization from the radio source M87* black hole, published by the Event Horizon Telescope Collaboration, revealed a linearly polarized emission on event-horizon scales which is thought to be produced by synchrotron radiation [1]. Synchrotron radiation is the electromagnetic radiation emitted when charged particles travel in curved paths. The radiation produced in this way has a characteristic polarization and the frequencies generated can range over a large portion of the electromagnetic spectrum.
By means of astrophysical models and their numerical simulations, the collected observational data provides a great opportunity to understand the signature and mechanisms in M87*.
Astrophysical jet outflows of ionised matter emitted as an extended beam along the axis of rotation in M87*. The blue light from the jet emerging from the bright active galactic nuclei AGN core, towards the lower right, is due to synchrotron radiation. NASA, The Hubble Heritage Team (STScI/AURA) – HubbleSite.
Comparing Polarization Patterns: Schwarzschild Black Hole vs. Wormhole
Employing the simplified model of a magnetized fluid ring which orbits in the equatorial plane and emits synchrotron radiation, a research team from Sofia University, in Bulgaria, simulated the observable polarization in wormhole geometries for a range of physical parameters and compared it with the Schwarzschild black hole case. To be able to reproduce the observed polarization of M87*, they focused mainly on equatorial magnetic fields, which were assumed constant in magnitude.
This model manages to reproduce the basic features of the observable polarization, using few parameters: the radius of the ring and its tilt with respect to the observer, the velocity of the fluid in the local rest frame and the magnitude and the direction of the magnetic field. With this model, researchers explored the polarization signatures in the spacetime of horizonless compact objects, looking as well for qualitatively new features of the polarized images which could distinguish observationally the exotic compact objects by means of these polarization measurements.
They probed as well how sensitive is the observable polarization to the space-time geometry and how effectively they can use its structure to determine the physical nature of the compact objects at the galactic centers. Their research aims at knowing whether the non-black hole nature of spacetime would leave imprints on the properties of the polarization, as they would like to isolate effects which can be attributed primarily to the absence of an event horizon by selecting geometries possessing similar structure of the circular geodesics as the Schwarzschild black hole.
In their work, authors describe the physical model of the linear polarization, which results from synchrotron radiation propagating in curved spacetime and the computational procedure for obtaining its observable image, presenting the simulated images for the linear polarization in wormhole geometry and discussing their properties in comparison to the Schwarzschild black hole. They considered direct images at different inclination angles, as well as the strongly lensed indirect images.
By performing a range of simulations in static wormhole spacetime for various inclination angles and magnetic field directions, authors investigated how the polarization properties of the fluid ring model are influenced by the spacetime geometry, concluding that the direct gravitational lensing around wormholes can lead to a similar polarization picture as for black holes for small inclination angles, as shown in the figure below. The direct equatorial emission in wormhole spacetimes can reproduce the polarization data for M87* in a comparable way as the corresponding model in the Schwarzschild spacetime.
Fig 1: (taken from preprint): Polarization in equatorial magnetic field for wormholes with different redshift parameter α. Each color represents the observable polarization of the orbits located at r = 6M (outer ring) and r = 4.5M (inner ring) for a particular wormhole solution with redshift parameter α ∈ [0, 3]. The polarization for the Schwarzschild black hole is given by a black dotted line as a reference. The inclination angle is θ = 20◦.
To assess quantitatively the variation in the polarization picture for the two types of compact objects, the deviation of the polarization intensity and its direction were analyzed in depth. Based on the analysis of a certain class of wormhole geometries, authors of the study conclude that at small inclination angles it could be difficult to distinguish wormhole from black hole spacetimes by their direct polarized images. Strongly lensed indirect images provide more reliable probes of the underlying spacetime, as well as characteristic effects such as the detection of the polarized radiation from the region across the wormhole throat.
The results provided by Nedkova et al. suggest that if wormholes exist, their signatures are very similar to black holes in a range of angles, raising the possibility we’ve seen examples of this mind-blowing phenomenon without knowing it. Maybe some of the signatures that we have detected and that we thought belonged to Black holes, could actually be from wormholes.
The researchers think that it should be possible to distingush wormholes from black holes by noting subtle differences in their polarization patterns, intensities and also in their radii.
