Spectral lines of dirty wormholes

TL;DR

This work investigates how a surrounding thick shell of matter modifies the absorption properties of dirty wormholes constructed by grafting Schwarzschild spacetimes. It combines null geodesic analysis, embedding diagrams, and scalar-field dynamics to track how new light rings and quasibound states alter absorption spectra. The main finding is that, although the environment can introduce additional resonant modes and shift low-frequency spectral lines, the positions of most spectral lines remain tied to the central object's fingerprints, with heavy shells potentially shrinking shadows in phantom-matter cases. These results refine how exotic compact objects might be distinguished from black holes via spectroscopy, even when embedded in realistic astrophysical environments.

Abstract

Astrophysical objects like black holes are usually surrounded by matter in the form of accretion disks or jets of matter. These astrophysical scenarios are expected to introduce novel phenomenology in the scattering of particles and fields. Wormholes are viable candidates for exotic compact objects that can mimic some black hole properties. Hence, it is natural to wonder what would happen if the central astrophysical object were a wormhole, instead of a black hole. We investigate the astrophysical environment effect on the absorption of a massless scalar field by a dirty wormhole surrounded by a thick shell of matter. We study null geodesics around these dirty wormholes and analyze under which conditions new pairs of light rings can appear. The presence of new stable light rings allows new quasibound states in the spacetime, apart from the ones trapped near the throat. Thus, the astrophysical environment can introduce deviations in the absorption bands. Remarkably, although heavy and dense distributions of matter are considered surrounding the wormhole, the position of most of the spectral lines in the absorption bands is preserved, indicating that the astrophysical environment cannot hide some fingerprints of the central object.

Figures (9)

• Figure 1: Embedding diagrams of dirty wormholes. The left panel depicts a dirty wormhole surrounded by normal matter, while the right panel represents a dirty wormhole surrounded by phantom matter.
• Figure 2: Effective potential for null geodesics of some dirty wormhole configurations. The top panels of each quadrant (plots with orange-like color scheme) correspond to thick shells with positive energy density, while the bottom panels of each quadrant (plots with blue-like color scheme) depict thick shell configurations with negative energy density. For comparison we also plot the corresponding thin-shell Schwarzschild wormhole case (black dashed line).
• Figure 3: Bending of light in dirty wormhole backgrounds. The dirtiness is represented by the gray strip and we are considering two values of the thick shell mass, namely $\Delta M=2.2M$ and $\Delta M=-3M$. Both cases introduce new light rings in the spacetime. In the positive thick shell mass case, the new unstable light ring is located outside the shell, while in the negative thick shell mass case, the new unstable light ring is located inside the thick shell. The unstable light rings are represented by dot-dashed circles, including the innermost ones at $r_\pm = 3M$, inherited from the thin-shell surgery. In the top row, the throat is located at $r_0=2.002M$, while in the bottom row the throat is located at $r_0=3M$, and we represent the throat by the solid circle. The light rays colored by the blue gradient represent geodesics scattered by the dirty wormhole (geodesics that do not cross the throat), while the light rays colored by the red gradient depict curves absorbed by the wormhole (geodesics that cross the throat). After crossing the throat, we choose dashed lines to represent the curves in the inner universe.
• Figure 4: Photon orbits in the outside (${\cal M}_+$) of a wormhole surrounded by a matter shell with $\Delta M/M=-1$. Null geodesics with $b>r_s+\Delta r_s$ do not feel any gravitational attraction or repulsion, therefore propagate as straight lines. Light rays that encounter the shell (gray strip) are bent and can be scattered back to the infinity of ${\cal M}_+$ (lines colored by blue gradient) or absorbed by the wormhole throat (lines colored by red gradient). The wormhole throat and the unstable photon spheres are represented by a solid circle and a dot-dashed circle, respectively.
• Figure 5: Massless scalar field's effective potential of some dirty wormhole configurations, for different multipole numbers $\ell$. Contrasting with the geodesic effective potential, depending on the thick shell distribution, regions of negative effective potential are present for small multipole numbers.