In 2016, LIGO detected gravitational waves, which are supposed to be signals of coalescence of two black holes. In 2019, the Event Horizon Telescope (EHT) collaboration produced the first-ever image of a black hole, which lies at the center of the M87 galaxy 55 million light-years from Earth. The image showed a bright ring with a dark center, which is the black hole’s shadow. These rapid advancements in observational technologies to detect black holes will also give us a chance to discover exotic compact objects such as boson stars, gravastars, wormholes, non-Abelian black holes, and braneworld black holes.
To detect such objects, it is necessary to understand theoretical predictions for observation in advance. For this purpose, observational consequences of boson stars, gravastars, wormholes, and braneworld black holes have been studied recent years.
Among many models of braneworld black holes, massless black holes, in which the curvature is produced only by a tidal effect, are observationally important because their gravitational lensing effects are characteristic and discriminative. Now, Ohgami and colleagues studied gravitational lensing by massless braneworld black holes in more detail. Specifically, they studied their microlensing and shadows, and discussed whether we can distinguish them from standard Schwarzschild black holes and Ellis wormholes by radio or electromagnetic observations.
First, they studied defection angles of light rays that pass around those objects. Previous work showed that both deflection angles of the braneworld black hole and the Ellis wormhole are proportional to α¯2, while that of the Schwarzschild black hole to α¯1. Ohgami et al. therefore speculated that the braneworld black hole and the Ellis wormhole may exhibit similar features in microlensing phenomena.
To elucidate observational consequences of those microlensing phenomena, they calculated images of an optical source object behind a lens object for the three models and their light curves. They found that for the braneworld black hole as well as for the Ellis wormhole, luminosity reduction appears just before and after amplification. This means that, observations of such reduction would indicate the lens object is either a braneworld black hole or a wormhole, though it is difficult to distinguish one from the other by microlensing solely.
Thus, they next analyzed the optical images of the braneworld black hole surrounded by optically thin dust and compared them with those of the Ellis wormhole. Because the spacetime around the braneworld black hole possesses unstable circular orbits of photons, a bright ring appears in the image, just as in Schwarzschild spacetime or in the wormhole spacetime. This indicates that the appearance of a bright ring does not solely confirm a braneworld black hole, a Schwarzschild, nor an Ellis wormhole. However, they found that only for the wormhole the intensity inside the ring is larger than the outsider intensity. Their results mean that observations of shadows would distinguish black holes from Ellis wormholes.
They therefore concluded that, with future high-resolution very long baseline interferometry observations of microlensing and shadows together, we could identify the braneworld black holes if they exist.
Reference: M. Kuniyasu, K. Nanri, N. Sakai, T. Ohgami, R. Fukushige, S. Koumura, “Can we identify massless braneworld black holes by observations?”, Phys. Rev. D 97, 104063 – Published 29 May 2018. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.97.104063
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