Can we identify massless braneworld black holes by observations?

TL;DR

This paper explores how to observationally distinguish massless braneworld black holes from similar objects like wormholes and Schwarzschild black holes through microlensing signatures and shadow imaging, highlighting potential observational markers.

Contribution

It introduces combined analysis of microlensing and shadow images to differentiate braneworld black holes from wormholes and standard black holes, proposing a method for future observational identification.

Findings

Luminosity reduction occurs before and after amplification in microlensing.

Bright rings appear in shadow images around braneworld black holes and wormholes.

Inner intensity differences in shadows can distinguish wormholes from black holes.

Abstract

For an extension of the previous work on gravitational lensing by massless braneworld black holes, we investigate their microlensing phenomena and shadows and discuss how to distinguish them from standard Schwarzschild black holes and Ellis wormholes. Microlensing is known as the phenomenon in which luminosity amplification appears when a bright object passes behind a black hole or another massive object. We find that, for the braneworld black hole as well as for the Ellis wormhole, there appears luminosity reduction just before and after the amplification. This means that observation of such a 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. Therefore, we next analyze the optical images, or shadows of the braneworld black hole surrounded by optically thin dust, and compare them to 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, we find that only for the wormhole is the intensity inside the ring larger than that the outsider intensity. Therefore, with future high-resolution observations of microlensing and shadows together, we could identify the braneworld black holes if they exist.