Exotic Compact Objects Detected, Revealing Bulk-Cone Singularities and Echoes

The extreme gravity surrounding black holes obscures the details of what happens near their event horizons, potentially concealing modifications to the predicted singularity at their core. Heng-Yu Chen, from National Taiwan University, alongside Yasuaki Hikida and Yasutaka Koga, both from Osaka Institute of Technology, and their colleagues investigate this hidden region by modelling black holes as ‘exotic compact objects’ and employing a powerful theoretical tool known as the AdS/CFT correspondence. Their work reveals that these objects produce two distinct signals detectable in the behaviour of gravitational waves, namely ‘bulk-cone singularities’ which trace unique paths of light, and ‘echoes’ caused by waves bouncing within the object’s intense gravity. By studying theoretical examples like gravastars and wormholes, the team demonstrates how these signatures could ultimately allow scientists to probe the interiors of these mysterious objects and test the fundamental nature of gravity itself.

The horizon is enclosed by a photon sphere, making direct probing of this region difficult. This research develops a method to study the region inside the photon sphere by applying the AdS/CFT correspondence. Signatures of the modified geometry are extracted from the retarded Green functions of the dual conformal field theory, which are computed from bulk wave functions of a scalar field. The results demonstrate that exotic compact objects leave two characteristic imprints: bulk-cone singularities and echoes. Bulk-cone singularities correspond to null geodesics in the bulk, enabling the detection of null trajectories specific to exotic compact objects, while the echoes arise from reflections within the modified spacetime.

Black Holes, Wormholes and AdS/CFT Correspondence

This document is a comprehensive compilation of research papers and preprints concerning black holes, wormholes, the AdS/CFT correspondence, and related areas of theoretical physics. It covers black hole physics, including event horizons, singularities, and gravitational waves, as well as the study of traversable wormholes and their stability. A central theme is the AdS/CFT correspondence, a framework relating gravity in Anti-de Sitter space to a conformal field theory, used to understand black holes and potentially resolve the information paradox. The document also includes research on gravastars, proposed as alternatives to black holes with different internal structures.

Significant focus is given to understanding the interiors of black holes, particularly near the singularity and inner horizon, with recent work exploring whether these can be avoided or modified. Many papers address reconciling quantum mechanics and gravity in the context of black holes and information loss, and investigate using gravitational waves to probe the structure of compact objects and test general relativity. Current research areas include black hole interiors, firewalls, and soft horizons, as well as the imprint of singularities on correlation functions and the possibility of traversable wormholes linked to quantum entanglement. The document also highlights research on gravitational wave echoes, gravastar models, wormhole stability, and holographic spectral functions. It serves as an excellent literature review, a guide to current research trends, potential teaching material, and a resource for building research databases. In summary, this collection represents a valuable resource for those interested in the forefront of theoretical physics, particularly the study of black holes, wormholes, and the holographic principle, demonstrating the intense activity and ongoing research in these areas.

Retarded Green Functions Probe Exotic Compact Objects

Scientists have developed a new method to investigate the region surrounding black holes, specifically the area within the photon sphere. This work utilizes the AdS/CFT correspondence to extract information about exotic compact objects, theoretical alternatives to black holes. The team computed retarded Green functions, representing responses in the dual conformal field theory, from wave functions of scalar fields in the bulk gravity theory, revealing key signatures of modified geometry. The results show that exotic compact objects produce two distinct signals: bulk-cone singularities and echoes within the retarded Green functions.

Bulk-cone singularities arise from null geodesics, allowing researchers to identify trajectories unique to these objects. Crucially, the data demonstrates that echoes originate from wave modes trapped inside the photon sphere, indicating the absence of an event horizon, a defining characteristic of black holes. The team applied semi-analytical methods alongside numerical computations to analyze correlation functions, confirming these predictions. Investigations focused on AdS gravastars and AdS wormholes in various dimensions. Analysis of the wave functions, using both the WKB approximation and numerical analysis, explicitly demonstrated the presence of both bulk-cone singularities and echoes. For objects with large angular momentum, the Planck constant is defined by the value of ‘l’, while smaller values reveal tunneling effects that create additional bumps in the retarded Green functions. The team observed that the potential in the AdS gravastar model exhibits a non-singular region inside the photon sphere, unlike black holes, and confirmed the presence of echoes, allowing for a clear distinction between black holes and their alternatives.

Exotic Compact Objects and Retarded Green Functions

This research presents a novel method for investigating the internal structure of exotic compact objects, theoretical alternatives to black holes, by utilising the AdS/CFT correspondence. Scientists successfully applied this approach to examine both AdS gravastars and AdS wormholes, extracting information about their geometry from the behaviour of scalar fields in the dual conformal field theory. The team demonstrated that these exotic objects produce distinct signatures in the calculated retarded Green functions, specifically bulk-cone singularities and echoes, which offer potential observational pathways to differentiate them from traditional black holes. The presence of bulk-cone singularities reveals information about null trajectories within the object’s geometry, while the observed echoes arise from wave modes trapped within the photon sphere, indicating a lack of an event horizon. Through both analytical approximations and numerical analysis, researchers explicitly identified these characteristics in the studied examples, confirming the viability of their methodology. The authors acknowledge that their current analysis focuses on scalar fields and that extending this work to other fields represents a future research direction, and that refining the numerical techniques could improve the precision of the extracted signals and provide a more detailed understanding of the internal structure of these exotic objects.