The recent advancements in black hole imaging have opened a new era of probing horizon-scale physics with electromagnetic radiation. However, a feature of the observed images, a bright ring encircling a relatively dark region, has not sufficiently proved the existence of event horizons. It thus requires extreme care when studying the possibility of using such image features to examine quantum effects that may change the classical picture of black holes slightly or drastically. In this work, we investigate the image of a horizonless compact object, whose interior metric satisfies the 4D semi-classical Einstein equation non-perturbatively for the Planck constant, and whose entropy agrees with the Bekenstein-Hawking formula. Although the absence of an event horizon allows light rays to pass through the dense interior, the extremely strong redshift significantly darkens the image, making it almost identical to the classical black-hole image. In particular, if there is light emission a bit inside the surface of the object, the intensity around the inner shadow is slightly enhanced, which could be a future observable prediction to characterize the object. We also find through a phenomenological parameter that the image is further darkened due to interactions inside. Thus, the image is consistent with current observations, and the object could be a candidate for black holes in quantum theory.
Gravitational-wave Echoes Beyond the Hubble Time from Quantum Black Holes
RIKEN iTHEMS（理化学研究所 iTHEMS）；Department of Physics, Kindai University（近畿大学物理系）；School of Engineering Science, Kochi University of Technology（高知工科大学工程科学学部）
Analytic semiclassical backreaction of a Schwarzschild black hole in a finite cavity: horizon shift, temperature renormalization, and canonical stability in the Hartle-Hawking State
Centre for Theoretical Physics, The British University（英国大学理论物理中心）；Department of Mathematics and Statistics, Imam Mohammad Ibn Saud Islamic University (IMSIU)（伊马姆穆罕默德·本·沙特伊斯兰大学数学与统计系）；Faculty of Physics, Babeş-Bolyai University（巴比什-博雅依大学物理学院）；Astronomical Observatory（天文台）
Transfer observables of rotating acoustic black holes from ray tracing: shadow centroid, redshift asymmetry and flux imbalance
The Assam Royal Global University（阿萨姆皇家全球大学）；Al-Hussein Bin Talal University（侯赛因·本·塔拉勒大学）；Universidade Federal da Paraíba（帕拉伊巴联邦大学）；Universidade Federal do Maranhão（马拉尼昂联邦大学）
Testing the Spacetime Geometry of Sgr A* with the Relativistic Orbit of S2 star
Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo（圣保罗大学天文、地球物理和大气科学研究所）；P. D. Patel Institute for Applied Sciences, Charusat University（Charusat大学P.D. Patel应用科学学院）；CENTRA, Departamento de Física, Instituto Superior Técnico – IST, Universidade de Lisboa – UL（里斯本大学高等理工学院物理系CENTRA中心）
