• New models challenge the singularity concept in black holes, proposing 'regular black holes' and 'black hole mimickers' as alternatives.
• These models suggest observable differences in photon rings, gravitational waves, and thermal emissions compared to standard black holes.
• The research aims to bridge general relativity and quantum mechanics by resolving the singularity problem.

The standard model of black holes includes a singularity, a point of infinite density where the laws of physics break down. This has led researchers to explore alternative models: regular black holes, which replace the singularity with an inner horizon or wormhole throat, and black hole mimickers, which lack an event horizon altogether. These models, born from theoretical discussions and workshops, could have observable differences from standard black holes, offering a path to link general relativity and quantum mechanics.

• What: Researchers are proposing and exploring theoretical models of black holes that do not contain a singularity, a point of infinite density, including regular black holes and black hole mimickers.
• Who: The Event Horizon Telescope (EHT) collaboration, LIGO/Virgo/KAGRA collaborations, Stefano Liberati (SISSA), Karl Schwarzschild, Albert Einstein, researchers at an IFPU workshop.
• When: The origin of the mystery dates back to Einstein's general relativity published in 1915, and Schwarzschild's solution in 1916. Gravitational waves detected from 2015 onwards. EHT images captured in 2019 and 2022. A recent paper has been accepted by the Journal of Cosmology and Astroparticle Physics.
• Where: Theoretical research and observations are conducted globally, with data coming from various telescopes and detectors.

The exploration of regular black holes and black hole mimickers represents a significant shift in how physicists understand these enigmatic objects. The singularity, a cornerstone of the standard black hole model, poses a fundamental problem by suggesting the breakdown of known physical laws. By proposing singularity-free alternatives, researchers aim to reconcile general relativity with quantum mechanics, potentially unlocking new insights into the nature of spacetime. The success of this endeavor hinges on the ability to identify and interpret subtle observational signatures that distinguish these models from standard black holes. Upcoming high-resolution imaging and gravitational wave measurements will play a crucial role in testing these theories.

Regular black holes, and especially mimickers, are never exactly identical to standard black holes... even outside the horizon.

We are entering an era where a vast and unexplored landscape is opening up before us.

The challenge to the black hole singularity represents a fascinating frontier in modern physics. By proposing alternative models like regular black holes and mimickers, scientists are pushing the boundaries of our understanding of gravity and spacetime. While definitive evidence remains elusive, the potential for future observations to distinguish between these models offers a tantalizing glimpse into a new era of black hole research, one that could ultimately bridge the gap between general relativity and quantum mechanics.