History
In 1915, Einstein published his seminal work on general relativity. Also known as the general theory of relativity and Einstein’s theory of gravity; considered the fundamental description of gravitation in modern physics. General relativity refines Newton’s law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or four-dimensional spacetime. Only a year later in 1916, German physicist Karl Schwarzschild devised an exact solution to Einstein’s equations, which implied the existence of extreme objects now known as black holes. These are objects with mass so concentrated that nothing—not even light—can escape their gravitational pull, hence the term “black”.
A decades-long debate subsequently ensued. In the 1960s, it became clear that spacetime curvature becomes truly infinite at the centre of a black hole; forming a “singularity” where the laws of physics would seemingly cease to apply. If this singularity were tangible, rather than just a mathematical artifact, it would imply that general relativity breaks down under these extreme conditions.
Despite the ongoing debate around singularities, scientific evidence for the existence of black holes continued to grow. Finally culminating in a number of milestone discoveries that, deservedly, landed their investigators with a Physics Nobel Prize. Notably, in 2017 Rainer Weiss (MIT), Barry Barish and Kip Thorne (Caltech) for their contributions to the Laser Interferometer Gravitational Wave Observatory (LIGO) and observations of gravitational waves. LIGO uses lasers to measure incredibly minute changes in distance caused by passing gravitational waves. These waves are ripples in spacetime produced by massive cosmic events like merging black holes or neutron stars. Then in 2020, Roger Penrose (Oxford), Reinhard Genzel (Max Planck Institute for Extra-terrestrial Physics, University of California, Berkeley) and Andrea M. Ghez (University of California, Los Angeles) jointly won the Nobel Prize “for the discovery that black hole formation is a robust prediction of the general theory of relativity.” and “for the discovery of a supermassive compact object at the center of our galaxy.”, respectively. Together, their discoveries were ground-breaking, proving black holes can exist within the framework of Einstein’s theory of relativity and that one does exist at the centre of the Milky Way.
Other key moments include the extraordinary images captured by the Event Horizon Telescope (EHT) in 2022. Displaying some of the clearest images of a supermassive black hole in our own galaxy labelled Sagittarius A. However these observations have so far not provided definitive answers about the nature of singularities.
Towards a Non-singular Paradigm of Black Hole Physics
A new paper published in the Journal of Cosmology and Astroparticle Physics, the outcome of work carried out at the Institute for Fundamental Physics of the Universe (IFPU) in Trieste, Italy. It must be noted that aspects of the paper is a truly collaborative effort. It is neither the work of a single research group nor a traditional review article. It emerged from a set of discussions among leading experts, all brought together during a dedicated IFPU workshop. The paper is a synthesis of the ideas presented and debated in those sessions. It describes two alternative models, proposes observational tests and explores how further research could also contribute to the development of a theory of quantum gravity.
Researchers have long been seeking a new paradigm, one in which a solution to the singularity problem is solved by quantum effects that gravity must exhibit under such extreme conditions. Carballo-Rubio et al., 2025 explored the notion of models of black holes without singularities. The study begins by outlining three main black hole models. The first being the current “standard” black hole predicted by classical general relativity, with both a singularity and an event horizon. A “regular” black hole, which eliminates the singularity but retains the horizon. Thirdly, the black hole “mimicker”, which reproduces the external properties of a black hole but has neither a singularity nor an event horizon.
The paper goes on to theorise how regular and mimicker black holes might form, how they could possibly transform into one another and what kind of observational tests might one day distinguish them from standard black holes.
To date, observations have been ground-breaking, however questions still remain unanswered. With assistance from researchers and institutions such as the LIGO lab and EHT, gravitational waves from black hole mergers have been documented and images of the shadows of two black holes (M87 and Sagittarius A) have also been collected. However these observations focus only on the exterior, they provide no insight into the presence of singularities at the centre of this astronomical phenomenon.
On the positive side, regular and mimicker black holes are never exactly identical to standard black holes. Even outside the horizon, minute differences can distinguish between them. Therefore, observations that probe these regions could also tell us something about their internal structure. Through the use of increasingly sophisticated instruments and as yet familiar observational techniques the ability to measure subtle deviations from the predictions of Einstein’s theory may very well provide this insight. For instance, high-resolution imaging of mimickers by the Event Horizon Telescope could reveal unexpected details in the light bent around these objects.
Other potential observational channels may include data from gravitational waves; offering subtle anomalies compatible with non-classical spacetime geometries. Or thermal radiation from the surface of a horizonless objects i.e. a mimicker, could provide promising evidence relating to the construction of our universe.
The process of discovery seemingly follows it’s own intrinsic formula. Theory guides observation, observation refines theory; ultimately hypotheses are ruled out and thereby lead to discovery. Significant advances in theoretical understanding and numerical simulations are to be expected of black hole physics; laying groundwork for new observational tools with the aforementioned alternative models in mind. Despite the current lack of knowledge, this line of research holds enormous potential in the development of a quantum theory of gravity. A long standing goal of physicists that bridges the gap between two pillars of modern physics: General Relativity and Quantum Mechanics. Often referred to as the “Theory of Everything”; an all encompassing theory to explain everything from the smallest subatomic particle to the largest structures in the known universe.
Full Article
Towards a Non-singular Paradigm of Black Hole Physics
Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser, Julio Arrechea, Carlos Barceló, Alfio Bonanno, Johanna Borissova, Valentin Boyanov, Vitor Cardoso, Francesco Del Porro, Astrid Eichhorn, Daniel Jampolski, Prado Martín-Moruno, Jacopo Mazza, Tyler McMaken, Antonio Panassiti, Paolo Pani, Alessia Platania, Luciano Rezzolla, Vania Vellucci
