Black holes: Beyond the singularity

(Press-News.org) “Hic sunt leones,” remarks Stefano Liberati, one of the authors of the paper and director of IFPU. The phrase refers to the hypothetical singularity predicted at the center of standard black holes — those described by solutions to Einstein’s field equations. To understand what this means, a brief historical recap is helpful.

In 1915, Einstein published his seminal work on general relativity. Just a year later, German physicist Karl Schwarzschild found an exact solution to those 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”.

From the beginning, however, problematic aspects emerged and sparked a decades-long debate. In the 1960s, it became clear that spacetime curvature becomes truly infinite at the center of a black hole: a singularity where the laws of physics — or so it seems — cease to apply. If this singularity were real, rather than just a mathematical artifact, it would imply that general relativity breaks down under extreme conditions. For much of the scientific community, invoking the term “singularity” has become a kind of white flag: it signals that we simply don’t know what happens in that region.

Despite the ongoing debate around singularities, scientific evidence for the existence of black holes has continued to grow since the 1970s, culminating in major milestones such as the 2017 and 2020 Nobel Prizes in Physics. Key moments include the first detection of gravitational waves in 2015 — revealing the merger of two black holes — and the extraordinary images captured by the Event Horizon Telescope (EHT) in 2019 and 2022. Yet none of these observations has so far provided definitive answers about the nature of singularities.

Unknowable territory

And this brings us back to the “leones” Liberati refers to: we can describe black hole physics only up to a certain distance from the center. Beyond that lies mystery — an unacceptable situation for science. This is why researchers have long been seeking a new paradigm, one in which the singularity is “healed” by quantum effects that gravity must exhibit under such extreme conditions. This naturally leads to models of black holes without singularities, like those explored in the work of Liberati and his collaborators.

One of the interesting aspects of the new paper is its collaborative origin. It is neither the work of a single research group nor a traditional review article. “It’s something more,” explains Liberati. “It emerged from a set of discussions among leading experts in the field — theorists and phenomenologists, junior and senior researchers — all brought together during a dedicated IFPU workshop. The paper is a synthesis of the ideas presented and debated in the sessions, which roughly correspond to the structure of the article itself.” According to Liberati, the added value lies in the conversation itself: “On several topics, participants had initially divergent views — and some ended the sessions with at least partially changed opinions.”

Two non-singular alternatives

During that meeting, three main black hole models were outlined: the standard black hole predicted by classical general relativity, with both a singularity and an event horizon; the regular black hole, which eliminates the singularity but retains the horizon; and the black hole mimicker, which reproduces the external features of a black hole but has neither a singularity nor an event horizon.

The paper also describes how regular black holes and mimickers might form, how they could possibly transform into one another, and, most importantly, what kind of observational tests might one day distinguish them from standard black holes.

While the observations collected so far have been groundbreaking, they don’t tell us everything. Since 2015, we’ve detected gravitational waves from black hole mergers and obtained images of the shadows of two black holes: M87* and Sagittarius A*. But these observations focus only on the outside — they provide no insight into whether a singularity lies at the center.

“But all is not lost,” says Liberati. “Regular black holes, and especially mimickers, are never exactly identical to standard black holes — not even outside the horizon. So observations that probe these regions could, indirectly, tell us something about their internal structure.”

To do so, we will need to measure subtle deviations from the predictions of Einstein’s theory, using increasingly sophisticated instruments and different observational channels. For example, in the case of mimickers, high-resolution imaging by the Event Horizon Telescope could reveal unexpected details in the light bent around these objects — such as more complex photon rings. Gravitational waves might show subtle anomalies compatible with non-classical spacetime geometries. And thermal radiation from the surface of a horizonless object — like a mimicker — could offer another promising clue.

A promising future

Current knowledge is not yet sufficient to determine exactly what kind of perturbations we should be looking for, or how strong they might be. However, significant advances in theoretical understanding and numerical simulations are expected in the coming years. These will lay the groundwork for new observational tools, designed specifically with alternative models in mind. Just as happened with gravitational waves, theory will guide observation — and then observation will refine theory, perhaps even ruling out certain hypotheses.

This line of research holds enormous promise: it could help lead to the development of a quantum theory of gravity, a bridge between general relativity — which describes the universe on large scales — and quantum mechanics, which governs the subatomic world.

“What lies ahead for gravity research,” concludes Liberati, “is a truly exciting time. We are entering an era where a vast and unexplored landscape is opening up before us.”


 

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