Accretion dynamics of thin and thick disks in charged Kalb-Ramond black holes

In the vast, dark theater of the cosmos, black holes are the ultimate anchors, warping space and time so severely that not even light can escape their grasp. For decades, our best map of this extreme gravity has been Einstein's theory of General Relativity, which has passed every test thrown at it, from the ripples of colliding stars to the first direct images of a black hole's shadow. Yet, scientists know this map is incomplete. It cannot explain what happens at the very center of a black hole, where the laws of physics seem to break down, nor can it easily unite with the rules of the quantum world. To fix this, researchers explore "modified" theories of gravity, asking what would happen if the fundamental symmetry of our universe—Lorentz invariance, which ensures that the laws of physics look the same regardless of how you are moving or oriented—were slightly broken. One such theory involves a hidden field called the Kalb-Ramond field, which, if it exists, would leave a subtle fingerprint on the fabric of spacetime.

A team of physicists recently set out to see if this hidden field could be detected by watching how matter swirls around a black hole. They focused on a specific, electrically charged black hole solution within this modified gravity framework. Their goal was twofold: first, to see if this strange black hole could still look like the ones we have already photographed, and second, to predict how the superheated gas falling into such a black hole would behave differently than in standard Einsteinian gravity. By comparing their theoretical models with the actual images captured by the Event Horizon Telescope of the black hole at the center of our galaxy, Sagittarius A*, they were able to narrow down the possible properties of this hidden field and reveal how it would change the glow of the accretion disk, the ring of superheated matter that feeds the black hole.

The researchers began by testing whether their theoretical black hole could mimic the real thing. They calculated the size of the "shadow" a black hole would cast against the bright background of surrounding gas, a feature that depends heavily on the black hole's mass, its electric charge, and the strength of the Lorentz symmetry-breaking effect. In the standard, uncharged version of this theory, the allowed range for the symmetry-breaking parameter was already quite tight. However, the team discovered that adding an electric charge to the mix created a kind of cosmic camouflage. The charge and the symmetry-breaking effect could balance each other out, allowing a much wider range of values for the hidden field to still produce a shadow size that matches the observations of Sagittarius A*. This finding expanded the possible values for the symmetry-breaking parameter significantly, suggesting that if this field exists, it could be stronger or weaker than previously thought, and that some of these configurations might even exist without a traditional event horizon, appearing instead as a naked singularity.

With these new constraints in place, the team turned their attention to the accretion disk itself, modeling two different types of gas flows: a thin, flat disk and a thick, donut-shaped torus. For the thin disk, they calculated how much energy the gas would radiate as it spiraled inward, heating up due to friction and gravity. They found that the presence of the symmetry-breaking field acts like a dimmer switch on the black hole's brightness. When the field has a positive value, the disk becomes hotter and brighter, emitting more energy, particularly in the high-frequency ultraviolet range. When the value is negative, the disk cools down and dims. This effect is amplified when the black hole is also electrically charged, making the combined influence of charge and the hidden field a powerful driver of the disk's luminosity. The efficiency with which the black hole converts falling matter into light also changes, with positive values of the field making the conversion more efficient.

The story becomes even more complex when looking at the thick, donut-shaped disks, which are often found in the early stages of black hole feeding or in systems where the gas is too hot to flatten out. Here, the researchers found that the symmetry-breaking field reshapes the very geometry of the gas cloud. It shifts the critical orbits where the gas can stably circle the black hole, effectively compressing the region where a stable, confined disk can exist. In the standard Einsteinian model, there is a specific range of angular momentum that allows the gas to form a stable, self-contained torus with a distinct inner edge and outer boundary. The presence of the Kalb-Ramond field narrows this safe zone. If the gas falls outside this new, tighter range, it cannot hold together; instead of forming a neat donut, the gas becomes unconfined, spreading out into open, unstable configurations that can escape into space or plunge directly into the black hole.

Ultimately, the study reveals that the hidden Kalb-Ramond field is not just a minor tweak to the laws of gravity but a force capable of fundamentally altering the appearance and behavior of black hole accretion flows. It changes how bright the disk shines, how hot it gets, and even the shape of the gas cloud itself. While the shadow of the black hole might look similar to what we expect from standard physics, the light emitted by the swirling gas carries a distinct signature of this symmetry breaking. The researchers suggest that by carefully observing the spectrum of light from these disks—specifically looking for shifts in the high-frequency emissions and the precise structure of the gas flow—astronomers may one day be able to distinguish between a standard black hole and one influenced by this exotic field. This work does not prove that the field exists, but it provides a clear roadmap for how to look for it, turning the violent, swirling chaos around a black hole into a laboratory for testing the deepest laws of the universe.