New Concepts in Black Hole Physics with Cosmological Implications

In the recent decade, progress in black hole (BH) physics has been driven by landmark experimental achievements — from the first observation of gravitational waves from a BH merger in 2016 to the direct imaging of the supermassive BH in Galaxy M 87 by the Event Horizon Telescope (EHT) collaboration in 2019, followed by the image of Sagittarius A* at the center of the Milky Way in 2022. These breakthrough discoveries have established BHs as fully astrophysical objects of unprecedented cosmological significance, motivating a new wave of theoretical investigations at the interface of BH physics and cosmology.

Current theoretical research on BHs spans both classical and quantum phenomena, with a growing interest in the physics of BH interiors. Although the properties of BH interiors cannot be directly verified or tested on classical grounds, it has been recognized that these interiors constitute anisotropic cosmological models. This brings a new and compelling perspective linking BH physics with cosmological implications: the interior dynamics of a BH can serve as a laboratory for studying expanding and contracting cosmological models. In this context, recent studies of the spontaneous creation and annihilation of particle pairs inside Schwarzschild Black (and White) Holes connect naturally to the broader research program of Cosmological Gravitational Particle Production, where the role of anisotropy in the dynamics of quantized fields is of particular importance.

Another area of growing interest concerns particle collisions in the vicinity of BH horizons. The Bañados-Silk-West (BSW) effect demonstrates that the center-of-mass energy of two-particle collisions near a BH horizon can, in principle, be made unbounded. Recent outcomes, however, appear to lead to a kinematical censorship hypothesis: although these collision energies may be made arbitrarily large, they cannot be truly infinite. A deeper understanding of this kinematical bound remains an open and significant question.

Beyond interior physics, two phenomena related to the exterior of BHs merit special attention. The first is the concept of a collapse of Fermi spheres of electrons and protons in the plasma of the accretion disc near the BH horizon. The second is the recent report of a stable photon sphere, which opens intriguing questions about possible “quantized states” of photon orbits around specific BHs.

The aim of this Research Topic is to illuminate the updated state of the art in selected areas of BH physics that carry cosmological implications. Research interests include but are not limited to the following topics:

- Anisotropic Cosmological Models: the interior of BHs as a laboratory of expanding and contracting cosmological models

- Cosmological Gravitational Particle Production inside Black Holes: the role of anisotropy in the dynamics of fields quantized inside BHs

- Two-Particle Collisions in the Vicinity of BH Horizons: toward a deeper meaning behind the kinematical censorship hypothesis

- Specific Phenomena in the Exterior of Black Holes: theoretical and observational implications of (a) stabilization of a photon sphere and (b) possible collapse of a Fermi sphere

Specific theoretical and observational questions — including the effect of interior anisotropy on particle production and the energy release in particle collisions near BH horizons — are expected to receive substantial attention. The final scope and depth of the collection will naturally depend on the contributions of the participating experts.