arXiv subjects

Marcos Riojas

Publications and source records attributed to Marcos Riojas.

13 recordsLinked to original sources

arXiv2026

On Black Holes Surrounded by Radiation: I. Classical Considerations

We consider spherically symmetric static solutions to Einstein's equations describing a Schwarzschild black hole enveloped by a thick shell of orbiting massless particles with zero radial pressure. The orbiting gas is ultra-compact and ultra-relativistic, and can be viewed as the marginally stable limit of a stable Einstein cluster. These solutions, which we refer to as "hillingar black holes", extend the photon sphere into a region of arbitrary depth. We compare these objects to black holes surrounded by other gases and note they have numerous special properties at the classical level; in particular, they appear optically indistinguishable from ordinary black holes to observers at infinity. We speculate concerning the possibility that these objects (or others much like them) might exist in nature, and whether they might be observable despite their similar outward appearance to ordinary black holes. We examine their thermodynamics and stability in companion papers.

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arXiv2026

On Black Holes Surrounded by Radiation II: Thermodynamics

In a companion paper we considered a Schwarzschild black hole of mass $m$ enveloped by a thick "ocean" of massless particles that extends the black hole's photon sphere into a region of finite depth. There we showed that this "hillingar black hole", of ADM mass $M$, optically mimics an ordinary black hole of the same mass. Here we find it also mimics the black hole thermodynamically: the formal assumption of thermal equilibrium implies the system has the same temperature and entropy as an ordinary black hole of mass $M$. We check this result carefully using multiple methods; a further method and indications of metastability are given by one of us in a companion paper. In AdS space, the mimicry does not hold, and the solutions have a richer structure. While it is far from clear that these systems are models for more realistic ones, we note possible connections with black hole evolution. In particular, assuming thermal equilibrium can be established and maintained, an HBH in a cavity of radius $\geq 3M$ can evaporate, potentially posing the information puzzle in a small finite volume.

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arXiv2026

As Cold as a Black Hole: Extended Photon Spheres

It is widely believed that self-gravitating radiation cannot reach thermal equilibrium with a black hole in asymptotically flat spacetime. We describe a marginally stable continuum exception to the standard instability argument. Our main observation is that the photon sphere controls the Israel junction conditions (IJCs), the Tolman-Oppenheimer-Volkoff (TOV) equation, and finite-radius black hole thermodynamics: the IJCs and TOV equation are equivalent at zero radial pressure, the inverse specific heat at the photon sphere is proportional to $-\Lambda$, and adding shells at fixed total mass \textit{lowers} the asymptotic Hawking temperature if and only if the local specific heat is positive. Using these results, we show how to compute thermodynamic entropies without the Euclidean path integral. The exception described here results from companion work with M.J. Strassler, where we found that a "hillingar black hole" (HBH) mimics an ordinary Schwarzschild black hole of mass $M$, sharing its Hawking temperature, photon ring, and, in equilibrium, its coarse-grained entropy $S = 4 \pi M^2$. Here, we show these features are not tuned; they follow uniquely from joint mechanical and thermodynamic constraints. Conditions for thermodynamic mimicry and for the formation of extended photon spheres are found; for self-similar solutions they coincide. A one-parameter family of self-similar systems -- all of which, excepting the HBH, require massless walls at the edges of their extended photon spheres -- satisfies both conditions. This family includes "stiffest stars" and "frozen stars".

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arXiv2026

Seeing Page Curves and Islands with Blinders On

This paper summarizes recent discussions of the Page curve and the information paradox, and responds to the reasoning and examples from arXiv:2506.04311. We review arguments demonstrating that in quantum gravity the algebra of observables at infinity is complete, both in AdS and in asymptotically flat space. This completeness implies that the bulk Hilbert space in quantum gravity does not factorize along the radial direction, undermining a key common assumption in Hawking's argument for information loss and in initial derivations of the Page curve. As a consequence, in a standard theory of gravity, information does not ``emerge'' from a black hole in the manner suggested by the Page curve; rather, it is already encoded in asymptotic observables. Relatedly, the full black hole interior, and not just an ``island'', can be reconstructed from exterior data. Page curves and islands can be obtained by removing the Hamiltonian from the exterior algebra. This may be implemented operationally by restricting access to part of the asymptotic region (a detector with a ``blind spot'') or, in the special case of null infinity in asymptotically flat spacetimes, by formally discarding the Hamiltonian from the set of observables despite its physical accessibility. Such Page curves describe only the redistribution of information between measured and unmeasured degrees of freedom, rather than fundamental information recovery. Finally, Page curves and islands also arise when a black hole is coupled to a nongravitational bath, a setup that yields a nonstandard theory of gravity. We show how, even in this setting, the unusual localization of information in gravity provides a concrete physical mechanism for information transfer from the gravitational system into the bath.

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arXiv2024

Branes Screening Quarks and Defect Operators

Here we generalize a well-known computation and uncover a phase-transition, showing that Wilson lines do not necessarily exhibit Coulomb scaling laws in AdS/BCFT at zero temperature. The area difference between a surface that returns to the boundary, and one that plunges into the bulk, determines the potential between two quarks. This classic AdS/CFT calculation is naturally extended to Wilson surfaces associated to general p-form symmetries in boundary conformal field theories (BCFTs) by embedding a Karch-Randall (KR) brane in the geometry. We find (generalized) Coulomb law scaling in subregion size $\Gamma$ is recovered only above the critical angle for the brane, $\theta_{c,p}$. The potential between the two quarks (or defect operators) vanishes precisely when the surface connecting them ceases to exist at $\theta_{c,p}$. This screening effect, where the operators are fully screened below the critical angle, is a phase transition from Coulomb law to perimeter law with the brane angle $\theta_b$ acting as an order parameter. This effect is also explored at finite temperature where we introduce a new regularization procedure to obtain closed-form results.

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arXiv2023

The Photon Sphere and Response Functions in Holography

In this letter, we show the Unruh temperature of the photon sphere for an AdS$_4$-Schwarzschild black hole can be determined holographically from the retarded Green's function and is proportional to its circumference according to a boundary observer. We then sharpen the conjecture that the photon sphere, as seen by a boundary observer, is the spatial Fourier transform of the response function. The conjecture is found to be in excellent agreement with our numerics after certain long-lived excitations -- associated with geodesics traveling between boundary points -- are removed from the response, which is then controlled by the short-lived excitations of the AdS black hole.

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arXiv2023

The Photon Sphere and the AdS/CFT Correspondence

The AdS/CFT correspondence connects bulk fields $\phi$ to boundary operators $\mathcal{O}$ characterized by source frequency $\omega$ and angular momentum $l$. Here we explore their connection to massless particles with an impact parameter $b=\omega/l$. In the AdS Schwarzschild spacetime, these particles follow unstable orbits around the photon sphere -- with Lyapunov exponent $\lambda$ -- when $b$ is near a critical value. The behavior of the bulk field is obtained numerically and then studied using an analytic approach, which leads to a precise approximate formula for the amplitude of the bulk field $\phi$. This gives the correct qualitative behavior for the system, with the amplitude of the field taking the shape of an arrowhead in tortoise coordinates. The field behaves analogously to the massless particles, and the amplitude of $\phi$ diverges at the critical impact parameter when the source frequency takes the value $\omega \approx \lambda l$, where $\lambda$ is the Lyapunov exponent of the null geodesics. We find this transition occurs when $b = \lambda$. We show this is precisely when the first QNM becomes available, and obtain an approximate formula for the first few overtones.

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arXiv2023

Constraining braneworlds with entanglement entropy

We propose swampland criteria for braneworlds viewed as effective field theories of defects coupled to semiclassical gravity. We do this by exploiting their holographic interpretation. We focus on general features of entanglement entropies and their holographic calculations. Entropies have to be positive. Furthermore, causality imposes certain constraints on the surfaces that are used holographically to compute them, most notably a property known as causal wedge inclusion. As a test case, we explicitly constrain the Dvali--Gabadadze--Porrati term as a second-order-in-derivatives correction to the Randall--Sundrum action. We conclude by discussing the implications of these criteria for the question on whether entanglement islands in theories with massless gravitons are possible in Karch--Randall braneworlds.

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arXiv2023

Subregion Entropy for the Doubly Holographic Global Black String

We study the growth of entanglement entropy in a doubly holographic model of gravity for a spherical AdS black hole. Compared to previous work, which was limited to the case of planar black holes, this introduces an extra scale to the problem. This allows us to analyze the interplay between the reorganization of entanglement entropy due to island formation and the onset of the Hawking-Page phase transition and to find the appearance of a new critical black hole radius unrelated to the thermodynamics. We also find that the geometry of the Ryu-Takayanagi surface capturing the physics of islands exhibits drastically different behavior than in the planar case.

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arXiv2022

Jackiw-Teitelboim Gravity from the Karch-Randall Braneworld

In this letter, we show that Jackiw-Teitelboim (JT) gravity can be naturally realized in the Karch-Randall braneworld. Notably the role of the dilaton in JT gravity is played by the radion in a suitably orbifolded version of the setup. In the classical entanglement entropy calculation, there is an apparent degeneracy of Ryu-Takayanagi surfaces. We demonstrate how quantum fluctuations of the radion/dilaton resolves this would-be classical puzzle regarding entanglement wedge reconstruction.

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arXiv2021

Inconsistency of Islands in Theories with Long-Range Gravity

In ordinary gravitational theories, any local bulk operator in an entanglement wedge is accompanied by a long-range gravitational dressing that extends to the asymptotic part of the wedge. Islands are the only known examples of entanglement wedges that are disconnected from the asymptotic region of spacetime. In this paper, we show that the lack of an asymptotic region in islands creates a potential puzzle that involves the gravitational Gauss law, independently of whether or not there is a non-gravitational bath. In a theory with long-range gravity, the energy of an excitation localized to the island can be detected from outside the island, in contradiction with the principle that operators in an entanglement wedge should commute with operators from its complement. In several known examples, we show that this tension is resolved because islands appear in conjunction with a massive graviton. We also derive some additional consistency conditions that must be obeyed by islands in decoupled systems. Our arguments suggest that islands might not constitute consistent entanglement wedges in standard theories of massless gravity where the Gauss law applies.

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arXiv2021

Entanglement Phase Structure of a Holographic BCFT in a Black Hole Background

We compute holographic entanglement entropy for subregions of a BCFT thermal state living on a nongravitating black hole background. The system we consider is doubly holographic and dual to an eternal black string with an embedded Karch-Randall brane that is parameterized by its angle. Entanglement islands are conventionally expected to emerge at late times to preserve unitarity at finite temperature, but recent calculations at zero temperature have shown such islands do not exist when the brane lies below a critical angle. When working at finite temperature in the context of a black string, we find that islands exist even when the brane lies below the critical angle. We note that although these islands exist when they are needed to preserve unitarity, they are restricted to a finite connected region on the brane which we call the atoll. Depending on two parameters -- the size of the subregion and the brane angle -- the entanglement entropy either remains constant in time or follows a Page curve. We discuss this rich phase structure in the context of bulk reconstruction.

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arXiv2021

Information Transfer with a Gravitating Bath

Late-time dominance of entanglement islands plays a critical role in addressing the information paradox for black holes in AdS coupled to an asymptotic non-gravitational bath. A natural question is how this observation can be extended to gravitational systems. To gain insight into this question, we explore how this story is modified within the context of Karch-Randall braneworlds when we allow the asymptotic bath to couple to dynamical gravity. We find that because of the inability to separate degrees of freedom by spatial location when defining the radiation region, the entanglement entropy of radiation emitted into the bath is a time-independent constant, consistent with recent work on black hole information in asymptotically flat space. If we instead consider an entanglement entropy between two sectors of a specific division of the Hilbert space, we then find non-trivial time-dependence, with the Page time a monotonically decreasing function of the brane angle -- provided both branes are below a particular angle. However, the properties of the entropy depend discontinuously on this angle, which is the first example of such discontinuous behavior for an AdS brane in AdS space.

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