arXiv subjects

Ignacio A. Reyes

Publications and source records attributed to Ignacio A. Reyes.

14 recordsLinked to original sources

arXiv2025

Self-consistent solution to the semiclassical Einstein equations of a star

We present the interior solution for a static, spherically symmetric perfect fluid star backreacted by QFT in four dimensions invoking no arbitrary parameters. It corresponds to a constant energy density star and is fully non-perturbative. The space of solutions includes ultra-compact configurations that have neither singularities nor light rings inside the star and can exist arbitrarily close to the Schwarzschild limit, showing that the classical paradigm of astrophysics does not hold once QFT in curved space is taken into account.

gr-qc↗

arXiv2024

Rényi second laws for black holes

Hawking's black hole area theorem provides a geometric realization of the second law of thermodynamics and constrains gravitational processes. In this work we explore a one-parameter extension of this constraint formulated in terms of the monotonicity properties of Rényi entropies. We focus on black hole mergers in asymptotically AdS space and determine new restrictions which these Rényi second laws impose on the final state. We evaluate the entropic inequalities starting from the thermodynamic ensembles description of black hole geometries, and find that for many situations they set more stringent bounds than those imposed by the area increase theorem.

hep-th↗

arXiv2023

Relativistic dynamics of moving mirrors in CFT$_2$: quantum backreaction and black holes

There is a well-known correspondence between the physics of black hole evaporation and that of moving mirrors in QFT. However, most analyses in this subject rely on prescribed mirror trajectories. Here, we study the flat-space dynamics of $1+1$-dimensional Conformal Field Theories interacting with a relativistic boundary particle of mass $m$ acting as a perfect mirror. The trajectory of the latter is not fixed but follows its own relativistic equation of motion $F^μ=ma^μ$. For given initial conditions at past null infinity, we find the boundary particle's trajectory and the reflected energy-momentum of the quantum fields. For incoming vacuum states, the solution yields mirror orbits that correspond to extremal black holes. For the class of incoming states that produce orbits becoming null in finite proper time -- corresponding to the formation of a horizon -- at the classical level, the quantum backreaction avoids this endpoint rendering the mirror's velocity in lightcone coordinates finite. We investigate the behavior of the Averaged Null Energy Condition, which in this setup reduces to a boundary term.

hep-th↗

arXiv2023

Trace anomaly and compact stars

A widespread assumption states that quantum effects of matter in curved spacetimes become relevant only when the inverse curvature radius approaches the Planck mass $m_{\text{P}}$. We challenge this view by showing that, relying solely on universal properties of QFTs at high energies, the trace anomaly becomes macroscopic instead at energy scales $ \left( m/m_{\text{P}} \right)^{\frac{1-α}{2-α}} m_{\text{P}}$, where $m\ll m_{\text{P}}$ and $α<1$ are, respectively, the mass scale and exponent appearing in the matter equation of state at high energies. As an application, we consider compact stars close to their Buchdahl limit, and examine the propagation of scalar waves on the resulting backreacted geometries. At the aforementioned energy scale, the curvature becomes negative within a neighborhood of the the star's center, implying a qualitative change in the behavior of the associated spectrum of modes.

gr-qc↗

arXiv2023

Quantum Field Theory on compact stars near the Buchdahl limit

Very compact stars seem to be forbidden in General Relativity. While Buchdahl's theorem sets an upper bound on compactness, further no-go results rely on the existence of two light rings, the inner of which has been associated to gravitational instabilities. However, little is known about the role of quantum fields in these strong gravity regimes. Here, we consider the particularly simple model of a constant density star and we work in the probe approximation where the backreaction is ignored. We show that the trapping of modes inside the star leads the renormalized stress tensor of Conformal Field Theories to diverge faster than the classical source in the Buchdahl limit. This leads to the violation of the Null Energy Condition around the inner light ring. The backreaction of quantum fields in this regime therefore cannot be ignored. This happens as the star's surface approaches the Buchdahl radius $9GM/4$ rather than the Schwarzschild radius. The results are independent of the details of the interactions, but contain an ambiguity associated to the renormalization scheme.

gr-qc↗

arXiv2021

Moving mirrors, Page curves and bulk entropies in AdS$_2$

Understanding the entanglement of radiation in QFT has been a long standing challenge, with implications ranging from black hole thermodynamics to quantum information. We demonstrate how the case of the free fermion in $1 + 1$ dimensions reveals the details of the density matrix of the radiation produced by a moving mirror. Using the resolvent method rather than standard CFT techniques we derive the Rényi entropies, modular Hamiltonian and flow of the radiation, and determine when mirrors generate unitary transformations.

hep-th↗

arXiv2020

Quantum Extremal Islands Made Easy, Part II: Black Holes on the Brane

We discuss holographic models of extremal and non-extremal black holes in contact with a bath in d dimensions, based on a brane world model introduced in arXiv:2006.04851. The main benefit of our setup is that it allows for a high degree of analytic control as compared to previous work in higher dimensions. We show that the appearance of quantum extremal islands in those models is a consequence of the well-understood phase transition of RT surfaces, and does not make any direct reference to ensemble averaging. For non-extremal black holes the appearance of quantum extremal islands has the right behaviour to avoid the information paradox in any dimension. We further show that for these models the calculation of the full Page curve is possible in any dimension. The calculation reduces to numerically solving two ODEs. In the case of extremal black holes in higher dimensions, we find no quantum extremal islands for a wide range of parameters. In two dimensions, our results agree with arXiv:1910.11077 at leading order; however a finite UV cutoff introduced by the brane results in subleading corrections. For example, these corrections result in the quantum extremal surfaces moving further outward from the horizon, and shifting the Page transition to a slightly earlier time.

hep-th↗

arXiv2020

Resolving modular flow: a toolkit for free fermions

Modular flow is a symmetry of the algebra of observables associated to spacetime regions. Being closely related to entanglement, it has played a key role in recent connections between information theory, QFT and gravity. However, little is known about its action beyond highly symmetric cases. The key idea of this work is to introduce a new formula for modular flows for free chiral fermions in $1+1$ dimensions, working directly from the \textit{resolvent}, a standard technique in complex analysis. We present novel results -- not fixed by conformal symmetry -- for disjoint regions on the plane, cylinder and torus. Depending on temperature and boundary conditions, these display different behaviour ranging from purely local to non-local in relation to the mixing of operators at spacelike separation. We find the modular two-point function, whose analytic structure is in precise agreement with the KMS condition that governs modular evolution. Our ready-to-use formulae may provide new ingredients to explore the connection between spacetime and entanglement.

hep-th↗

arXiv2020

Quantum Extremal Islands Made Easy, Part I: Entanglement on the Brane

Recent progress in our understanding of the black hole information paradox has lead to a new prescription for calculating entanglement entropies, which involves special subsystems in regions where gravity is dynamical, called \textit{quantum extremal islands}. We present a simple holographic framework where the emergence of quantum extremal islands can be understood in terms of the standard Ryu-Takayanagi prescription, used for calculating entanglement entropies in the boundary theory. Our setup describes a $d$-dimensional boundary CFT coupled to a ($d$-1)-dimensional defect, which are dual to global AdS${}_{d+1}$ containing a codimension-one brane. Through the Randall-Sundrum mechanism, graviton modes become localized at the brane, and in a certain parameter regime, an effective description of the brane is given by Einstein gravity on an AdS${}_d$ background coupled to two copies of the boundary CFT. Within this effective description, the standard RT formula implies the existence of quantum extremal islands in the gravitating region, whenever the RT surface crosses the brane. This indicates that islands are a universal feature of effective theories of gravity and need not be tied to the presence of black holes.

hep-th↗

arXiv2019

The entanglement spectrum of chiral fermions on the torus

We determine the reduced density matrix of chiral fermions on the torus, for an arbitrary set of disjoint intervals and generic torus modulus. We find the resolvent, which yields the modular Hamiltonian in each spin sector. Together with a local term, it involves an infinite series of bi-local couplings, even for a single interval. These accumulate near the endpoints, where they become increasingly redshifted. Remarkably, in the presence of a zero mode, this set of points 'condenses' within the interval at low temperatures, yielding continuous non-locality.

hep-th↗

arXiv2019

The entanglement and relative entropy of a chiral fermion on the torus

We derive the entanglement entropy of chiral fermions on the circle at arbitrary temperature. The spin-sector contribution depends only on the total length of the entangling region, regardless of the configuration of the intervals. Thus three-partite information provides a global indicator for the spin boundary conditions. Together with the modular Hamiltonian, our results provide a systematic way of obtaining relative entropy on the torus.

hep-th↗

arXiv2018

Topological Complexity in AdS3/CFT2

We consider subregion complexity within the AdS3/CFT2 correspondence. We rewrite the volume proposal, according to which the complexity of a reduced density matrix is given by the spacetime volume contained inside the associated Ryu-Takayanagi (RT) surface, in terms of an integral over the curvature. Using the Gauss-Bonnet theorem we evaluate this quantity for general entangling regions and temperature. In particular, we find that the discontinuity that occurs under a change in the RT surface is given by a fixed topological contribution, independent of the temperature or details of the entangling region. We offer a definition and interpretation of subregion complexity in the context of tensor networks, and show numerically that it reproduces the qualitative features of the holographic computation in the case of a random tensor network using its relation to the Ising model. Finally, we give a prescription for computing subregion complexity directly in CFT using the kinematic space formalism, and use it to reproduce some of our explicit gravity results obtained at zero temperature. We thus obtain a concrete matching of results for subregion complexity between the gravity and tensor network approaches, as well as a CFT prescription.

hep-th↗

arXiv2017

A short review on Noether's theorems, gauge symmetries and boundary terms

This review is dedicated to some modern applications of the remarkable paper written in 1918 by E. Noether. On a single paper, Noether discovered the crucial relation between symmetries and conserved charges as well as the impact of gauge symmetries on the equations of motion. Almost a century has gone since the publication of this work and its applications have permeated modern physics. Our focus will be on some examples that have appeared recently in the literature. This review is aim at students, not researchers. The main three topics discussed are (i) global symmetries and conserved charges (ii) local symmetries and gauge structure of a theory (iii) boundary conditions and algebra of asymptotic symmetries. All three topics are discussed through examples.

hep-th↗

arXiv2016

A scale dependent black hole in three-dimensional space-time

Scale dependence at the level of the effective action is a generic result of quantum field theory. Allowing for scale dependence of the gravitational couplings leads to a generalization of the corresponding field equations. In this work, those equations are solved by imposing the "null energy condition" in three-dimensional space time with stationary spherical symmetry. The constants of integration are given in terms of the classical BTZ parameters plus one additional constant, that parametrizes the strength of the scale dependence. The properties such as asymptotics, horizon structure, and thermodynamics are discussed. It is found that the black hole entropy shows a remarkable transition from the usual "area~law" to an "area~$\times$~radius" law.

hep-th↗