Superselection Sectors of Gravitational Subregions

Motivated by the problem of defining the entanglement entropy of the graviton, we study the division of the phase space of general relativity across subregions. Our key requirement is demanding that the separation into subregions is imaginary---i.e., that entangling surfaces are not physical. This translates into a certain condition on the symplectic form. We find that gravitational subregions that satisfy this condition are bounded by surfaces of extremal area. We characterise the 'centre variables' of the phase space of the graviton in such subsystems, which can be taken to be the conformal class of the induced metric in the boundary, subject to a constraint involving the traceless part of the extrinsic curvature. We argue that this condition works to discard local deformations of the boundary surface to infinitesimally nearby extremal surfaces, that are otherwise available for generic codimension-2 extremal surfaces of dimension $\geq$ 2.Comment: 17 pages. v4: minor corrections, accepted in JHE

Gravity duals of boundary cones

The replica trick defines Renyi entropies as partition functions on conically singular geometries. We discuss their gravity duals: regular bulk solutions to the Einstein equations inducing conically singular metrics at the boundary. When the conical singularity is supported on a flat or spherical surface, these solutions are rewritings of the hyperbolic black hole. For more general shapes, these solutions are new. We construct them perturbatively in a double expansion in the distance and strength of the conical singularity, and extract the vacuum polarisation due to the cone. Recent results about the structure of logarithmic divergences of Renyi entropies are reproduced ---in particular, $f_b\neq f_c$. We discuss in detail the dynamical resolution of the singularity in the bulk. This resolution is in agreement with a previous proposal, and indicates a non-minimal settling to the `splitting problem': an apparent ambiguity in the holographic entropy formula of certain theories with higher derivatives.Comment: v3: 31 pages. Typos corrected and some clarifications. Matches publishe

Generalized gravitational entropy without replica symmetry

We explore several extensions of the generalized entropy construction of Lewkowycz and Maldacena, including a formulation that does not rely on preserving replica symmetry in the bulk. We show that an appropriately general ansatz for the analytically continued replica metric gives us the flexibility needed to solve the gravitational field equations beyond general relativity. As an application of this observation we study Einstein-Gauss-Bonnet gravity with a small Gauss-Bonnet coupling and derive the condition that the holographic entanglement entropy must be evaluated on a surface which extremizes the Jacobson-Myers entropy. We find that in both general relativity and Einstein-Gauss-Bonnet gravity replica symmetry breaking terms are permitted by the field equations, suggesting that they do not generically vanish.Comment: 24 pages, 3 figures. v3: fixed some more typos, v2: fixed minor typo

Exact Gravitational Wave Signatures from Colliding Extreme Black Holes

The low-energy dynamics of any system admitting a continuum of static configurations is approximated by slow motion in moduli (configuration) space. Here, following Ferrell and Eardley, this moduli space approximation is utilized to study collisions of two maximally charged Reissner--Nordstr{\"o}m black holes of arbitrary masses, and to compute analytically the gravitational radiation generated by their scattering or coalescence. The motion remains slow even though the fields are strong, and the leading radiation is quadrupolar. A simple expression for the gravitational waveform is derived and compared at early and late times to expectations.Comment: 6 page

Exposar l'altre o exposar-se a un mateix

En aquest text, Joan Bestard, professor i director del Departament d'Antropologia Social de la Universitat de Barcelona, reflexiona sobre les mutacions dels museus d’etnografia. Compara els primers museus amb els jardins botànics de les grans metròpolis i considera que les noves tendències museogràfiques, tal i com queden reflectides en el nou museu parisenc Quai Branly, ja no mostren a un Altre distant, sinó que ens inciten a reflexionar sobre nosaltres mateixos i evoquen la nostra relació amb els altres

Parentiu i nació: Relacions concretes i idees abstractes

Seguint Schneider (1969), considero que en les societats modernes euroamericanes el domini cultural de la nació i el del parentiu són definits i estructurats en termes idèntics, és a dir, en termes d'una substància natural i d'un codi de conducta. Els símbols del parentiu americà, diu Schneider, proporcionen als parents una solidaritat difusa i duradora, de la mateixa manera que els símbols de la nació proporcionen igualment als anomenats "nacionals" una solidaritat difusa i duradora. En aquest article miro d'expandir aquest argument amb relació a la definició moderna de nació. Analitzo la contradicció manifesta d'una definició de nació o bé en termes purament cívics o bé de jus soli o bé de jus sanguinis. Comparo aquestes definicions amb la naturalesa híbrida del parentiu com un domini que ens serveix per anar de la naturalesa a la cultura, com també de l'esfera privada a l'esfera pública. Atès aquest aspecte dual del parentiu, ha estat un instrument intel·lectual excel·lent per moure´s des de les relacions concretes dels llaços genealògics fins a les idees més abstractes de la societat. Considero el parentiu un element per naturalitzar categories socials, produir nocions socials de temps i de localitat, com també idees culturals d'identitat i de diferència