GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Abraham I. Harte,
“Gravitational lensing beyond geometric optics: II. Metric independence”,
Gen Relativ Gravit (2019) 51: 160.
https://doi.org/10.1007/s10714-019-2646-7

Madeleine Burkhart and Daniel Pollack,
“Causal geodesic incompleteness of spacetimes arising from IMP gluing”,
Gen Relativ Gravit (2019) 51: 139.
https://doi.org/10.1007/s10714-019-2621-3

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Topical Collection “Testing the Kerr spacetime with gravitational-wave and electromagnetic observations”

We are please to inform you that the Topical Collection on “Testing the Kerr spacetime with gravitational-wave and electromagnetic observations”, edited by Emanuele Berti, has been completed and is free-to-read until 11 December 2019.

One of the Holy Grails of observational astronomy is to confirm the prediction that black holes in the Universe are described by the Kerr solution of Einstein’s field equations of general relativity. This Topical Collection provides a status report of theoretical and experimental progress towards confirming the “Kerr paradigm” through X-ray astronomy, gravitational lensing, stellar tidal disruption events, superradiance, and gravitational-wave observations of black hole binary mergers.

Berti, E.: Topical collection: Testing the Kerr spacetime with gravitational-wave and electromagnetic observations (EDITORIAL), Gen. Relativ. Gravit. 51(11), 140 (2019).
https://doi.org/10.1007/s10714-019-2622-2

We hope that you will publish your next article in GRG and look forward to working with you soon!

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Shamik Banerjee,
“Symmetries of free massless particles and soft theorems”,
Gen Relativ Gravit (2019) 51: 128.
https://doi.org/10.1007/s10714-019-2609-z

Pierre Martin-Dussaud,
“A primer of group theory for Loop Quantum Gravity and spin-foams”,
Gen Relativ Gravit (2019) 51: 110.
https://doi.org/10.1007/s10714-019-2583-5

Guillaume Bossard and Severin Luest,
“Microstate geometries at a generic point in moduli space”,
Gen Relativ Gravit (2019) 51: 112.
https://doi.org/10.1007/s10714-019-2584-4

Joan Garcia i Tormo and Marika Taylor,
“One point functions for black hole microstates”,
Gen Relativ Gravit (2019) 51: 89.
https://doi.org/10.1007/s10714-019-2566-6

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Topical Collection “The Fuzzball Paradigm”

Call for papers contributing to special article collection

We solicit manuscripts on “The Fuzzball Paradigm” for inclusion in a Topical Collection of the journal General Relativity and Gravitation.

The fuzzball proposal is a paradigm for black holes. In the conventional picture of a black hole, the region around the horizon is in the vacuum state. In the fuzzball paradigm, string theory effects modify the interior of the black hole up to the horizon scale. The proposal posits that the radiation leaving from a non-extremal fuzzball carries information just like radiation from a piece of burning coal. In recent years, these ideas have been widely explored by a number of authors. This Topical Collection plans to explore the ideas and advances in this area.

Topics to be included in this collection include, but are not limited to:

– Fuzzball solutions and their properties
– D1-D5 CFT
– Smooth solutions of alternative theories of gravity
– Black Hole information
– AdS/CFT and the fuzzball proposal
– Classical and quantum properties of black hole microstates

Articles with possibly indirect implications for the fuzzball proposal are also welcome.

Authors are invited to submit through the website https://www.editorialmanager.com/gerg/. Please indicate that your manuscript is intended for inclusion in the special issue “T.C. : The Fuzzball Paradigm”. For preparation, please follow the instructions for authors available at https://www.springer.com/journal/10714

Papers will be published continuously and will appear (as soon as accepted) on the journal website. All submitted papers will be refereed according to the usual high standards of the journal.

We look forward to receiving your submission!

Samir D. Mathur (Guest Editor), David Turton (Guest Editor), or Amitabh Virmani (GRG Associate Editor).

GRG Editor’s Choice free-to-read for GR22/Amaldi13

On the occasion of the GR22/Amaldi13 conference, all recent GRG Editor’s Choice articles will be free-to-read during July 2019!

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Alan A. Coley,
“Mathematical general relativity”,
Gen Relativ Gravit (2019) 51: 78.
https://doi.org/10.1007/s10714-019-2559-5

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Kastytis Zubovas, Andrew R. King,
“The M–$\sigma$ relation between supermassive black holes and their host galaxies”,
Gen Relativ Gravit (2019) 51: 65.
https://doi.org/10.1007/s10714-019-2549-7

Sudipta Sarkar,
“Black hole thermodynamics: general relativity and beyond”,
Gen Relativ Gravit (2019) 51: 63.
https://doi.org/10.1007/s10714-019-2545-y

Brian Allen,
“Sobolev stability of the Positive Mass Theorem and Riemannian Penrose Inequality using inverse mean curvature flow”,
Gen Relativ Gravit (2019) 51: 59.
https://doi.org/10.1007/s10714-019-2542-1

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Golden Oldie: The theoretical significance of experimental relativity by R.H. Dicke (1964)

The Golden Oldies series of the journal General Relativity and Gravitation reprints important papers in general relativity theory that were published 30 or more years ago and are either hard to get hold of, or were originally printed in a language other than English.

The latest work has been republished on 30 April 2019:

Dicke, R.H., Republication of: The theoretical significance of experimental relativity, Gen Relativ Gravit (2018) 51: 57. https://doi.org/10.1007/s10714-019-2509-2​​​​​​​

An editorial note by P.J.E. Peebles and a a brief biography by M.A.H. MacCallum are freely available at https://doi.org/10.1007/s10714-019-2508-3

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Eric Addison, Miguel Gracia-Linares, Pablo Laguna, Shane L. Larson,
“Busting up binaries: encounters between compact binaries and a supermassive black hole”,
Gen Relativ Gravit (2019) 51: 38.
https://doi.org/10.1007/s10714-019-2523-4

Eva Hackmann, Arnab Dhani,
“The propagation delay in the timing of a pulsar orbiting a supermassive black hole”,
Gen Relativ Gravit (2019) 51: 37.
https://doi.org/10.1007/s10714-019-2517-2

Thomas P. Kling, Eric Grotzke, Kevin Roebuck, Harry Waite,
“Examining the Kerr metric through wave fronts of null geodesics”,
Gen Relativ Gravit (2019) 51: 32.
https://doi.org/10.1007/s10714-019-2518-1

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Stone, N.C., Kesden, M., Cheng, R.M. et al.,
“Stellar tidal disruption events in general relativity”,
Gen Relativ Gravit (2019) 51: 30.
https://doi.org/10.1007/s10714-019-2510-9

Osorio Morales, J. and Santillan, O.P.,
“The existence of smooth solutions in q-models”
Gen Relativ Gravit (2019) 51: 29.
https://doi.org/10.1007/s10714-019-2507-4

Chakraborty, S. and Parattu, K.,
“Null boundary terms for Lanczos-Lovelock gravity”,
Gen Relativ Gravit (2019) 51: 23.
https://doi.org/10.1007/s10714-019-2502-9

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight article

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Harte, Abraham I.,
“Gravitational lensing beyond geometric optics: I. Formalism and observables”,
Gen Relativ Gravit (2019) 51: 14.
https://doi.org/10.1007/s10714-018-2494-x

Abstract:
The laws of geometric optics and their corrections are derived for scalar, electromagnetic, and gravitational waves propagating in generic curved spacetimes. Local peeling-type results are obtained, where different components of high-frequency fields are shown to scale with different powers of their frequencies. Additionally, finite-frequency corrections are identified for a number of conservation laws and observables. Among these observables are a field’s energy and momentum densities, as well as several candidates for its corrected “propagation directions”.

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

General Relativity and Gravitation welcomes Pablo Laguna

Introducing our new Editor-in-Chief to succeed Abhay Ashtekar

Publisher and editors of the journal General Relativity and Gravitation (GRG) welcome Pablo Laguna who is taking over the responsibilities of Editor-in-Chief from Abhay Ashtekar, who served on this post from 2012–2018.

We gratefully acknowledge Abhay Ashtekar’s contributions to GRG and the community as a whole. Roy Maartens will continue to jointly lead the journal together with Pablo Laguna, who has been an Editorial Board member since 2013.

Pablo Laguna is Professor and Chair of the School of Physics and member of the Center for Relativistic Astrophysics in the College of Sciences at the Georgia Institute of Technology in Atlanta, U.S.A. His research is in computational astrophysics and numerical relativity. He is using computers to solve the Einstein’s equations of General Relativity to study astrophysical phenomena were the curvature of space-time plays a fundamental role. He is currently focusing on computer simulations to investigate 1) the merger of black holes and/or neutron stars binary systems and their emission of gravitational waves, and 2) the disruption of stars by tidal forces from massive black holes.

General Relativity and Gravitation is led by a high-profile international editorial board and publishes carefully peer-reviewed research papers on all theoretical and experimental aspects of modern general relativity and gravitation research and its extensions. Its Golden Oldies series makes historically relevant seminal papers available to a wide readership. Published under the auspices of the International Society on General Relativity and Gravitation (ISGRG).

GRG Editor’s Choice: recent highlight article

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Chiang-Mei Chen, Jian-Liang Liu, James M. Nester,
“Quasi-local energy from a Minkowski reference”
https://doi.org/10.1007/s10714-018-2484-z

Abstract:
The specification of energy for gravitating systems has been an unsettled issue since Einstein proposed his pseudotensor. It is now understood that energy-momentum is quasi-local (associated with a closed 2-surface). Here we consider quasi-local proposals (including pseudotensors) in the Lagrangian–Noether–Hamiltonian formulations. There are two ambiguities: (1) there are many possible expressions, (2) they depend on some non-dynamical structure, e.g., a reference frame. The Hamiltonian approach gives a handle on both problems. The Hamiltonian perspective helped us to make a remarkable discovery: with an isometric Minkowski reference a large class of expressions—namely all those that agree with the Einstein pseudotensor’s Freud superpotential to linear order—give a common quasi-local energy value. Moreover, with a best-matched reference on the boundary this is the Wang–Yau mass value.

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight article

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Abhay Ashtekar, Miguel Campiglia and Alok Laddha,
“Null infinity, the BMS group and infrared issues”
https://doi.org/10.1007/s10714-018-2464-3

Abstract:
There has been a recent resurgence of interest in the structure of the gravitational field at null infinity, sparked by new results on soft charges and infrared issues related to the S matrix theory in perturbative quantum gravity. We summarize these developments and put them in the broader context of research in the relativity community that dates back to several decades. In keeping with intent of this series, this overview is addressed to gravitational scientists who are not experts in this specific area.

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Golden Oldies by Lichnerowicz / Doroshkevich and Novikov

The Golden Oldies series of the journal General Relativity and Gravitation reprints important papers in general relativity theory that were published 30 or more years ago and are either hard to get hold of, or were originally printed in a language other than English.

Two recent works have been republished in October 2018:

Lichnerowicz, A., Republication of: Propagators, commutators and anti-commutators in general relativity, Gen Relativ Gravit (2018) 50: 145. https://doi.org/10.1007/s10714-018-2433-x

Editorial note by Giampiero Esposito is freely available at https://doi.org/10.1007/s10714-018-2440-y

Doroshkevich, A.G. and Novikov, I.D., Republication of: Mean density of radiation in the Metagalaxy and certain problems in relativistic cosmology, Gen Relativ Gravit (2018) 50: 138. https://doi.org/10.1007/s10714-018-2441-x

Editorial note by Ruth Durrer is freely available at https://doi.org/10.1007/s10714-018-2440-y

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserve special attention:

Jolin, Shan W. & Rosquist, Kjell
“Analytic analysis of irregular discrete universes”
https://doi.org/10.1007/s10714-018-2436-7

Newman, Ezra T.
“Classical mechanics via general relativity and Maxwell’s theory: a bit of magic”
https://doi.org/10.1007/s10714-018-2424-y

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserve special attention:

Henric Krawczynski
“Difficulties of quantitative tests of the Kerr-hypothesis with X-ray observations of mass accreting black holes”
https://doi.org/10.1007/s10714-018-2419-8

Ivan Agullo
“Primordial power spectrum from the Dapor–Liegener model of loop quantum cosmology”
https://doi.org/10.1007/s10714-018-2413-1

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserve special attention:

Jeremy D. Schnittman
“The collisional Penrose process”
https://doi.org/10.1007/s10714-018-2373-5

Claus Kiefer, David Wichmann
“Semiclassical approximation of the Wheeler–DeWitt equation: arbitrary orders and the question of unitarity”
https://doi.org/10.1007/s10714-018-2390-4

Hideyoshi Arakida
“Light deflection and Gauss–Bonnet theorem: definition of total deflection angle and its applications”
https://doi.org/10.1007/s10714-018-2368-2

Emanuele Berti, Kent Yagi, Nicolas Yunes
“Extreme gravity tests with gravitational waves from compact binary coalescences: (I) inspiral–merger”
https://doi.org/10.1007/s10714-018-2362-8

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

Three recent articles deserve special attention (free-to-read access until April 15, 2018):

Giorgio Sarno, Simone Speziale and Gabriele V. Stagno, 2-vertex Lorentzian spin foam amplitudes for dipole transitions, Gen Relativ Gravit (2018) 50: 43. https://doi.org/10.1007/s10714-018-2360-x

Pedro V. P. Cunha and Carlos A. R. Herdeiro, Shadows and strong gravitational lensing: a brief review, Gen Relativ Gravit (2018) 50: 42. https://doi.org/10.1007/s10714-018-2361-9

Dario Brooks, Paul-Christopher Chavy-Waddy, Alan A. Coley, Adam Forget, Daniele Gregoris, Malcolm A. H. MacCallum and David D. McNutt, Cartan invariants and event horizon detection, Gen Relativ Gravit (2018) 50: 37. https://doi.org/10.1007/s10714-018-2358-4

Frank Schulz
Publishing Editor GRG

GRG Editor’s Choice: Higher curvature gravities cannot be bootstrapped

Deser, S., Higher curvature gravities, unlike GR, cannot be bootstrapped from their (usual) linearizations, Gen Relativ Gravit (2017) 49: 149. https://doi.org/10.1007/s10714-017-2308-6

Editor’s Choice (Research Article)
First Online: 07 November 2017

“This paper extends the author’s original contribution from 1970 to a broad class of theories that modify general relativity. The result is a paper that provides a continuation of the 1970 story. The author’s style and commitment to exposing the structural elements of the procedure make for a very nice article that reads well and serves a useful and illuminating purpose in the ‘extensions of GR’ literature.”

Abstract:
We show that higher curvature order gravities, in particular the propagating quadratic curvature models, cannot be derived by self-coupling from their linear, flat space, forms, except through an unphysical version of linearization; only GR can. Separately, we comment on an early version of the self-coupling bootstrap.

GRG Editor’s Choice:
In each volume of GRG, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

GRG Editor’s Choice: Notable articles in General Relativity & Gravitation

In each volume of the journal General Relativity & Gravitation, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

Tanatarov, I.V. & Zaslavskii, O.B., Collisional super-Penrose process and Wald inequalities, Gen Relativ Gravit (2017) 49: 119. https://doi.org/10.1007/s10714-017-2281-0

Abstract:
We consider collision of two massive particles in the equatorial plane of an axially symmetric stationary spacetime that produces two massless particles afterwards. It is implied that the horizon is absent but there is a naked singularity or another potential barrier that makes possible the head-on collision. The relationship between the energy in the center of mass frame Ec.m. and the Killing energy E measured at infinity is analyzed. It follows immediately from the Wald inequalities that unbounded E is possible for unbounded Ec.m. only. This can be realized if the spacetime is close to the threshold of the horizon formation. Different types of spacetimes (black holes, naked singularities, wormholes) correspond to different possible relations between Ec.m. and E. We develop a general approach that enables us to describe the collision process in the frames of the stationary observer and zero angular momentum observer. The escape cone and escape fraction are derived. A simple explanation of the existence of the bright spot is given. For the particular case of the Kerr metric, our results agree with the previous ones found in Patil et al. (Phys Rev D 93:104015, 2016).

Sakovich, A. & Sormani, C., Almost rigidity of the positive mass theorem for asymptotically hyperbolic manifolds with spherical symmetry, Gen Relativ Gravit (2017) 49: 125. https://doi.org/10.1007/s10714-017-2291-y

Abstract:
We use the notion of intrinsic flat distance to address the almost rigidity of the positive mass theorem for asymptotically hyperbolic manifolds. In particular, we prove that a sequence of spherically symmetric asymptotically hyperbolic manifolds satisfying the conditions of the positive mass theorem converges to hyperbolic space in the intrinsic flat sense, if the limit of the mass along the sequence is zero.

GRG Editor’s Choice: Cosmological solutions with charged black holes

Bibi, R., Clifton, T. and Durk, J., Cosmological solutions with charged black holes, Gen Relativ Gravit (2017) 49: 98. doi:10.1007/s10714-017-2261-4

Open Access | Editor’s Choice (Research Article)
First Online: 30 June 2017

In each volume of the journal General Relativity & Gravitation, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

Abstract:
We consider the problem of constructing cosmological solutions of the Einstein–Maxwell equations that contain multiple charged black holes. By considering the field equations as a set of constraint and evolution equations, we construct exact initial data for N charged black holes on a hypersphere. This corresponds to the maximum of expansion of a cosmological solution, and provides sufficient information for a unique evolution. We then consider the specific example of a universe that contains eight charged black holes, and show that the existence of non-zero electric charge reduces the scale of the cosmological region of the space. These solutions generalize the Majumdar–Papapetrou solutions away from the extremal limit of charged black holes, and provide what we believe to be some of the first relativistic calculations of the effects of electric charge on cosmological backreaction.

The authors:
Jessie Durk is currently a PhD student, researching theoretical cosmology with Dr Timothy Clifton, lecturer at Queen Mary University London. Rashida Bibi is a faculty member of the International Islamic University, Islamabad.

GRG Editor’s Choice articles free to read

On the occasion of the upcoming GR21 conference, selected articles from the journal General Relativity and Gravitation are now free-to-read until July 31, 2016:
http://bit.ly/294Uh4J

In each volume, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community. We would also like to introduce Springer’s new article interface, optimized for easy reading on mobile devices.

General Relativity and Gravitation is a journal devoted to all aspects of modern gravitational science, and published under the auspices of the ISGRG. The journal publishes research letters and papers, invited review articles on all theoretical and experimental aspects of modern general relativity and gravitation, as well as book reviews and historical articles of special interest.

We welcome your submissions, and see you at GR21!

Topical Collection: The First Century of General Relativity: GR20/Amaldi10

We cordially invite you to read the Topical Collection of articles
The First Century of General Relativity: GR20/Amaldi10
published in the journal General Relativity and Gravitation.

An outstanding collection of 37 articles contributed by prominent international experts, commemorating the First Century of General Relativity!
Guest Editors: Jerzy Lewandowski, Bala Iyer, Sheila Rowan

Visit: http://news.springer.com/re?l=D0In5v6jrI6h42bfsI9
Enjoy your free access to all articles through May 11, 2015!

Message from the Editors-in-Chief:
The discovery of general relativity by Albert Einstein one hundred years ago was quickly recognized as a supreme triumph of the human intellect. To paraphrase Hermann Weyl, “wider expanses and greater depths were suddenly exposed to the searching eye of knowledge, regions of which there was not even an inkling”. To mark this centenary, General Relativity and Gravitation has published a special Topical Collection of articles.

We would like to thank all the contributors, and the guest editors Jerzy Lewandowski, Bala Iyer, Sheila Rowan for their valuable help in bringing this collection together.

A. Ashtekar and R. Maartens
Editors-in-Chief