Pauli defuses the fermionic black-hole bomb

Fermionic vacuum around Kerr black holes spontaneously decays to form a co-rotating Fermi sea.

Antonin Coutant and Peter Millington

With the direct observation of gravitational waves produced in black-hole and neutron-star mergers by LIGO (the Laser Interferometry Gravitational-Wave Observatory), we have entered an exciting new era of multi-messenger astronomy.  For the first time, we are able to determine the properties of some of the most violent events in our universe, testing our theories of gravity and particle physics in extreme regimes.

coutant

Antonin Coutant is a post-doctoral fellow in the Acoustic Laboratory of Le Mans University

We often think of black holes as giant sinks, which swallow up anything that passes nearby and from which nothing can escape.  However, this picture is not quite right, as Stephen Hawking and others have shown.  In 1971, Roger Penrose discovered a process that allows rotational energy to be extracted from black holes.  Most astrophysical black holes are expected to spin on their axes, due to their formation from the collapse of initially asymmetric or rotating matter distributions.  Understanding how these black holes lose angular momentum is of major interest for gravitational-wave astrophysics and, at the same time, can provide constraints on new models of fundamental physics. A peculiar process of angular-momentum loss is induced by the quantum vacuum of fermionic particles: a co-rotating sea of fermions forms spontaneously around the black hole, extracting some of its rotational energy.

Rotating black holes are described theoretically by the Kerr metric, after Roy Kerr, who found this solution to Albert Einstein’s equations of General Relativity in 1963.  One peculiarity of this solution is the existence of the ergoregion, where physical objects are forced to co-rotate with the black hole.  To extract the black hole’s rotational energy and angular momentum, the Penrose process exploits the unusual properties of the ergoregion.  Specifically, a classical particle incident on the ergoregion can back-scatter inelastically, with the ejected particle having an increased energy.  For scattering waves, a similar process leads to the phenomenon of superradiance: an incident wave can be back-scattered with increased amplitude.  This effect has recently been observed in a water-wave analogue.  Now, if we can arrange for the reflected wave to be directed back towards the black hole after each back-scatter, its amplitude will grow exponentially.

millington

Peter Millington is a Research Fellow in the Particle Cosmology Group at the University of Nottingham.

In quantum theory, massive particles also behave as waves, and massive particles can become trapped near black holes.  A scalar field (describing a spin-zero boson), with Compton wavelength comparable to the size of the black hole, will scatter in the ergoregion and undergo superradiance.  Modes that are trapped near the black hole can then scatter repeatedly, leading to an instability known as the black-hole bomb.  If such light scalar fields exist in nature, this instability affects the population density of certain angular momenta of black holes, allowing observations to set limits on the masses of these fields.

The black-hole bomb instability cannot occur for fermionic fields (having half-integer spin), due to Wolfgang Pauli’s exclusion principle, which prevents more than one fermion being in any given state.  However, rotating black holes emit a steady radiation of massless fermions in the same frequency range as superradiance would be expected for bosons.  This is known as the UnruhStarobinsky radiation, discovered by William Unruh and Alexei Starobinsky.  When the fermions are massive, the steady radiation is replaced by an instability, corresponding to the decay of the quantum vacuum to a non-trivial state: the Kerr-Fermi sea, where certain fermion modes that co-rotate with the black hole are populated by extracting its rotational energy and angular momentum.

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Introducing CQG’s new Editor-in-Chief

I am very honored to assume the position of Editor-in-Chief of Classical and Quantum Gravity, following ten very successful years by Clifford Will.

gabriela gonzalez

Gabriela González, CQG’s new Editor-in-Chief, is a professor at Louisiana State University and a member of the LIGO Scientific Collaboration

During Cliff’s term, there were very exciting developments in the field, including precision cosmology, new astrophysics and discoveries of gravitational waves – and the journal was there to provide insight and quality articles. The journal has now 15 “renowned” papers with more than 500 citations (according to inspirehep.net), with half of those in the last 10 years, in topics ranging from “Holographic methods for condensed matter physics”, “Loop Quantum Cosmology”, to details of the LIGO and Virgo gravitational detectors and their discoveries. It is this diversity of topics which has made the journal a pillar of the community, thanks to the efforts of the Editor-in-Chief, the Editorial Board, and the excellent IOP editorial team (Adam Day, 2009-2017 and Holly Young until 2019). This is quantified in the journal impact factor, which is very competitive, as well as in the fast turn-around for reviewing and publishing.

There have been many changes in the last decade which have all helped this success: the introduction of focus sections (not just issues), brief review articles, reviewer awards, an open access policy, and an advisory panel, among others. Following the times, Classical and Quantum Gravity has a presence in social media, especially through this CQG+ blog, started by Adam Day.  The journal has also acquired a physical presence in many conferences in the field to keep in touch with latest developments, and sponsors two important awards for young scientists, the IOP Gravitational Physics Group Thesis prize and the ISGRG Bergmann–Wheeler Thesis Prize. The journal prides itself on having very diverse article authors, with diversity understood in the broadest sense: geography, gender, age, and expertise area among others.

I am very humbled to occupy a position that six eminent scientists held before (H. Nicolai, G. Gibbons, K. Stelle, M. MacCallum, R. Wald and C. Will), and will help the journal continue to grow and succeed in a rapidly evolving field. It is my goal to maintain the highest standards for the journal, as we broaden the range of articles – “gravity” is at the core of exciting theory and experiment with expanding frontiers at cosmologically large and small quantum scales.

Professor Gabriela González

Improved constraint on the primordial gravitational-wave density

Florent Robinet and Sophie Henrot-Versille

Sophie Henrot-Versillé and Florent Robinet are research associates at the Laboratoire de l’Accélérateur Linéaire d’Orsay

Many cosmological models predict the existence of a stochastic Gravitational-Wave (GW) background produced just after the universe was born. As gravitational waves do not interact with matter, their detection would give us a unique and pristine probe to study the very first instants of the Universe: when it was 50 orders of magnitude younger than its age at the epoch of the photon decoupling. Such a detection would be as important as the discovery of the Cosmological Microwave Background (CMB). CMB studies tell us what the universe looked like when it became optically thin (~300,000 years after the Big Bang). They help us to establish the standard ΛCDM model of cosmology and to understand the important role of inflation. Continue reading

When coupling to matter matters

Claudia De Rham

Claudia de Rham is an assistant professor at Case Western Reserve University working on cosmology and particle physics and is particularly interested in models of modified gravity and their embedding within consistent field theory frameworks.

How does matter couple in theories involving several metrics? We unveil the possibility for a new effective metric.

While the theory of general relativity will mark its 100 year anniversary next fall, the realization that the expansion of our universe may currently be accelerating has opened up the door for a series of investigations to understand the behavior of gravity at large distances – as large as the current observable Universe or about 1010 light years. Among the different possible modifications of gravity explored in the past decade, theories of gravity which involve several metrics have played a crucial role. The idea that gravity could be the outcome of several interacting metrics is of course not a new concept and such theories have been explored for more than 70 years, but their consistent realization has only been derived very recently in the past few years, and we are finally reaching a stage where we can understand more precisely how matter couples to gravity in such theories. Continue reading

Video: Hunting for gravitational waves using pulsars

Louise Mayor

Louise Mayor is features editor of Physics World

As features editor of Physics World magazine, my search for stories to share with our readers takes me far and wide – from nuclear reactors to the quietest lab in the world. But sometimes I need look no further than the very office in which I work. That’s because I share my workplace with the staff behind nearly 70 journals published by IOP Publishing. So it was that one lunchtime earlier this year, I got chatting to Adam Day, publisher of Classical and Quantum Gravity (CQG).

Day began telling me about a method of detecting gravitational waves I’d not heard of before, and in no time at all I was hooked. First proposed in the 1970s, the method involves Continue reading

Achieving resonance in the Advanced LIGO gravitational-wave interferometer

Alexa Staley

Alexa Staley is a PhD candidate at Columbia University in the City of New York, and has been working as a graduate student at the LIGO Hanford Observatory in Richland, WA.

The next generation gravitational wave interferometers, known as Advanced LIGO, located in Hanford, WA and Livingston, LA have been installed and are in the process of achieving a sensitivity required for the first direct detection of a gravitational wave. The goal of their design is to measure a gravitational strain as small as 4×10–24/√Hz, requiring a length resolution of approximately 10–19 rms within a 100 Hz bandwidth. This high sensitivity demands multiple optical cavities to enhance the response Continue reading

Black holes against the universe – particle and photon orbits in McVittie spacetimes

Brien Nolan

Brien Nolan is a Senior Lecturer in the School of Mathematical Sciences, Dublin City University

Black holes have a potential technological application that is frequently overlooked: they allow you to look at the back of your own head. This could be useful for checking that your tie is properly tucked into your shirt collar, or – perhaps more relevant for physicists – that your pony tail is straight. This technology relies on the fact that there exist circular photon orbits in all members of the Kerr-Newman-de Sitter family of spacetimes for which the parameters (mass, charge and cosmological constant) correspond to a black hole.

The question arises as to whether this characteristic feature of electro-vac Continue reading

Holographic entanglement obeys strong subadditivity

Aron Wall

Aron Wall is a member of the School of Natural Sciences at the Institute for Advanced Study. In his spare time he blogs at Undivided Looking. He was the 2013 recipient of the Bergmann-Wheeler thesis prize, which is sponsored by Classical and Quantum Gravity.

Gauge-gravity duality allows us to calculate properties of certain quantum field theories (QFT) from classical general relativity. One famous piece of this conjecture, due to Ryu and Takayanagi, relates the entanglement entropy in a QFT region to the area of a surface in the gravitational theory. In addition to being a clue about quantum gravity, this proposal is one of the few tools which allow us to calculate entanglement entropy analytically. Since the entanglement entropy is of increasing interest for field theory and condensed matter applications, it is important to check if the conjecture is true.

One important property of the entropy is strong subadditivity (SSA). This quantum inequality says that the sum of the entropies in two regions is always greater than the sum of the entropies of their union and intersection. My article uses proof Continue reading

Black holes as beads on cosmic strings

Amjad Ashoorioon and Robert Mann

Amjad Ashoorioon (left) is a Senior Research Associate at the physics department of Lancaster University in the United Kingdom. Robert B. Mann (right) is a Professor of Physics and Applied Mathematics at the University of Waterloo, Ontario, Canada.

Cosmic strings have been a source of fascination in cosmology since Tom Kibble first proposed their existence 40 years ago. Like an imperfection in a solidifying crystal, a cosmic string is a thread of energy that might have formed in the early universe during a symmetry breaking phase transition. Twenty years ago Ruth Gregory pointed out that a black hole could have a cosmic string as a single “hair”.   Turning this idea around, in this article we have proposed that a Continue reading

New focus issue: Advanced interferometric gravitational wave detectors

Peter Shawhan and Marie-Anne Bizouard

Peter Shawhan is an Associate Professor of Physics at the University of Maryland, USA. His primary research area is in the analysis of data from gravitational wave detectors and connections with astrophysical events.
Marie-Anne Bizouard is a research fellow at CNRS, Laboratoire de l’Accélérateur Linéaire, Orsay, France. She is an experimental physicist working on gravitational wave searches with ground based interferometric detectors.

The quest to detect gravitational waves directly has seen great advances over the past five decades, with the earlier resonant “bar” detectors being surpassed in sensitivity by large laser interferometers in the last decade.  The first generation of interferometric detectors proved the viability of the approach, progressively improving sensing and control techniques and running up against the fundamental limitations of their designs.  Along the way, many searches for gravitational wave signals were carried out and published, but none achieved the milestone of detecting a clear gravitational-wave signal.

All of that is about to change.  The lessons learned from the first full-scale interferometric detectors fed into the design of advanced detectors which are now being constructed and commissioned and will soon begin collecting data.  Higher laser power, Continue reading