Author Archives: Claire Fullarton

Waves, stability and exotic black holes

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Jake Dunn and Dr Claude Warnick from the Pure Mathematics group at Imperial College, London tell us all about their research using the Klein-Gordon equation to study black holes.

Jake Dunn

Jake Dunn is a PhD student at Imperial College, London.
Claude Warnick is a Lecturer in Pure Mathematics at Imperial College, London.

There is a long standing conjecture in the theory of general relativity that the final state of the gravitational collapse of a star should be a stationary black hole modelled by the Kerr solution. To this date there remains no mathematical proof of this statement, and it seems that we may have to wait a while before this result can be established. Even the simpler problem of black hole stability is a considerable mathematical challenge.

We may think of a stationary black hole as Continue reading →

Book Review: The Singular Universe and the Reality of Time

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JulianBarbour

Julian Barbour is an independent theoretical physicist and Visiting Professor in Physics at the University of Oxford. He has specalized in the relational aspects of dynamics.

Review of “The Singular Universe and the Reality of Time” by Roberto Mangabeira Unger and Lee Smolin

The Singular Universe is effectively two separate books held together by some common ideas. Roberto Mangabeira Unger is a philosopher, social and legal theorist and politician who helped to bring about democracy in Brazil and has twice been appointed as its Minister of Strategic Affairs (in 2007 and 2015). According to Wikipedia (current entry), “his work begins from the premise that no natural social, political or economic arrangements underlie individual or social activity.” A similar spirit informs his approach to cosmology. Lee Smolin is of course well known as one of the creators of loop quantum gravity and as the author of several popular-science books. For brevity, I shall refer to the authors as RMU and LS. The book is over 500 pages in length. The first part, by RMU, is more than twice the length of LS’s and could have been shortened without loss of essential content. There is a final 20-page section detailing differences of view, which are substantial in some cases because RMU advocates a much greater break with the conventional approach to science than LS.

The two authors are agreed that a new ‘historical’ approach to cosmology is needed. For RMU, the mere fact that the universe has been shown to have a history is enough to indicate that the methods hitherto used to study the universe must be radically modified. LS argues for a new approach because of our failures to understand the history and properties of the universe as so far discovered. He points out that, Continue reading →

Insights from the Boundary: black holes in a magnetic universe

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Hari Kunduri

Hari Kunduri

Following from the seminal work of Dain, a great deal is now known concerning geometric inequalities relating the area, charge, and angular momentum of axisymmetric black hole horizons in (possibly dynamical) spacetimes.  A key feature of these results is that they are quasi-local: they depend on spacetime only near the horizon itself and so are not sensitive to the asymptotic behaviour of the geometry.

For Einstein-Maxwell theory the celebrated uniqueness theorems tell us under certain conditions, that the Kerr-Newman (KN) family of solutions are the only stationary, axisymmetric and asymptotically flat black hole spacetimes. These are the model geometries that originally motivated the inequalities. However if we relax the condition of asymptotic flatness there are many other families of black hole solutions. While in general these will not contain event horizons (whose standard definitions require flat or AdS asymptotics) they still contain singularities and Killing horizons. In this paper we focussed Continue reading →

TianQin: a space-borne gravitational wave detector

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Attendees at the third workshop on the TianQin science mission

Attendees at the third workshop on the TianQin science mission

Gravitational waves can paint a completely new picture of the Universe. Promising advances in technology may make it possible to detect the minute wobbling of spacetime in the next few years. Estimates show that ground-based gravitational wave detectors, such as Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) or Advanced Virgo will probably see several hundred events by 2020. These ground-based instruments will be complemented by space-borne detectors. These are sensitive to a much richer set of sources, including compact binary star systems in our own Milky Way, supermassive black holes consuming stars, and binary supermassive black holes in distant galactic nuclei. Dozens of proposals have been put forward for space-borne gravitational wave detectors, among which the most studied are LISA (Laser Interferometric Space Antenna) and its evolved version, eLISA. The European space agency has picked “Gravitational Universe” as the science theme for its 3rd large science mission L3; if chosen, eLISA might be launched in 2034.

In our paper, we describe the preliminary concept of a newly proposed space-borne gravitational wave detector, TianQin. In old Chinese legend, the lives of the gods in heaven are very similar to the lives of people on the ground (apart from the fact that they can fly, perform other miracles, and are presumably much happier). They also play music using instruments such as a Chinese zither. A zither on the ground is called “Qin”, and one in heaven is “TianQin”. Bearing this name, our experiment is metaphorically seen as Continue reading →

Perfect accordance of the gravitational and the electromagnetic field in 3D

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Maro Cvitan (assistant professor at the University of Zagreb) (left), Predrag Dominis Prester (associate professor at the University of Rijeka) (centre) and Ivica Smolić (assistant professor at the University of Zagreb) (right)

Maro Cvitan (assistant professor at the University of Zagreb) (left), Predrag Dominis Prester (associate professor at the University of Rijeka) (centre) and Ivica Smolić (assistant professor at the University of Zagreb) (right)

Does a physical field have to share the symmetries of the ambient spacetime?

Open a typical textbook on classical electrodynamics and you will find numerous examples of wrinkled and twisted electromagnetic fields, in a sheer contrast with the maximally symmetric Minkowski spacetime they inhabit. These, however, are the weak fields which do not “bend” the spacetime “fabric”. Once we allow the field to interact with the spacetime geometry via gravitational field equations, the symmetry constraints become much more stringent. When the answer to the opening question is affirmative we say that the field inherits the spacetime symmetries. Symmetry inheritance is not only used as a convenient assumption in a choice of the ansatz, but is also an important ingredient of Continue reading →

The importance of being Melvin

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The authors, Jennie Traschen and David Kastor, enjoy the wit and humor of Oscar Wilde.

The authors, Jennie Traschen and David Kastor, enjoy the wit and humor of Oscar Wilde. The image above has been obtained from the Wikimedia website, where it is stated to have been released into the public domain. It is included within this blog post on that basis.

Like Oscar Wilde’s famous 1895 play, our recent CQG article “Melvin Magnetic Fluxtube/Cosmology Correspondence,” features an intricate interplay of dual and concealed identities. While our paper lacks the biting wit of Wilde’s dialogue, e.g.

“I do not approve of anything that tampers with natural ignorance. Ignorance is like a delicate exotic fruit; touch it and the bloom is gone. The whole theory of modern education is radically unsound. Fortunately in England, at any rate, education produces no effect whatsoever,”

our revelations regarding true identity do play out on a more vast, indeed a cosmic stage.

Melvin’s solution to the Einstein-Maxwell equations describes a static bundle of magnetic flux-lines bound together by self-gravity. Originally discovered in 1963, it has a rich and influential history. In 1964, Thorne studied the stability of what he called “Melvin’s Magnetic Universe.” Its resistance to gravitational collapse was an important clue leading to the formulation of his well-known hoop conjecture. In 1975, Ernst showed that Continue reading →

Life-altered cosmologies

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Jay Olson

Jay Olson (lecturer at Boise State University) seeks to minimize a convenient reserve of free energy.

A few assumptions regarding life and technology translate into new cosmological solutions.

For the universe as a whole, will the next several billion years be any different from the last several billion years? What kinds of things could make it different? Something like a phase transition or a big rip would definitely break up the monotony, but that kind of thing seems unlikely to happen any time soon. Barring that, we can expect cosmic acceleration to push a bit harder, galaxies to get a bit dimmer, black holes to get a bit fatter.  It’s mostly a boring, predictable, stable time for the cosmos.

Then again, there is something a little different happening now. It hardly seems worth mentioning. It takes a while for the universe to produce enough heavy elements to form earthlike planets. And then, judging by our Continue reading →

Accreting onto almost Kerr-de Sitter black holes

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Read the full article for free* in Classical and Quantum Gravity:
Bondi-type accretion in the Reissner-Nordström-(anti-)de Sitter spacetime
Filip Ficek 2015 Class. Quantum Grav. 32 235008
arXiv: 1509.07005
*until 30/12/15

Filip Ficek

Filip Ficek is a graduate student in Theoretical Physics at Jagiellonian University.

In spite of numerous investigations, accretion flows onto the Kerr black hole are still not fully understood, especially for radially dominated flows, where aside from a very specific case of an ultra-hard fluid, general solutions are not known. Some insight may be provided by considering a simpler problem instead, namely spherically symmetric, steady accretion in Reissner-Nordström spacetimes. It is well known that rotating Kerr black holes and charged Reissner-Nordström black holes feature similar horizon and causal structures. In fact, it is common to treat a Reissner-Nordström black hole as a toy model of an astrophysical black hole. If we also take into account the cosmological constant, we may suppose, that accretion solutions in Reissner-Nordström-(anti-)de Sitter spacetime will Continue reading →

Quantum fluctuations of geometry in a hot Universe

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Read the full article for free* in Classical and Quantum Gravity:
Quantum fluctuations of geometry in a hot Universe
Iwo Bialynicki-Birula 2015 Class. Quantum Grav. 32 215015
arXiv:1501.07405
*until 23/12/15

Quantum uncertainty relations force the components of the Riemann curvature tensor to fluctuate

Iwo Bialynicki-Birula

Iwo Bialynicki-Birula is a member of the Polish Academy of Sciences working at the Center for Theoretical Physics in Warsaw. His main interests are classical and quantum electrodynamics, quantum mechanics, and general relativity. He has published 4 books and 184 papers, 44 of them were coauthored by his wife Zofia, also a theoretical physicist. All his papers are available on his website.

My paper published in Classical and Quantum Gravity, extends the concept of zero-point field fluctuations from electromagnetism to gravity with the use of an uncommon but very convenient tool: the Wigner function.

Electromagnetism is perhaps the most studied and the best understood part of physics. We have an almost perfect theory of electromagnetic phenomena both at the classical and at the quantum level. In contrast, our understanding of gravitational phenomena is not satisfactory even at the classical level. However, there are similarities between electromagnetism and gravitation that may help in exploration of quantum gravity. These similarities Continue reading →

Classifying noise transients in advanced gravitational-wave detectors

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Read the full article in Classical and Quantum Gravity (Open Access):
Classification methods for noise transients in advanced gravitational-wave detectors   Jade Powell, Daniele Trifirò, Elena Cuoco, Ik Siong Heng and Marco Cavaglià 2015 Class. Quantum Grav. 32 215012
arXiv:1505.01299

Jade Powell

Jade Powell is a PhD student at the University of Glasgow

A careful analysis of detector noise is necessary to determine whether a real gravitational-wave signal exists in the data of Advanced LIGO and Virgo. Instrumental and environmental disturbances can produce non-astrophysical triggers in science data, so called “glitches”. These glitches may reduce the duty cycle of the interferometers, and they could lead to a false detection if they occur simultaneously in multiple detectors. In the initial science runs of LIGO and Virgo a glitch could be classified by looking at an image of its time series waveform or spectrogram. This proved to be a slow and inefficient method for characterising a large number of glitches. To solve this problem the detector characterization team proposed a challenge for the fast automatic classification of Continue reading →