Yearly Archives: 2015

Black-hole superradiance and the hunt for dark matter

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Paulo Pani

Paolo Pani is a Marie Curie Fellow at Sapienza University of Rome and FCT Researcher at Instituto Superior Tecnico in Lisbon. His research interests include black holes, foundations of General Relativity and relativistic astrophysics. He is co-author of the book “Superradiance” (Springer-Verlag), now in press.

Little is known about dark matter, despite the numerous searches for its constituents. Fortunately, everything falls in the same way, so possible imprints of dark matter can be found in gravitational fields. In particular, if ultralight bosons exist in nature, they would make spinning black holes unstable. How does such instability evolve in realistic scenarios? And what can it teach us about the existence of dark matter?

In our recent CQG paper, we take the first step to address these questions by studying how a light scalar field grows near Continue reading →

Interview with Patricia Schmidt, winner of the 2015 GPG thesis prize

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Patricia Schmidt

Patricia Schmidt is a Postdoctoral Scholar in TAPIR at Caltech

What was the most interesting thing that happened during your PhD?

The most interesting thing was that literally every day, you would do something new. Even looking at the same set of equations again and again you would get new ideas and new insights.

Were there any big surprises?

The biggest surprise was that it actually worked out! We didn’t have a path set out to reach this result when we started, so who would have thought when Mark and I set out that we would actually have exactly Continue reading →

Book Review: On the topology and future stability of the universe, by Hans Ringström

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Mihalis Dafermos

Mihalis Dafermos is Professor of Mathematics at Princeton University

This impressive new book is first and foremost an original and thought-provoking contribution to the study of cosmology in research monograph form, in the best tradition of the kind of deep mathematical work which has played a crucial role in the development of the subject. At the same time, the book doubles as a dependable introduction and  reference for several foundational results in the analysis of the Einstein equations and relativistic kinetic theory which are hard to find elsewhere but which form the basis of so much current (and hopefully, future!) work.

Both these roles are most welcome.

Let me first discuss what this Continue reading →

Designing the future of gravitational wave astronomy: Choosing the best sites for the next generation of gravitational wave detectors

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Fig 1

A new view of the world map, with the black areas indicating allowable sites for building future generation gravitational wave detectors. The other coloured areas are excluded for various reasons.

Imagine you could time travel to decades after the first detections of gravitational waves by ground-based interferometers: someone has already had the call from Stockholm, a series of amazing gravitational wave discoveries have been reported and the watching world is going wild about gravitational wave astronomy.  Such momentous events would certainly trigger the demand for even more sensitive and powerful gravitational wave detectors to drive forward this exciting new field of observational astronomy. But the immediate question would be: where to put these multi-billion dollar instruments?  Future generations of gravitational wave detectors, like the proposed European Einstein Telescope, would be very expensive to build, so choosing the most favourable sites in which to build them will be a crucial issue. Our work, published in Classical and Quantum Gravity, explores the question of Continue reading →

Subtleties of holographic entanglement

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Fischetti Marolf Wall

Sebastian Fischetti (left) is a graduate student of Professor Don Marolf (middle) at UCSB. Aron Wall (right) is a member of the School of Natural Sciences at the Institute for Advanced Study.

One of the most useful features of gauge/gravity duality is that it converts difficult problems in certain types of gauge theories into (relatively) simple geometric problems in gravity in one higher dimension. For example, the Hubeny-Rangamani-Takayagani (HRT) conjecture says that Continue reading →

Gravitational lensing by black holes in astrophysics and in Interstellar

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Interstellar's accretion disc, with and without Doppler shift.

Interstellar‘s accretion disc, with and without Doppler shift. Figure 15 a, c from “Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar” Oliver James et al 2015 Class. Quantum Grav. 32 065001

New insights into the effects of black holes from the team responsible for the Oscar®-winning visual effects of Interstellar.

Depicting a super-massive black hole in the movie Interstellar presented a new challenge to our visual effects team at Double Negative. Luckily the Executive Producer was theoretical physicist Kip Thorne who ended up working closely with us to create a new computer code, DNGR: Double Negative Gravitational Renderer. This code traces the path of light past a spinning black hole (Kerr metric) whose immense gravity warps space and time in its vicinity. A hot disk of gas orbiting the hole appears to Continue reading →

And now for something completely different…

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Nils Andersson

Nils Andersson is Head of the Southampton University Gravity group. He is mainly focussed on problems involving the modelling of neutron stars and understanding various related astrophysical phenomena, from pulsar glitches to magnetar giant flares. Away from the office, he writes science-inspired books for kids and occasionally blows his own trumpet in public.

Each and every trade has its favourite tools, some more powerful than others. A plumber would not get by without a good wrench, a carpenter needs a hammer, a mechanic a screwdriver and so on. Theoretical physicists prefer action principles.

This preference is natural given that many of the phenomena we are interested in are associated with deviations from some minimum energy equilibrium state. It is well known that, once you understand a problem from the variational point-of-view, you have a very powerful tool at your hands. However, it is also generally accepted that this approach is restricted to conservative systems.

Our recent paper in Classical and Quantum Gravity challenges this consensus view. Working in the framework of classical general relativity (no extra dimensions and fancy stuff here!), we develop Continue reading →

The spin limit of colliding black holes

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Geoffrey Lovelace

Geoffrey Lovelace is an Assistant Professor of Physics at California State University, Fullerton. As member of Fullerton’s Gravitational-Wave Physics and Astronomy Center and the Simulating eXtreme Spacetimes collaboration, his research interests focus on using computer simulations to model colliding black holes and neutron stars and the gravitational waves they emit.

A single black hole’s size limits its spin. Do colliding black holes obey this limit?

In our recent paper, published in Classical and Quantum Gravity, we take a first look at how supercomputer simulations can help reveal the answer.

A black hole is an object whose gravity is so strong that nothing, even light, can escape from inside its horizon. An isolated, uncharged black hole can be completely described by just two numbers: its spin and its horizon surface area. All of the black hole’s properties then follow from Kerr’s solution of Einstein’s equations.

Kerr’s solution implies that a single black hole can spin no faster than its horizon area times a constant: spinning any faster would destroy the horizon. Astronomers have found evidence that some black holes spin very close to the limit (but still below it). Mathematical relativists have proven that this spin limit is obeyed not only by Continue reading →

Ambitwistor Strings and Soft Theorems

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Arthur Lipstein

Arthur Lipstein is a Postdoctoral Research Associate at University of Hamburg/DESY.

Ambitwistor string theories are a family of chiral (holomorphic) string theories whose target space is the space of complexified null geodesics in a general space-time. Like conventional strings, they are critical in 10 dimensions and describe supergravity, but unlike conventional strings, they do not admit a tower of higher massive modes (and are correspondingly not thought to be ultraviolet finite). They provide a natural generalization of the twistor-strings of Witten, Berkovits and Skinner to arbitrary dimension and their correlators give rise to the beautiful formulae for gravitational and Yang-Mills scattering amplitudes in all dimensions recently discovered by Cachazo, He and Yuan (CHY). Ambitwistor strings were also used to obtain new formulae in four dimensions by the authors of this article.

In a recent series of papers, Strominger and Continue reading →

Towards 3.5PN accurate polarizations for compact binaries

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Guillame Faye

Guillaume Faye is Chargé de Recherche (researcher)

The gravitational-wave observatory Advanced LIGO is now in its commissioning stage and preparing for its first scientific runs in early 2015. It will be soon followed by the Advanced Virgo detector. Being reasonably optimistic, one can expect the first detections to occur by 2018. The most likely sources to be observed are coalescences of two compact objects. As both detection and
parameter estimation rely on matched filtering techniques, several programs to compute accurate waveform models using various approximation techniques are being pursued. Notably, the post-Newtonian perturbative approach, where all quantities of interest are expanded in powers of Continue reading →