Strong Gravity

Strong Gravity research at Perimeter Institute is devoted to understanding both the theoretical and observational aspects of systems in which gravity is very strong (i.e., spacetime is highly curved or dynamical],. On one hand, this means studying extreme astrophysical systems, like black holes and neutron stars, as well as making and testing predictions for existing and forthcoming gravitational wave detectors, electromagnetic telescopes, and particle astrophysics experiments. On the other hand, it also includes a range of non-astrophysical topics, such as the instabilities of higher-dimensional black holes or the dynamics of strongly-coupled quantum field theories (via holography). The goal of strong gravity researcher is to test the validity of Einstein's theory of gravity, constrain proposed alternatives, understand the most extreme astrophysical systems, and investigate the ways in which highly curved or dynamical spacetimes are linked with a range of other problems in fundamental physics.

Displaying 553 - 564 of 639

PHYS 781 - Lecture 15

Niayesh Afshordi University of Waterloo
November 06, 2014

PIRSA:14110053
Cosmology

Quantum Gravity

Strong Gravity
PHYS 781 - Lecture 14

Niayesh Afshordi University of Waterloo
November 04, 2014

PIRSA:14110049
Cosmology

Quantum Gravity

Strong Gravity
Spinning Black Holes and the Membrane Paradigm

Robert Penna Massachusetts Institute of Technology (MIT)
October 16, 2014

PIRSA:14100007
Strong Gravity
Probing (beyond) general relativity with compact binaries and gravitational waves

Leo Stein Cornell University
October 02, 2014

PIRSA:14100003
Strong Gravity
Conformal field theories at non-zero temperature: operator product expansions, Monte Carlo, and holography
September 19, 2014

PIRSA:14090072
Quantum Fields and Strings

Condensed Matter

Strong Gravity
Force-Free Electrodynamics around Extreme Kerr Black Holes

Maria J. Rodriguez Utah State University
September 11, 2014

PIRSA:14090065
Strong Gravity
A mechanism for vortex annihilation in two dimensional superfluid turbulence

Andrew Lucas University of Colorado Boulder
May 22, 2014

PIRSA:14050139
Strong Gravity
Gravity waves from Kerr/CFT

Achilleas Porfyriadis Harvard University
May 15, 2014

PIRSA:14050009
Strong Gravity
Black holes without Lorentz symmetry

Thomas Sotiriou University of Nottingham
May 08, 2014

PIRSA:14050002
Strong Gravity
Geometric inequalities for black holes

Sergio Dain Universidad Nacional de Córdoba
May 01, 2014

PIRSA:14050030
Strong Gravity
The characteristic structure of the Weyl curvature tensor and its utilization

Fan Zhang Beijing Normal University
April 24, 2014

PIRSA:14040139
Strong Gravity
Compact objects as dark-matter probes

Paolo Pani Instituto Superior Tecnico - Departamento de Física
April 17, 2014

PIRSA:14040078
Strong Gravity

PHYS 781 - Lecture 15

Niayesh Afshordi University of Waterloo
November 06, 2014

PIRSA:14110053
Cosmology

Quantum Gravity

Strong Gravity
PHYS 781 - Lecture 14

Niayesh Afshordi University of Waterloo
November 04, 2014

PIRSA:14110049
Cosmology

Quantum Gravity

Strong Gravity
Spinning Black Holes and the Membrane Paradigm

Robert Penna Massachusetts Institute of Technology (MIT)
October 16, 2014

PIRSA:14100007
Strong Gravity
There are about nine astrophysical black holes with measurements of the black hole's spin via the continuum fitting method. Several of these black holes drive powerful jets, which appear to extract the black hole's rotational energy. I will discuss the theory behind these observations, with a particular focus on the black hole membrane paradigm. The membrane paradigm is useful on a practical level for understanding black hole jets. However, it may also be related to fundamental physics through holography and AdS/CFT. This suggests new opportunities for interactions between fundamental physics and astrophysics.
Probing (beyond) general relativity with compact binaries and gravitational waves

Leo Stein Cornell University
October 02, 2014

PIRSA:14100003
Strong Gravity
General relativity enjoys phenomenal success in agreeing with experiments and observations, but it must break down at some point. Astrophysics can give guidance for what type of theory may correct general relativity, if we know which phenomenology to look for. I will discuss the possible corrections to the structure of compact objects, the binary problem, and observations with pulsar timing and gravitational wave detection. These corrections are computed in specific effective theories, such as Einstein-dilaton-Gauss-Bonnet (EdGB) and dynamical Chern-Simons (dCS) gravity, and we may further generalize the common aspects of these theories to build a parametrized framework which captures a large number of effective field theories.
Conformal field theories at non-zero temperature: operator product expansions, Monte Carlo, and holography
September 19, 2014

PIRSA:14090072
Quantum Fields and Strings

Condensed Matter

Strong Gravity
We discuss properties of 2-point functions in CFTs in 2+1D at finite temperature. For concreteness, we focus on those involving conserved flavour currents, in particular on the associated conductivity. At frequencies much greater than the temperature, ω >> T, the ω dependence of the conductivity can be computed from the operator product expansion (OPE) between the currents and operators which acquire a non-zero expectation value at T > 0. Such results are found to be in excellent agreement with quantum Monte Carlo studies of the O(2) Wilson-Fisher CFT. Results for the conductivity and other observables are also obtained in vector 1/N expansions. We match these large ω results to the corresponding correlators of holographic representations of the CFT: the holographic approach then allows us to extrapolate to small ω/T. Other holographic studies implicitly only used the OPE between the currents and the energy-momentum tensor, and this yields the correct leading large ω behavior for a large class of CFTs. However, for the Wilson-Fisher CFT a relevant “thermal” operator must also be considered, and then consistency with the Monte Carlo results is obtained without a previously needed ad hoc rescaling of the T value [1]. We also use the OPE to prove sum rules obeyed by the conductivity. **In collaboration with A. Katz, S. Sachdev and E. Sørensen** [1] WWK, E. Sørensen, S. Sachdev, Nat. Phys. 10, 361 (2014)
Force-Free Electrodynamics around Extreme Kerr Black Holes

Maria J. Rodriguez Utah State University
September 11, 2014

PIRSA:14090065
Strong Gravity
Plasma-filled magnetospheres can extract energy from a spinning black hole and provide the power source for a variety of observed astrophysical phenomena. These magnetospheres are described by the highly nonlinear equations of force-free electrodynamics, or FFE. Typically these equations can only be solved numerically. In this talk I will explain how to analytically obtain several infinite families of exact solutions of the full nonlinear FFE equations very near the horizon of a maximally spinning black hole, where the energy extraction takes place.
A mechanism for vortex annihilation in two dimensional superfluid turbulence

Andrew Lucas University of Colorado Boulder
May 22, 2014

PIRSA:14050139
Strong Gravity
Recent numerical simulations [1] have suggested that two dimensional superfluid turbulence is characterized by a direct cascade of energy to small length scales, in contrast to the inverse cascade of normal fluids, where energy is transported to large length scales. This direct cascade is characterized by many vortex-antivortex annihilation events. Recent experimental work [2] on Bose-Einstein condensates appears to demonstrate qualitatively similar physics. I will discuss recent work in progress towards identifying the physical mechanism underlying this direct cascade, using techniques of effective field theory.
Gravity waves from Kerr/CFT

Achilleas Porfyriadis Harvard University
May 15, 2014

PIRSA:14050009
Strong Gravity
Astronomical observation suggests the existence of near-extreme Kerr black holes whose horizons spin at nearly the speed of light. Properties of diffeomorphisms imply that the dynamics of the high-redshift near-horizon region of near-extreme Kerr, which includes the innermost-stable-circular-orbit (ISCO), is governed by an infinite-dimensional emergent conformal symmetry. This symmetry may be exploited to analytically, rather than numerically, compute a variety of potentially observable processes. In this talk I will show how we compute and study the conformal transformation properties of the gravitational radiation emitted by an orbiting massive object in the large-redshift near-horizon region. I will also use conformal symmetry of the near-horizon region to compute the gravitational radiation produced during the plunge phase following the object's crossing of the ISCO.
Black holes without Lorentz symmetry

Thomas Sotiriou University of Nottingham
May 08, 2014

PIRSA:14050002
Strong Gravity
Our current definition of what a black hole is relies heavily on the assumption that there exists a finite maximum speed of propagation for any signal. Indeed, one is tempted to think that the notion of a black hole has no place in a world with infinitely fast signal propagation. I will use concrete examples from Lorentz-violating gravity theories to demonstrate that this naive expectation is not necessarily true.
Geometric inequalities for black holes

Sergio Dain Universidad Nacional de Córdoba
May 01, 2014

PIRSA:14050030
Strong Gravity
A geometric inequality in General Relativity relates quantities that have both a physical interpretation and a geometrical definition. It is well known that the parameters that characterize the Kerr-Newman black hole (angular momentum, charge, mass and horizon area) satisfy several important geometric inequalities. Remarkably enough, some of these inequalities also hold for dynamical black holes. This kind of inequalities, which are valid in the dynamical and strong field regime, play an important role in the characterization of the gravitational collapse. They are closed related with the cosmic censorship conjecture. Also, variants of these inequalities are valid for ordinary bodies. In this talk I will review recent results in this subject.
The characteristic structure of the Weyl curvature tensor and its utilization

Fan Zhang Beijing Normal University
April 24, 2014

PIRSA:14040139
Strong Gravity
The talk will summarize some results relating to clarifying the physical significance of the characteristic structure of the Weyl curvature tensor, and proposals for its utilization. I will begin by showing how null force-free or vacuum electrodynamic solutions experience reduced scattering by propagating along the principal null directions (GPNDs) of the spacetime, as if they were the flatter directions of the curvature tensor. Building on this observation, I will use complexified sectional curvatures to develop an analogy between the Weyl tensor and the second fundamental form of an immersed surface, and argue that, despite their name, the GPNDs are more akin to the asymptotic directions rather than the principal directions. I will also identify the ``true'' principal directions and curvatures, which has so far been under-utilized. I will then move on to propose usage for these quantities in topics related to the peeling theorem, the large scale geometry of the spacetime, the transition from the merger phase of binary black hole merger into the quasinormal mode ringdown phase, and perturbation of generic background spacetimes.
Compact objects as dark-matter probes

Paolo Pani Instituto Superior Tecnico - Departamento de Física
April 17, 2014

PIRSA:14040078
Strong Gravity
In the last few years the possibility of constraining dark matter with astrophysical observations of compact objects, such as white dwarfs, neutron stars and black holes, has been explored. The ultra-high density interior of neutron stars and the strong-curvature regions near massive black holes make these objects unique laboratories to test weakly-interacting particles. In the first part of this talk I will review some recent work on the potential of compact objects as dark-matter detectors, including the consequences of dark-matter accretion on compact stars and superradiant black-hole instabilities triggered by ultralight bosons. In the second part I will discus how observations of neutron stars can be used to constrain the dark-matter fraction in primordial black holes. In a close encounter with a neutron star, a primordial black hole can get gravitationally captured due to accretion, dynamical friction and tidal heating. Within a short time, the black hole is trapped inside the star and disrupts it by rapid accretion. The mere existence of old neutron stars limits the abundance of primordial black holes in the only mass range that was still unconstrained, thus suggesting that these objects cannot be the dominant dark matter constituent.

Title	Speaker(s)	Date	Talk Type	Info link
PHYS 781 - Lecture 15	Niayesh Afshordi	2014-11-06	Course	View details
PHYS 781 - Lecture 14	Niayesh Afshordi	2014-11-04	Course	View details
Spinning Black Holes and the Membrane Paradigm	Robert Penna	2014-10-16	Scientific Series	View details
Probing (beyond) general relativity with compact binaries and gravitational waves	Leo Stein	2014-10-02	Scientific Series	View details
Conformal field theories at non-zero temperature: operator product expansions, Monte Carlo, and holography		2014-09-19	Scientific Series	View details
Force-Free Electrodynamics around Extreme Kerr Black Holes	Maria J. Rodriguez	2014-09-11	Scientific Series	View details
A mechanism for vortex annihilation in two dimensional superfluid turbulence	Andrew Lucas	2014-05-22	Scientific Series	View details
Gravity waves from Kerr/CFT	Achilleas Porfyriadis	2014-05-15	Scientific Series	View details
Black holes without Lorentz symmetry	Thomas Sotiriou	2014-05-08	Scientific Series	View details
Geometric inequalities for black holes	Sergio Dain	2014-05-01	Scientific Series	View details
The characteristic structure of the Weyl curvature tensor and its utilization	Fan Zhang	2014-04-24	Scientific Series	View details
Compact objects as dark-matter probes	Paolo Pani	2014-04-17	Scientific Series	View details

Strong Gravity

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