Strong Gravity

This series consists of talks in areas where gravity is the main driver behind interesting or peculiar phenomena, from astrophysics to gravity in higher dimensions.

Seminar Series Events/Videos

Currently there are no upcoming talks in this series.

Hydrodynamics and the eightfold way to dissipation

Thursday Nov 12, 2015
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This is the first of two talks on recent advances in our understanding of hydrodynamics as a generic theory of near-thermal dynamics of density matrices. In this talk I will focus on the structure of the hydrodynamic gradient expansion subject to the Second Law of thermodynamics. I will present an eightfold classification scheme and an explicit solution at all orders in derivatives of hydrodynamic transport consistent with the Second Law.

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Near-Horizon Magnetospheres of Rapidly Spinning Black Holes

Thursday Oct 29, 2015
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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. Typically these equations can only be solved numerically, but they become amenable to analytic solution in the extremal limit when the black hole achieves maximal angular momentum and an infinite-dimensional conformal symmetry emerges in the high-redshift region near its horizon.

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Exploring astrophyiscs with gravitational waves

Thursday Oct 22, 2015
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Advanced LIGO has recently started operating, with the promise that discoveries of gravitational wave transients will begin within in the next few years. As astrophysical observatories, LIGO and similar experiments may inform our knowledge of a variety of topics, including heavy element formation, dynamical capture of black holes, and the neutron star equation of state. In this talk, I will highlight recent efforts to quickly identify and distribute transients found with LIGO, and explore some of the astrophysics questions we hope to address.

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Conformal defects in gravity - backreacted Dirac delta sources

Thursday Oct 15, 2015
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In this talk I will describe numerical constructions of gravitational

duals of theories deformed by localized Dirac delta sources for scalar

operators. We perform two different constructions, one at zero and

the other at nonzero temperature. Surprisingly we find that imposing the preservation of scale

invariance at zero temperature requires the bulk scalar self-interaction potential to be

the one found in a certain Kaluza-Klein compactification of 11D supergravity.

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Thursday Sep 24, 2015
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We introduce the notion of a local shadow for a black hole and determine its shape for the particular case of a distorted Schwarzschild black hole. Considering the lowest-order even and odd multiple moments, we compute the relation between the deformations of the shadow of a Schwarzschild black hole and the distortion multiple moments. For the range of values of multiple moments that we consider, the horizon is deformed much less than its corresponding shadow, suggesting the horizon is more `rigid'.

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The gravity of fundamental fields

Thursday Sep 10, 2015
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Scalar fields are a useful proxy for other complex interactions, but also an attractive extension of General Relativity and a possible dark matter component. I will discuss some aspects of the gravitational interaction of scalar fields, in particular (i) the formation and growth of self-gravitating structures and their interaction with compact stars. and (ii) superradiance around black holes and how it can be used to constraint particle masses.

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Geometries of any dimension without twist and shear

Friday May 29, 2015
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We study a general class of D-dimensional spacetimes that admit a non-twisting and shear-free null vector field. This includes the famous non-expanding Kundt family and the expanding Robinson-Trautman family of spacetimes. In particular, we show that the algebraic structure of the Weyl tensor is I(b) or more special, and derive surprisingly simple conditions under which the optically privileged null direction is a multiple WAND. All possible algebraically special types, including the refinement to subtypes, are thus identified.

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Gravitational collapse in the ghost-free gravity

Thursday May 28, 2015
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We discuss a problem of a black hole formation in the ghost-free gravity. We demonstrate how a non-local modification of gravity equations regularizes static and dynamical solutions. We focus on the problem of a collapse of small masses in the ghost-free gravity, and demonstrate that there exists a mass gap for mini-black-hole formation in this model.

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Modified Gravity, Dark Matter and Black Hole Shadows

Thursday Apr 02, 2015
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A modified gravity (MOG) theory has been developed over the past decade that can potentially fit all the available data in cosmology and the present universe. The basic ingredients of the theory are described by an action principle determined by the Einstein-Hilbert metric tensor and curvature tensor. An additional massive vector field φµ is sourced by a gravitational charge $Q=\sqrt{\alpha G_N}M$, where $\alpha$ is a parameter, $G_N$ is Newton's gravitational constant and $M$ is the mass of a body.

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Tensor algebra made easy with Mathematica

Tuesday Mar 24, 2015
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Calculations in General Relativity inevitably involve tricky manipulations of tensor equations. In many cases, the tensor algebra involved is at best tedious and fraught with error, and at worst impossible. It is, however, ideally suited to implementation in a computer algebra system such as Mathematica. In this talk I will show how the xAct tensor algebra package can be used to make light work of difficult tensor calculations.

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RECENT PUBLIC LECTURE

Amber Straughn: A New Era in Astronomy: NASA’s James Webb Space Telescope

Speaker: Amber Straughn