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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.

Modified Gravity, Dark Matter and Black Hole Shadows

Jeudi avr 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

Mardi 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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Even a tiny cosmological constant casts a long shadow

Jeudi mar 19, 2015
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Surprisingly, several basic questions in classical and quantum gravity, which were resolved some 40-50 years ago for zero $\Lambda$, still remain open in the $\Lambda >0$ case. In particular, for $\Lambda >0$, we still do not have a satisfactory notion of gravitational radiation or Bondi 4-momentum in exact general relativity, nor a positive energy theorem. Similarly, the standard constructions of in' and out' Hilbert spaces that we routinely use (e.g. in the analysis of black hole evaporation) do not extend to the $\Lambda >0$ case.

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Kerr black holes with scalar hair

Jeudi mar 05, 2015
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The Kerr metric of vacuum general relativity is expected to describe astrophysical black holes. Boson stars, on the other hand, are one of the simplest gravitating solitons, suggested as astrophysical compact objects, black holes mimickers and as dark matter candidates. Kerr black holes with scalar hair, found in [1], continuously interpolate between these two types of, per se, physically interesting solutions.

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Quantum Extremal surfaces

Jeudi fév 26, 2015
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I will describe a new proposal for defining the holographic

entanglement entropy at subleading orders in N (on the boundary) or

hbar (in the bulk). This involves a new concept of "quantum extremal

surfaces" defined as the surface which extremizes the sum of the area

and the bulk entanglement entropy. This conjecture reduces to

previous conjectures in suitable limits, and satisfies some nontrivial

consistency checks. Based on arXiv:1408.3203

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Black hole chemistry: thermodynamics with Lambda

Jeudi fév 19, 2015
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The mass of a black hole has traditionally been identified with its energy. We describe a new perspective on black hole thermodynamics, one that identifies the mass of a black hole with chemical enthalpy, and the cosmological constant as thermodynamic pressure. This leads to an understanding of black holes from the viewpoint of chemistry, in terms of concepts such as Van derWaals fluids, reentrant phase transitions, and triple points. Both charged and rotating

black holes exhibit novel chemical-type phase behaviour, hitherto unseen.

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Optical frequency standards for gravitational wave detection

Jeudi fév 12, 2015
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Gravitational waves (GW) imprint apparent Doppler shifts on the frequency of photons propagating between an emitter and detector of light. This forms the basis of a method to detect mHz GW using Doppler velocimetry between pairs of satellites [1]. The crucial component in such GW detectors is the frequency standard on board the emitting and receiving satellites. I will discuss how recent developments in atomic clock technology have led to devices that could be sufficiently sensitive to probe astrophysically interesting sources.

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Macroscopic Quantum Objects

Jeudi déc 18, 2014
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With recent advancement of experimental physics, macroscopic objects, which are typically well-described by classical physics, can now be isolated so well from their environment, that their quantum uncertainties can be studied quantitatively. In the research field called “optomechanics”, mechanical motions of masses from picograms to kilograms are being prepared into nearly pure quantum states, and observed at time scales ranging from nanoseconds to milliseconds.

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The Incompleteness of HRT

Jeudi déc 11, 2014
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In AdS/CFT, the HRT prescription relates the entanglement entropy of a region of a CFT to the area of an extremal surface in the dual AdS spacetime. But there exists a class of spacetimes in which the HRT prescription is ill-defined. These spacetimes consist of planar AdS wormholes containing an inflating region. I will introduce these so-called AdS-dS-wormholes, discuss how the HRT prescription fails in them, and suggest possible modifications to remedy the problem.

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Testing the membrane paradigm with holography

Jeudi déc 04, 2014
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One version of the membrane paradigm states that as far as outside observers are concerned, black holes can be replaced by a dissipative membrane with simple physical properties located at the stretched horizon. We demonstrate that such a membrane paradigm is incomplete in several aspects. We argue that it generically fails to capture the massive quasinormal modes, unless we replace the stretched horizon by the exact event horizon, and illustrate this with a scalar field in a BTZ black hole background.

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RECENT PUBLIC LECTURE

The Most Wanted Particle

Speaker: Jon Butterworth