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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

avr 2 2015 - 1:00pm
Room #: 400
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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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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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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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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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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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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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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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Jeudi nov 27, 2014
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A prime challenge to our understanding of galaxy formation concerns the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in the current ΛCDM paradigm. This is usually accounted for by assuming that energetic feedback from evolving stars confines dwarf galaxy formation to relatively massive halos spanning a narrow mass range. I will highlight a number of observations that may be used to test this assumption and discuss the puzzles and challenges that arise from this analysis.

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Jeudi nov 06, 2014
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In the context of gauge/gravity duality, it has been suggested that the far-from equilibrium strongly coupled dynamics encountered in ultrarelativistic heavy-ion collisions may be modelled as the collisions of black holes in asymptotic anti-de-Sitter spacetimes. I will present results from the evolution of spacetimes that describe the merger of asymptotically global AdS black holes in 5D with an SO(3) symmetry. The initial trapped regions are sourced by scalar field collapse and we are able to evolve through the ensuing black hole merger as well as subsequent ring-down.

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Jeudi oct 16, 2014
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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.

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