# 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.

## 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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## Even a tiny cosmological constant casts a long shadow

Thursday 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

Thursday 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

Thursday Feb 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

Thursday Feb 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

Thursday Feb 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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## Effective potentials and morphological transitions for binary black-hole spin precession

Thursday Jan 08, 2015
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Generic binary black holes have spins that are misaligned with their orbital angular momentum. When the binary separation between the black holes is large compared to their gravitational radii, the timescale on which the spins precess is much shorter than the radiation-reaction time on which the orbital angular momentum decreases due to gravitational-wave emission. We use conservation of the total angular momentum and the projected effective spin on the precession time to derive an effective potential for BBH spin precession.

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## Macroscopic Quantum Objects

Thursday Dec 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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