HEP/GR Informal Seminar Series for Graduate Students
Place: CMS, Potter Room (B1.19)
Time: Wednesdays, 4.15 - 5pm (unless otherwise stated.)
Organisers: Mike Blake and Kenny Wong
A series of seminars aimed at graduate students in the High Energy Physics and General Relativity groups. It will contain talks by the students both on original research and as pedagogical introductions to more specialised topics.
All are welcome.
Lent 2013
13-March-2013: The wave equation as a poor man's linearisation of the Einstein
equations
Damon Civin, DAMTP
Abstract: The Einstein field equations (EFE) can be written in harmoniccoordinates as a system of quasilinear wave equations. This allows for thestudy of the (EFE) within the theory of hyperbolic PDE. In particular, this can be used to study well-posedness and stability of the (EFE) for a large class of initial data.
I'll focus on the linear wave equation, which is the prototype hyperbolic PDE. It can also be viewed as a "poor man's linearisation" of the (EFE). Therefore the study of boundedness and decay of solutions of the wave equation on a fixed black hole background are a first step towards stability of the background as a solution of the (EFE).
I'll start off by discussing this "poor man's linearisation" and recalling what well-posedness means. We'll then move onto the highlights of the proof of well-posedness the Cauchy problem for the wave equation.
Along the way, we'll run into some Sobolev spaces and energy estimates, and I will try to convince you of their power and naturality. Time permitting, I'll discuss the heuristics of my work on the linear stability of
subextremal Kerr-Newman spacetimes.
06-March-2013: D-branes
Arnab Rudra, DAMTP
*Please note Arnab's talk is 4pm in MR9*
Abstract: String Theory contains a lot of objects other than the fundamental strings. For example, D Branes which are extended objects in space-time, is one of the key ingredients in string theory. They break half of the space-time supersymmetry, ( ie. BPS states) and also play a crucial role in understanding dualities in string theory. In this talk, I will try to discuss the basics D-Brane physics and their interpretation in perturbative string theory.
04-Feb-2013: Modified Gravity - Great or Graveyard?
Jeremy Sakstein, DAMTP
*note start time is 4pm*
Abstract: The increasing need for dark phenomena to explain the cosmic acceleration and clumping of matter has lead to a recent resurgence of interest in modified gravity theories. Modified theories can help with these phenomena to some extent, however when one looks at our own solar system these theories are in blatant violation of our experiments. In this talk I will introduce these problems and show how it is possible to construct theories with interesting dynamics that can satisfy solar-system bounds. These theories posses "screening mechanisms" which have deep roots in the underlying symmetries of the field theories describing spin-2 particles.
30-Jan-2013: Quantum electrodynamics and the on-shell renormalization scheme
Kenny Wong, DAMTP
*note start time is 4pm*
Abstract: Renormalization gets too much bad press. My Cavendish friends feel uneasy about it. My pure mathmo friends say it's a complete joke. The problem is that renormalization can come across as a desperate attempt to rescue a sick theory - by "chopping off" infinities at will. That's nonsense. There's nothing artificial, unnatural or contrived in renormalization. Renormalization is a subtlety, not a fix. Field theory has no infinities, and if you calculate an infinity, you should question your physical reasoning rather than tweak the theory itself. I'm going to talk about something called the "on-shell renormalization scheme". It's conceptually different from the minimal subtraction renormalization scheme that we learned in AQFT. What's great about the on-shell scheme is that it makes the whole renormalization business feel natural and inevitable. I'll also talk about the famous precision tests of QED, including the anomalous magnetic moment of the electron that QED gets right to 11 significant figures. Finally I'll explain how to deal with the IR divergences in QED - and trust me, this is really, really beautiful.
Michaelmas 2012
10-Oct-2012: An AdS/CFT - Boson/Fermion Double Duality
Kenny Wong, DAMTP
Abstract: It is possible for a physical system to have two equivalent mathematical descriptions, each providing a different perspective on its properties. The AdS/CFT duality, for example, tells us that strongly-coupled systems can be described in terms of classical fields in one higher dimension. The dual description gives us an opportunity to answer questions about these systems that could be answered using conventional techniques in QFT. Bosonization is a duality between bosons and fermions in 1+1 spacetime dimensions. Since bosons are easier to handle than fermions, this duality is a valuable tool in string theory and condensed matter. The subject of this talk is a strongly-interacting system with a finite density of fermions. Using both AdS/CFT and bosonisation, one discovers that it has an elegant, intuitive description as a star of solitons in anti-de Sitter space.
17-Oct-2012: Large N Gauge Theory
Mike Blake, DAMTP
Abstract: At low energies QCD is strongly coupled and so an expansion in the coupling constant is not valid. In 1973 't Hooft proposed an imaginative alternative - one can study the theory via an expansion in the number of colors, an expansion in '1/N'. The study of the first terms of this expansion - large N gauge theory - lies at the heart of modern theoretical physics. In particular it underpins the AdS/CFT correspondence of Maldecena. In this talk I shall provide a pedagogical introduction to large N QCD following the classic approaches of Coleman and Witten. I shall then briefly review modern research into a web of large N equivalences that connect different gauge theories.
24-Oct-2012: A low energy effective introduction to string theory (and generalised geometry)
*note the unusual location of MR4*
Chris Blair, DAMTP
Abstract: My original hope for this seminar was to talk about the notion of "generalised geometry" and how it is applied in string and M-theory. Though this is still roughly going to be the goal of the talk, the principal focus will be to give a very basic but hopefully helpful introduction to some aspects of string theory (the closed bosonic string only, supersymmetry, branes etc will not be mentioned). The thinking behind this is to make the talk accessible to graduate students who don't work in string theory and haven't done the Part III course. The talk will be "low energy" in that you will not have to think very hard during it, and "effective" in that I will be giving an outline and ignoring subtleties of higher order. Time permitting I will be able to present enough stringy ideas to show why generalised geometry is interesting and useful.
31-Oct-2012: 3d Strings
Alasdair Routh, DAMTP
Abstract: String Theory is usually studied in D=26 (or 10), as in other dimensions Lorentz invariance is generally lost after quantisation. Recently, Paul Townsend and Luca Mezincescu showed that quantum theories of Lorentz invariant strings also exist in 3 dimensions and have interesting properties, and I have worked with them this year to extend these results.
I will discuss why traditional arguments about Lorentz invariance fail for D=3, why this case might be interesting, and what happens.
Very basic knowledge of bosonic string theory will be needed to get the most out of the talk, the quantisation of the bosonic string in D=26 should be enough.
07-Nov-2012: Quantum theory of massless spin 1 & spin 2 particles
*note the unusual location of MR4*
Arnab Rudra, DAMTP
Abstract: The role of symmetry in physics is to constrain the set of all possible theories that can describe the nature.These symmetry principles are much more powerful in the quantum version of a theory compared to their classical counterpart. For example, the consequence of Poincare symmetry which, by our present understanding, is an exact symmetry of nature, tells us that a massless relativistic quantum particle in 4 dimensions can have only 2 degrees of freedom. The purpose of this talk is to demonstrate how a quantum theory of massless particles is completely constrained by the symmetries of nature. More specifically, one can show that the theory of massless spin 1 particle has to be a yang-Mills theory and the theory of massless spin 2 particle has to be a theory of gravity.
14-Nov-2012: Energy and Stability in GR
*note start time is 4pm*
Joe Keir, DAMTP
Abstract: It is often stated that various positive energy theorems tell us something about the stability of certain spacetimes in general relativity. In this talk I will first explain why a naive interpretation of such a statement fails. I will then go on to show how energy can correctly be related to (linear) stability properties. I will construct the appropriate "energy" functional for use with linear perturbations of initial data, and briefly discuss some of its properties and the restrictions that must be made on the space of initial data perturbations. Along the way I will indicate how to prove the first law of black hole mechanics, for arbitrary variations, in an arbitrary diffeomorphism invariant theory. Finally, I will argue that this energy is related to stability and instability.
21-Nov-2012: Generalised geometry and 11-dimensional supergravity
*note start time is 4pm and the location is MR 4*
Emanuel Malek, DAMTP
Abstract: I will discuss how U-dualities arise in 11-dimensional supergravity and how generalised geometry allows a reformulation of the action so that it is manifestly invariant under dualities. If time permits, I will introduce the concepts of ''non-geometric'' backgrounds (U-folds).
28-Nov-2012: Symmetries of Inflation
Valentin Assassi, DAMTP
Abstract: First introduced to solve several shortcomings of the standard model of cosmology, inflation provides an elegant explanation for the origin of the cosmic microwave background anisotropies and the large scale structure. It is indeed believed that quantum fluctuations produced during inflation seeded the primordial inhomogeneity of our universe. In this talk, I will give a short review of inflation and explain how quantum fluctuations produced during inflation relate to the observed inhomogeneity in the universe. Then, after reviewing the symmetries of adiabatic fluctuations, I will show how these symmetries constrain the low-momentum physics of these perturbations. Finally, I will discuss the possibility of probing the spectrum of particles present during inflation through the statistics of the primordial fluctuations.