GRAVITY THEORY SEMINARS 2002
Abstracts:

Feb. 8, Fri., 1:30pm, Room 4208
David Mattingly, UMD

Astrophysical Constraints on Lorentz Breaking Dispersion Relations''

Current astrophysical observations can strongly constrain Lorentz breaking dispersion relations, even when the deviation from Lorentz symmetry is suppressed by the Planck energy. We discuss these constraints as well as new phenomena that may occur when Lorentz invariance is broken.

Feb. 8, Fri., 2:00pm, Room 4208
Jim van Meter, UC Davis

Post Minkowskian approach to binary black hole dynamics''

Mar. 4, Mon.  1:00pm, Room 4102
Akikazu Hashimoto, IAS Princeton

Observables of String Field Theory''

Mar. 22, Fri. 1:00pm, Room 4208
Bei-Lok Hu,  UMD

A Kinetic Theory Approach to Quantum Gravity''

We describe a kinetic theory approach to quantum gravity -- by which we mean a theory of the microscopic structure of spacetime, not necessarily equivalent to a theory obtained by quantizing general relativity. A figurative conception of this program is like building a ladder with two knotted poles: quantum matter field on the right and spacetime on the left. Each rung connecting the corresponding knots represent a distinct level of structure. The lowest rung is hydrodynamics and general relativity; the next rung is semiclassical gravity, with the expectation value of quantum fields acting as source in the semiclassical Einstein equation. We recall how ideas from the statistical mechanics of interacting quantum fields helped us identify the existence of noise in the matter field and its effect on metric fluctuations, leading to the establishment of the third rung: stochastic gravity, described by the Einstein-Langevin equation. Our pathway from stochastic to quantum gravity is via the correlation hierarchy of noise and induced metric fluctuations.
1. Deduce the correlations of metric fluctuations from correlation noise in the matter field;
2. Reconstruction of quantum coherence-- this is the reverse of decoherence -- from these correlation functions
3. Use the Boltzmann-Langevin equations to identify distinct collective variables depicting recognizable metastable structures in the kinetic and hydrodynamic regimes of quantum matter fields and how they demand of their corresponding spacetime counterparts.
This will give us a hierarchy of generalized stochastic equations -- call them the Boltzmann-Einstein hierarchy of quantum gravity -- for each level of spacetime structure, from the the macroscopic (general relativity) through the mesoscopic (stochastic gravity) to the microscopic (quantum gravity).

Apr. 5, Fri. 1:30pm, Room 4208
Stefano Liberati,  UMD

Towards the observation of Hawking radiation in Bose-Einstein condensates''

Acoustic analogues of black holes (dumb holes) are generated when a supersonic fluid flow forms a trapped region from which sound cannot escape. The surface of no return, the acoustic horizon, is qualitatively very similar to the event horizon of a black hole in general relativity. In particular Hawking radiation (a thermal bath of phonons with temperature proportional to the surface gravity'') is expected to occur. In this talk I shall consider a supersonic flow of a Bose-Einstein condensate. First I shall discuss how an effective geometry emerges for the propagation of phonons in these system. After I shall expose the opportunities and the pitfalls on the way to obtain an experimental confirmation of the analog Hawking effect.

Apr. 9, Tue. 12:30pm, Room 4208
Niall O'Murchadha,  University Collge, Cork, Ireland

Relativity without relativity: emergent time and the classical fields''

Jacobi introduced a parameter time action, called the Jacobi Principle, of the form sqrt{E -V }sqrt{T} in particle mechanics, where E is a constant, the total energy, V is the potential energy, and T the kinetic energy. He showed that it reproduced all of the standard equations but with an emergent time. I will consider an action quite like the Jacobi Principle but I will take the square roots of the individual kinetic energies of each particle and then sum. This is still reparametrisation invariant. This new action has a large set of constraints which are not mirrored by any symmetry of the action, and thus is not, in general, self-consistent. What it does do is act as a filter. If the particles do not interact everything works beautifully. The missing' symmetries appear in the solution because each particle trajectory turns out to be independently reparametrisable. The action has only a global time reparametrisation but the solutions have local time reparametrisation. This local square root action can be easily generalised to field theories and one can use it to pick out each of the standard fields of modern classical physics. If we choose the space of three-metrics as the configuration space we get general relativity, if we add a three-vector we get the Maxwell field, if we have several interacting vector fields, they must satisfy the Yang-Mills equations. In each case we go from a low symmetry action to a high symmetry solution just by demanding that the action is self-consistent.

Apr. 26, Fri.. 1:30pm, Room 4208

Matt Visser,  Washington University, St. Louis.

The quantum physics of chronology protection''

Classical general relativity (Einstein gravity) has many solutions'' that appear to be time machines''.  This makes most physicists very nervous. There are several ways of fixing things up. In particular, Stephen Hawking has formulated the chronology protection conjecture'' wherein quantum physics steps in to save the day (and so makes the universe safe for historians'').

I will assume the audience is familiar with Quantum Field Theory, and knows some basic General relativity.

May 10, Fri.. 1:30pm, Room 4208

Esteban Calzetta,  Buenos Aires University.

Thermalization in Large-N quantum field theory''

We shall discuss whether the next to leading order Large-N approximation correctly describes the thermalization process in quantum field theory. We shall consider a O(N) invariant scalar field theory with unbroken symmetry. We shall show that, in this approximation, the only translation invariant solutions to the Schwinger - Dyson equations are thermal. Together with the familiar Kadanoff-Baym approach to quantum kinetic theory, this shows that this approximation predicts thermalization, at least for certain initial conditions. This analytic result provides support for recent claims in the literature based on numerical evidence.

However, there is a glitch. If we adhere strictly to the next to leading order approximation, then the truncated theory, unlike the exact theory, allows thermalization with non vanishing chemical potential. To obtain the correct behavior, we must admit, within the perturbative Schwinger-Dyson equations, corrections to all orders in the density of states. But these are of the same order of magnitude than other terms previously discarded because of the Large N approximation.

We conclude that Large-N theory is a nice theoretical framework to study these issues, but probably not a realistic depiction of the phenomenon.

June 21, Fri.. 1:30pm, Room 420
8
Amit Ghosh, Penn State University.

Black hole entropy from semiclassical methods and  2nd law''

Symmetry based approaches to the black hole entropy are attractive because they are very general and do not depend on details of the quantization method. However we point out that, of the two available approaches, one faces conceptual problems (also emphasized by others), while the second contains certain technical flaws. We correct these errors and, within the new, improved scheme, calculate the entropy of 3-dimensional black holes. We find that, while the new symmetry vector fields are well-defined on the "stretched horizon", and lead to well-defined Hamiltonians satisfying the expected Lie algebra, they fail to admit a well-defined limit to the horizon. This suggests that, although the formal calculation can be carried out at the classical level, its real, conceptual origin probably lies in the quantum theory. We then discuss some open issues related to the 2nd law of black hole mechanics -- this is an on-going project.

Oct 4, Fri.. 1:30pm, Room 420
8
Eric Poisson, University of Guelph.

Radiation reaction in strong gravitational fields''

In this talk I will explore the fundamental physics of radiation reaction (the response of an orbiting body to the radiative loss of energy and angular momentum) in strong gravitational fields. After some motivational remarks I will introduce the concept of a gravitational self-force acting on a massive object moving in curved spacetime. I will then pull back and describe the physics of radiation reaction in the simpler context of electrodynamics, both in flat and curved spacetime. I will then return to the gravitational self-force and describe recent work (by me and many others) to compute it and use it in calculations of gravitational waves emitted by compact objects in orbit around massive black holes, one of the most promising type of  signals for detection by the Laser Interferometer Space Antenna.

Nov. 1, Fri.. 1:30pm, Room 420
8
Albert Roura, University of Maryland.

Einstein-Langevin equation as the large N limit for quantum metric fluctuations''

The so-called Einstein-Langevin equation has been proposed in order to extend the applications of the semiclassical theory of gravity to those situations in which the fluctuations of the stress tensor operator for the quantum matter fields are important. After briefly reviewing some aspects of nonequilibrium quantum field thoery, it will be explained how a stochastic description can be introduced to gain information on quantum properties of simple open systems. This will then be extended to the gravitational case by considering N quantum matter fields interacting with the metric perturbations around a given background spacetime. In particular, it will be shown that the correlation functions obtained using the Einstein-Langevin equation are equivalent to the leading order contribution in the large N limit to the correlation functions that would follow from a purely quantum treatment.

Nov 8, Fri.. 1:30pm, Room 420 8
Jong Hyuk Yoon, Konkuk University, Korea / U. of Chicago

Quasi-local dynamics of black holes''

A set of exact quasi-local conservation equations is derived from the Einstein's equations using the first-order (2,2)-formalism of general relativity of 4-dimensional spacetime. These equations are interpreted as quasi-local energy, linear momentum, and angular momentum conservation equations. In the asymptotic region of asymptotically flat spacetimes, it is shown that these quasi-local conservation equations reduce to the conservation equations of Bondi energy, linear momentum, and angular momentum, respectively. When restricted to the event horizon of a generic dynamical black hole, the quasi-local conservation equations coincide with the conservation equations studied by Thorne et al. All of these quasi-local quantities are expressed as invariant two-surface integrals, and geometrical interpretations in terms of the area of a given two-surface and  a pair of null vector fields orthogonal to that surface are given. Finally we apply these quasi-local conservation equations to study the dynamics of a generic black hole, and obtain the {\it dynamical} version of the first law of black hole mechanics, which states that the change in the internal quasi-local energy of a generic black hole is given by the positive-definite heat in-flux into the black hole minus the work done by the black hole itself in deforming and rotating its horizon.

Nov 15, Fri.. 1:30pm, Room 4208
Enric Verdaguer, University of Barcelona

`Cosmological Structure Formation in Stochastic Gravity''

Stochastic gravity describes the interaction of the gravitational field with quantum matter fields beyond the semiclassical limit. The Einstein-Langevin equations are introduced as a consistent set of equations for a first order correction to semiclassical gravity. The solutions of these equations  allow the computation of the symmetric two-point quantum correlations of the metric fluctuations induced by the quantum matter fields over the semiclassical background. This theory is particularly useful in the cosmological structure formation problem. A simple inflationary cosmological model driven by an inflaton field is described. It is shown that the two-point correlation functions for the gravitational fluctuations induced by the inflaton fluctuations lead to an almost scale invariant spectrum at large scales. We also comment on the application of the theory to the structure formation problem in an inflationary model not driven by an inflaton field such as the trace anomaly induced inflation model (Starobinsky inflation). The Einsten-Langevin equations for this problem have been computed.

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