**GRAVITY THEORY SEMINARS, Fall
2001**
*Abstracts:*

Sept. 7, Fri., 2:00pm, Room 4220
**Enric Verdaguer, **University
of Barcellona

**"Stochastic gravity and structure formation"**

Stochastic semiclassical gravity describes the interaction of the classical gravitational field with quantum matter fields beyond the semiclassical limit. The theory predicts that stochastic fluctuations of the gravitational field are induced by the the matter fields quantum fluctuations. We introduce the Einstein-Langevin equations as a consistent set of equations for a first order perturbative correction to semiclassical gravity. We then describe the application of the theory in a simple inflationary cosmological model, where the fluctuations of the inflaton field induce fluctuations in the gravitational field . We show that the correlation functions for the gravitational fluctuations lead to a scale invariant spectrum at large scales, in agreement with standard theories for cosmological structure formation.

Sept. 14, Fri., 2:00pm, Room 4220
**Bei-Lok Hu**, University
of Maryland

**"From Stochastic to Quantum Gravity: Applications
and Issues"**

Stochastic gravity is a newly discovered regime beyond semiclassical gravity, where in addition to the mean value of the stress energy tensor of quantum fields its fluctuations are also included as the source of a new stochastic semiclassical or Einstein-Langevin equation. This is the necessary foundation for asking questions concerning the validity of semiclassical gravity and the viability of inflationary cosmology based on the appearance and sustenance of a vacuum energy- dominated phase. It is also the proper framework for studying structure formation in the early universe, backreaction of Hawking radiation in late stage black hole dynamics, metric fluctuations and spacetime foams. In the first part of my talk I shall discuss applications of this new theory for treating quantum processes at the Planck scale. In the second part I shall address conceptual issues in stochastic in relation to quantum gravity highlighting fluctuations, correlations and coherence. Driven by the noise kernel which is the expectation value of the stress energy bitensor, stochastic gravity forms the lowest rung in the ladder to quantum gravity: The complete hierarchy of correlation functions recovers the full quantum theory. Thus the reconstruction of quantum coherence (the opposite of decoherence) becomes a central issue. We shall also examine the basis of the conventional point-defined quantum field theory from this new point-separated framework and redefine the meaning of regularization. We will also invoke ideas such as `semiclassical gravity as mesoscopic physics' and `general relativity as geometro-hydrodynamics' to ask meaningful questions to illuminate the pathway from stochastic to quantum gravity.

Sept. 21, Fri., 2:00pm, Room 4220
**Mike Ryan, **University
of Mexico

**"T for R and other cheap ways to make cosmologies"**

Since the Kantowski-Sachs cosmological metric is just Schwartzschild inside its horizon, it is natural to ask if this is applicable to other "black hole" metrics. It is, and several examples will be discussed. Other solution-generating techniques will also be mentioned.

Sept. 28, Fri., 2:00pm, Room 4220
**Erik Schnetter, **University
of Tuebingen

**"The Maya Project: Colliding Black Holes"**

The Maya project at Penn State is underway to provide the computational structure necessary to evolve black hole spacetimes in a stable manner. One long term goal is the simulation of two inspiralling black holes and the extraction of the produced gravitational radiation. At Maya's heart lies singularity excision; this enables us to use arbitrary slicings that do hit the singularity. After Maya's predecessor Agave managed to collide black holes, we went back to examining the single black hole case in some detail to gain more insight into the everpresent instabilities.

Oct. 5, Fri., 2:00pm, Room 4220
**Ted Jacobson**, University
of Maryland

**"Return of the Aether?"**

Recent cosmic ray observations and theoretical considerations both suggest the possibility that Lorentz invariance may not be a fundamental symmetry of nature. That is, there may be a preferred rest frame, defined by the substructure of spacetime. This talk will discuss approaches to incorporating such a preferred frame in an effective field theory within the framework of general relativity. Possible consequences for gravitation and cosmology will be presented.

Oct. 12, Fri., 2:00pm, Room 4220
**Larry Ford,** Tufts
University

**"Fluctuations of Spacetime Geometry and of
the Electromagnetic Field"**

This talk will discuss some of the physical effects of quantum fluctuations of the gravitational field. These fluctuations can arise either directly from the quantum nature of gravity, or indirectly from fluctuations of the stress tensor for matter fields, In either case, the operational meaning of these fluctuations is in the Brownian motion of test particles, both massive and massless. The Brownian motion of photons leads to the phenomenon of lightcone fluctuations, wherein the fixed lightcone of classical relativity is smeared out. The modification of the electric field fluctuations by a boundary (Casimir effect) will be discussed as an analog model for better understanding gravitational field fluctuations. Some results for the velocity and position fluctuations of charged and polarizable test particles near a conducting plate will be discussed.

Oct. 19, Fri., 2:00pm, Room 4220
**Hanno Sahlmann,** AEI
Potsdam

**"Classical behaviour in canonical quantum gravity:
Coherent states, random graphs, classical fields"**

Canonical (or loop-) quantum gravity represents a promising go at quantizing general relativity. With at least some parts of the theory well developed by now, it is a major challenge to understand whether the theory admits a regime where the gravitational field behaves almost classical. In this talk we will give an introduction to the framework of canonical quantum gravity. We will then propose a scheme to treat a classical field coupled to the quantized gravitational field in a semiclassical state and discuss the necessary ingredients. The free scalar field will serve as an example. We will conclude with a critical review of the scheme presented.

Oct. 26, Fri., 1:30pm, Room 4208
**Stefano Liberati,**University
of Maryland

**"Constraining Lorentz breaking dispersion relations
with cosmological observations"**

The structure of spacetime at the Planck scale
can lead to a breakdown of Lorentz invariance at high energies in the form
of non linear dispersion relations for fundamental particles. We discuss
some possible interactions allowed or influenced by modified dispersion
relations and analyze the observational constraints which can be imposed
using current astrophysical observations. We shall not assume any a priori
equality between the coefficients determining the amount of Lorentz violation
for different particle species. We shall

focus instead on constraints from three high
energy processes involving photons and electrons: photon decay, photo-production
of

electron-positron pairs, and vacuum Cerenkov
radiation. We find that cubic momentum terms in the dispersion relations
are strongly

constrained.

Nov. 2, Fri., 1:30pm, Room 4208

John Baker, NASA/Goddard

**"The Lazarus Project: Calculating Binary Black
Hole Coalescence Waveforms"**

We report on the first gravitational radiation
waveform calculations based on astrophysically plausible initial data.
These are carried out

using a new technique that combines the full
numerical approach to solve Einstein's equations in the truly non-linear
regime, with linearized perturbation theory around the final distorted
single black hole at later times. Starting with non-spinning binary black
holes from near the innermost stable circular orbit, we find that about
3% of the system's total mass-energy is emitted as gravitational waves
during coalescence.

Nov. 9, Fri., 1:30pm, Room 4208
**Marco Cavaglia', **MIT

**"M-theory cosmology in a nutshell"**

In superstring theories duality relations provide
a much richer setting than Einstein's theory for investigating cosmological
models. Observational cosmology is becoming more and more efficient in
constraining cosmological parameters. In this context, the
investigation of string cosmology is essential to test whether superstring
theory (or M-theory) really does describe our universe or is just an elegant
mathematical construction. In this talk we review the basic features of
superstring cosmology and discuss a simple cosmological model derived from
M-theory. Three assumptions naturally lead to a pre-big bang scenario:

(a) 11-dimensional supergravity describes the
low-energy world;

(b) non-gravitational fields live on a three-dimensional
brane;

(c) asymptotical past triviality.

Nov. 16, Fri., 1:30pm, Room 4208

F.W. Stecker, NASA/GSFC
(Lab. for High Energy Astrophysics)

**"The propagation of extragalactic high energy
gamma-rays and constraints on the breaking of Lorentz invariance"**

A class of active galaxies called "blazars" have been discovered to be sources of high energy gamma-radiation. Of these, some of the so-called BL Lacertae objects have been discovered to emit gamma-rays up to multi-TeV energies. Such gamma-rays are expected to interact with the low energy photons produced in galaxies and emitted into intergalactic space - particlularly the infrared photons from the reradiation of starlight photons by interstellar dust. These interactions result in the annihilation of the gamma-rays and the associated production of electron-positron pairs.

Recent space-based observations of this extragalactic infrared background by the COBE satellite, as well as infrared studies of the galaxies themselves, have given us a good idea of what the spectrum of the low energy photon background is which attenuates high energy gamma-rays coming from cosmic distances. Observations of multi-TeV blazar spectra which contain information about pair-production annihilation interactions can then be used to test for evidence of the breaking of Lorentz invariance, which would inhibit the attenuation of cosmic gamma-rays.

Nov. 30, Fri., 1:30pm, Room 4208
**Jim Stone, Chris Reynolds, Cole Miller,**University
of Maryland

**"Astrophysical black holes: three mini-talks"**

Jim Stone: MHD Simulations of accretion flows around black holes

It is now possible to compute from first principles the structure and evolution of accretion flows around black holes over several decades in radius using direct numerical MHD simulations. I will describe results from both 2D and 3D simulations, and compare and contrast these results with previous steady-state solutions.

Chris Reynolds: Probing the immediate environment of supermassive black holes

Strong-field General Relativity is now an observational science. Modern X-ray telescopes allow us to study the immediate environment of the massive black holes which reside at the centers of probably all galaxies. I will briefly describe these studies and discuss possible future research directions.

Cole Miller: Gravitational waves from black holes in globular clusters

As ground-based and space-based gravitational wave detectors are constructed, interest is being focused on astrophysical sources of gravitational radiation. I will discuss the unique properties and future challenges of a new proposed class of sources: intermediate-mass black holes in globular clusters.

Dec. 7, Fri., 1:30pm, Room 4208
**Mike Ryan, **National University of Mexico

**"The geometrical formulation of quantum mechanics"**

Numerous variations on possible geometries of the state space of non-relativistic quantum mechanics have appeared in the literature. This seminar will focus on these ideas, where reduced state space (CP^N) is given a Kahler structure. This formalism, while it does not extend ordinary quantum mechanics, provides insight into the structure of state spaces, and may, in the future, point the way toward new quantum theories.

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