GR17 - Plenary Lectures
Miguel
Alcubierre (Mexico City)
- Status of Numerical
Relativity
I briefly discuss the status ans prospects of current research in numerical relativity. The talk will focus both on recent developments in the theoretical framework that underlines numerical work in relativity and also on recent results in the simulation of astrophysical systems, with particular empashis in simulations of black hole binaries. In the context of the theoretical developments, I will discuss important areas of reseach related to the choice of the formulation of the Einstein equations used for numerical simulations, on the choice of gauge conditions, and on the important problem of the choice of boundary conditions.
- Loop Quantum
Gravity, Quantum Geometry and Spin Foams
The last few years have seen the techniques of loop quantum gravity applied to a growing number of problems. They have been used to compute the entropy of various sorts of black holes, including those with non-minimally coupled matter, for which the entropy is not proportional to the area. Perhaps more importantly, loop quantum gravity has given us a clear {\it picture} of the quantum geometry of the horizon, which accounts for the microscopic degrees of freedom responsible for black hole entropy. There are tantalizing connections to Hod's work on quasinormal modes, but these remain mysterious. Combined with traditional ideas on quantum cosmology, loop quantum gravity has led to new insights on how quantization can eliminate the singular behavior of geometry at the big bang - perhaps with testable consequences. Predictions of Lorentz symmetry violation may also be experimentally testable, but these remain controversial. Finally, the dynamics of the theory is being studied with the help of spin foam models. My talk will survey all this work with a bare minimum of technical details.
- Results from GW Experiments (LIGO, VIRGO, GEO, TAMA, bars)
This talk with review the status of both the resonant bar detectors and gravitational wave interferometers worldwide. The present status of searches for gravitational waves from the different types of sources will be reviewed and the prospects for the future will be discussed.
Lars Bildsten (KITP,
UC Santa Barbara)
- Advances in
Relativistic Astrophysics
The advent of more sensitive X-ray telescopes in space has allowed for new discoveries, as well as detailed measurements of the properties of compact objects. These breakthroughs include the first gravitational redshift measurement of a neutron star, evidence that accreting neutron stars have a limit to their rate of rotation, and data on neutron star cooling that constrains their interior properties. All of these raise new puzzles for the theoretical astrophysics community, and provide new opportunities for the gravitational wave detectors presently online. The masses of stellar mass black holes are now regularly measured to be in the 7-15 solar mass range, and there are indirect indication of rapid rotation. I will close by showing some of the remarkable data from the recently discovered double radio pulsar, PSR J0737-3039.
- Recent Results in
Mathematical Relativity
I will review the latest results in the global structure of solutions of the vacuum Einstein equations. More particularly, I will briefly describe new constructions of solutions of the constraint equations, and discuss the improved understanding of asymptotic properties of the solutions, new stability results, as well as progress in strong cosmic censorship.
Prof. Chrusciel's participation is sponsored by Classical and Quantum Gravity.
- The information paradox for black holes
The Euclidean path integral over all topologically trivial metrics can
be done by time slicing and so is unitary when analytically continued to the Lorentzian.
On the other hand, the path integral over all topologically non-trivial metrics is
asymptotically independent of the initial state. Thus the total path integral is unitary
and information is not lost in the formation and evaporation of black holes. The way the
information gets out seems to be that a true event horizon never forms, just an apparent
horizon.
(A transcript of this talk and a commentary are available at this URL. Opinions expressed are those of the author; this link appears with his kind permission.)
- Gravity and Cosmology in Braneworlds
In the last few years, a lot of attention has been devoted to the possibility that our observable universe is a $1+3$ submanifold (the "brane"), embedded in a higher dimensional spacetime (the "bulk"), with ordinary matter trapped on the brane.
Focusing on models with our universe as a self-gravitating brane in a five-dimensional "bulk" spacetime, i.e. with a single extra dimension, this talk will give an overview of their gravitational and cosmological properties.
Donald
Marolf (UC Santa Barbara)
- Is holography a principle?
The AdS/CFT correspondence of string theory is an impressive conjecture relating gravitational dynamics in a high dimension to non-gravitational dynamics in a lower dimension. Detailed evidence in its favor continues to increase with the passage of time. Various arguments involving black holes have in the past been used to make the controversial suggestion that something similar should be true of string theory more generally, and in fact of any theory of quantum gravity. This talk will review the situation, emphasizing modern developments including both recent further support for AdS/CFT and recent discussions of loopholes in the more general arguments.
- Advanced
Gravitational-Wave Detection Technologies
I will introduce the technical challenges of future gravitational-wave detectors and plans for improving the sensitivity by over an order of magnitude compared to first-generation detectors. I will describe the path to quantum-limited interferometers and prospects for going beyond the quantum limit.
- The Cosmological Tests
The $\Lambda$CDM cosmology passes tests that have become quite demanding. This means that we have, among other things, significant empirical checks of general relativity theory extrapolated by more than fifteen orders of magnitude from the length scales of the precision tests. This deeply impressive advance will be further improved by better understanding of the physics of structure formation that underlies modern versions of the cosmological tests. From the half dozen models for structure formation that were under discussion a decade ago the community has converged on the Cold Dark Matter picture, because it is readily adjusted to fit the observations, and the community has agreed to accept Einstein's cosmological term, $\Lambda$, or a component in the stress-energy tensor that acts like it, to avoid a conflict between the evidence for small mean mass density and small space curvature. We know these choices go beyond curve fitting, because there are some heavily redundant constraints, but there are also open issues. One is whether the old ideas about structure formation might still apply, as subdominant but not negligible corrections to the present standard model. Another is the physics of the dark matter and dark energy, which in the standard model amount to 95\%\ of the total mass. Physics in the remaining 5\%\ is simple, at least in comparison to the spectacular complexity of its expression. In the standard cosmology physics in the dark sector is so very simple that its expression is simple. Is this the way it really is, or are we getting away with a crude approximation within still quite schematic empirical constraints? The next generation of cosmological tests will advance the studies of such issues.
- Relativity and Astrophysics with LISA
We describe how LISA can be used for testing and discovery in astrophysics and relativity. Particular emphasis will be placed on how it can be used to develop an understanding the evolution, properties, and static and dynamic space-time structure of black holes. We will also briefly describe what LISA, and a follow-on mission currently under study, can teach us about cosmology and the early universe.
- The Gravitational Self-Force
The gravitational self-force describes the effect of a particle's own field on its motion; while the motion is geodesic in the test-mass limit, it is accelerated to first-order in the particle's mass. I will review the foundations of the self-force, and show how an infinite field can be unambiguously decomposed into a singular piece that exerts no force, and a smooth remainder that is responsible for the acceleration. I will also describe the recent effort, by a number of workers, to compute the self-force in the case of a small mass moving in the field of a much more massive black hole.
The context of this work is provided by the Laser Interferometer Space Antenna, which will be sensitive to low-frequency gravitational waves. Among the sources for this detector is the motion of small compact objects around massive (galactic) black holes. To calculate the waves emitted by such systems requires a detailed understanding of the motion, beyond the test-mass approximation.
Joe Polchinski (UC
Santa Barbara)
- Cosmic F and D Strings
It is possible that superstrings, as well as other one-dimensional branes, could have been produced in the early universe and then expanded to cosmic size today. I discuss the conditions under which this will occur, and the signatures of these strings. Such cosmic superstrings could be the brightest objects visible in gravitational wave astronomy, and might be distinguishable from gauge theory cosmic strings by their network properties.
- Quantum Computation
A quantum computer, which processes a quantum state rather than classical bits, could easily perform calculations that would take far longer than the age of the universe on today’s supercomputers. But constructing practical quantum computers will be tremendously challenging. A particularly daunting difficulty is that quantum computers are far more susceptible to making errors than conventional digital computers. The theory of fault-tolerant quantum computation prescribes how a robust subsystem of a larger quantum system can be constructed and reliably manipulated. A large-scale fault-tolerant quantum computer will be a highly unusual quantum many-body system, having much in common with topological quantum field theory and loop quantum gravity.
- Black Holes in
Active Galactic Nuclei
It has long been suspected that black holes are implicated in some of the most powerful and energetic cosmic phenomena, but the evidence has firmed up in recent years. Stellar-mass holes, associated with cosmic x-ray sources and (probably) gamma-ray bursts, form as the endpoint of the evolution of massive stars. Even more interestingly, the centres of almost all galaxies seem to be the hosts of supermassive holes, of millions or even billions of solar masses. These holes are believed to be the outcome of successive mergers of smaller holes, and also to have gained mass via accretion of gas and capture of stars. The latter two processes provide the energy sources for active galactic nuclei and quasars. Relativistic jet-like outflows are generated close to the holes, perhaps by mechanisms that extract energy from their spin. The challenge for astrophysics is to understand the formation of these huge holes, and to relate them to the hierarchical processes that gave rise to galaxies; also to account for the phenomena associate with active galactic nuclei.
For relativists, the greatest interest lies in seeking evidence that these holes are indeed described by the Kerr metric, as theory predicts. So far, the progress towards this latter goal is limited: the best evidence for general relativity still comes from 'weak field' tests. But there are genuine hopes of discovering phenomena that will probe the strong-gravity regime.
- The implications
for cosmology of WMAP data
The primary goal of the WMAP mission is to produce a high-fidelity all-sky polarization-sensitive map of the CMB radiation to determine the cosmology of our Universe. WMAP has produced a full sky map of the microwave sky in 5 frequencies, with a resolution a factor 30 higher than the previous full sky map as produced by the COBE satellite in 1992. This is the cleanest picture of the early Universe; the structures on the CMB --the pattern of hot and cold spots-- carry information about the composition, geometry, age, etc. of our Universe.
We will review the implications for cosmology of WMAP data. The highlight is that cosmology now has a standard cosmological model. With only 6 parameters the model fits not only WMAP data remarkably well, but also a host of other astronomical observations.
*
The LOC is pleased to announce that as with previous editions of the conference, there
will be a special session funded by the Gravity Research Foundation consisting of the
lectures by the speakers indicated. The LOC thanks GRF and Louis Witten for this support.