Report to the 1999 General Assembly for 1996-99
AC.2, in its form as the International Society on General Relativity and Gravitation (founded 1971), is flourishing, with around 400 members, including an increasing proportion of life members. Its finances are stable and it was able to contribute a significant amount from its own resources towards participation in the conference GR15. It continues to publish the journal General Relativity and Gravitation by arrangement with Plenum Press.
The 15th triennial conference, GR15, was held at the Inter-University Center for Astronomy and Astrophysics (IUCAA), Pune, India, on Dec 16-21, 1997, and was, like previous meetings in the series, successful in attracting about 550 participants, including a very high proportion from developing countries (especially, of course, India itself). With IUPAP and AC.2 support and other sponsors, about 200 of these were financially aided in whole or part. The program contained 17 plenary lectures and there were 50 timetabled sessions for contributed papers.
A special session was sponsored by the Gravity Research Foundation, and attracted three excellent speakers chosen to broaden the focus of the meeting, Profs. M. Berry, M. Longair and R. Narayan. Another highlight was a public lecture by Roger Penrose which attracted an audience of 1500 and an additional 600 who could not be fitted in and were instead offered a video repeat two days later. The Proceedings volume has already been published by IUCAA. The next conference, GR16, will be in 2001 in Durban, South Africa.
In general and committee meetings at GR15, AC.2 decided to take a number of initiatives including sponsorship of the email information distribution previously run unofficially by M. MacCallum, setting-up a Web page (http://www.maths.qmw.ac.uk/grgsoc/), and the possible reprint of valuable but unobtainable materials and other publishing initiatives.
The quality and quantity of material in the GRG journal is improving, and it is now available electronically. One new venture of considerable value there is the reprint, with commentary, of important old papers not readily accessible otherwise.
The FORTH foundation has agreed with AC.2 arrangements for the future management of the Xanthopoulos Prize. The prize for 1997 was awarded to M. Choptuik for his pioneering work on critical phenomena in gravitational collapse.
The main new developments were reviewed at GR15 and detailed accounts appear in the Proceedings (see above).
The observational evidence for black holes has become better, not only with new observations but with new theoretical understanding of the accretion mechanisms, which has led to improved data fits from advection-dominated flow models that work only if accreted material falls into a black hole rather than hitting a stellar surface. From these arguments and from earlier types of evidence for a large mass in a compact region, there are a number of examples of good black hole candidates. These include stellar mass black holes in X-ray sources within the Galaxy and the Large Magellanic Cloud, and supermassive black holes in active galactic nuclei (AGNs). Galaxies with strong black hole candidates at their centres include the Milky Way, M87, and NGC 4258 (where there is a water maser which has been probed to 0.3 light years from the centre).
Interferometric gravitational-wave detectors are now under construction in the US (LIGO) and Europe (VIRGO and GEO-600), and there may be others of similar size in Japan (TAMA) and Australia (AIGO). A number of cryogenic detectors are also in use or development. The newer detectors, in particular LIGO, should, for the first time, reach levels of sensitivity sufficient to detect the anticipated astrophysical sources (and it will therefore be embarassing to theorists if they see nothing).
The value of the detectors will be much enhanced if accurate predictions of radiation output of the possible sources can be made which can then be used as templates with which to filter the data. The theoretical work required is concerned with simulations of black hole and compact object coalescence and collision rather than the black hole accretion discussed above: inspiralling black holes seem to be especially promising as detectable sources, and coalescing black holes may be related to the recently discovered gamma-ray bursters. Important advances in related analytic and numerical studies have been made.
The numerical studies of colliding or orbiting black holes have been boosted by the Grand Challenge, in which supercomputers are being used to provide much improved simulations. This has entailed advances in numerical methods for the problems, and input for this has come from a coordinated effort of workers in many centres.
An unexpected advance in the theory of black hole formation has been the discovery, from numerical simulations, of critical phenomena in the behaviour of collapsing spherical systems close to the critical value for black hole formation. This was first observed in collapse of scalar fields and has subsequently been seen in a number of other studies with different assumptions.
Striking in the last few years have been the theoretical discoveries by many workers of new types of black holes. It had been assumed for a long time that the only parameters of a black hole in equilibrium are mass, electric (and magnetic monopole) charge, and angular momentum. Since the turn of the decade black hole types with other parameters have become known: colored black holes, black holes with Skyrme number, black holes with nonabelian magnetic monopole, black holes hybridized with cosmic strings, and so on. These are interesting, not so much for their astrophysical implications (which are doubtful), nor for their stablity (as most are unstable) but because they tie neatly together ideas from gravitation and from particle physics. And the Skyrme black hole, stable to all appearances, may be another object physicists will have to reckon with.
In cosmology, there are new (but somewhat conflicting and hence still uncertain) measurements of the main parameters of expansion and its deceleration (Ho and q), in particular from observations of supernovae of type Ia. The well-known COBE data on the microwave background will be supplemented in coming years by new satellite date from the MAP and Planck projects. This, with the Hubble space telescope data, should give accurate measurements of the main cosmological parameters. Present data seem to challenge the standard view of the last few years, that the universe consists principally of cold dark matter which underwent a period of inflation early in the Big Bang, and there have been substantial amounts of recent theoretical work on the effects of different matter contents, and on refining the calculations of structure formation. In particular detailed modelling of angular variations of the microwave background has been carried out. There are also important consequences of studies of gravitational lensing, especially the use of weak lensing (where no multiple images are produced, but only distortions of the single image) in observations of distant objects, which can be used to infer cosmological parameters. This relies on statistical analyses of large numbers of weakly-lensed objects.
There has been exciting new progress in several of the approaches towards a quantum theory of gravity, most spectacularly in developments of superstring theory. Here dualities have been found which relate strong field and weak field limits, allowing results in the non-perturbative regime to be deduced from perturbation theory, and which show that the various superstring theories possible can all be regarded as aspects of some general underlying theory, known as M-theory. Within this context, the dynamics of D-branes (D-dimensional objects in some higher-dimensional space) have been extensively studied and lead to surprisingly successful recovery of black hole phenomena including black hole entropy. Other approaches which have continued to progress include those based on Ashtekar's loop variables.