The Institute of Cosmology and Gravitation hosted a two-week international conference at the end of September on the subject of brane-world gravity. The conference began with a three-day meeting which was followed by a workshop; about 80 delegates attended. The programme and slides from most of the talks can be found on the conference website, the URL for which is http://www.icg.port.ac.uk/brane06/
Invited speakers were Cliff Burgess, Cedric Deffayet, Gary Gibbons, Ruth Gregory, Panagiota Kanti, David Langlois, James Lidsey, Kei-ichi Maeda, Nick Mavromatos, Lefteris Papantanopoulos, Valery Rubakov, Misao Sasaki, Tetsuya Shiromizu, Jiro Soda, Kellogg Stelle and Takahiro Tanaka.
Brane-world models have been studied intensively for the last decade. Originally motivated by the existence of branes in string theory, brane-worlds have been of interest to the particle physics community because they offer new ways to explain hierarchies, and because of the new phenomenology for colliders and cosmic ray showers resulting from the possibility of a low Planck mass. They have also inspired relativists and cosmologists because they represent a very geometrical way to modify gravity and to change the cosmological history of the universe. The conference focussed on the gravitational and cosmological aspects of brane-worlds, the aim being to review recent progress in the field and to spark discussions and collaborations on the outstanding issues.
The themes discussed in the meeting were cosmology and the evolution of cosmological perturbations in brane-worlds, the Dvali-Gabadadze-Porrati (DGP) model and its possible problems with ghosts, the nature of black holes in brane-worlds and possible collider signatures, possible solutions to the cosmological constant problem using six-dimensional brane-worlds, and links between the phenomenological models and fundamental physics ideas like string theory. The meeting ended with a discussion of the outstanding issues, identifying projects for study during the workshop and beyond. About half of the delegates remained for the workshop. The workshop involved two talks each day with time in between for delegates to discuss the themes raised and form collaborations.
Some interesting subjects and outstanding questions were discussed, resulting in an advance in understanding and new collaborations. Effective actions are very useful tools in higher-dimensional physics, but it is important to understand in which circumstances they are effective; for Kaluza-Klein theories this is entirely understood but for non-homogeneous configurations there are additional subtleties. Understanding the quantum vacuum state for the early universe in the Randall-Sundrum model with inflation on the brane is important for predicting possible cosmological signatures; it was argued by some that the initial state could be and by others that it must be very close to the usual four-dimensional result. Perhaps most contentious was the issue of ghost states in the DGP model. Some delegates presented work showing that the model has a ghost state either in the spin-two or spin-zero sector, but it was argued by others that this does not necessarily invalidate the model because the energy scale associated is on the limit of where one can trust an effective four-dimensional description.
The local organising committee (Kazuya Koyama, Andrew Mennim and Sanjeev Seahra) would like the thank David Langlois, Roy Maartens, Kei-ichi Maeda, Lefteris Papantanopoulos, Misao Sasaki and David Wands for their help in the organisation of the conference; and the Institute of Physics, and the Particle Physics and Astronomy Research Council for providing financial support.