Quantum Gravity, Black Holes and Strings

Date :From 2014-05-26 To 2014-07-04
Advisory committee :
Local coordinators :Cai, Rong-Gen (蔡荣根),Chen, Bin (陈斌),Feng, Bo (冯波), Li, Miao (李淼), Lu, Hong (吕宏), Lu, Jianxin (卢建新)
International coordinators :G.W. Gibbons (University of Cambridge, UK),K. Maeda (Waseda University, Japan),H. Nicolai (Max Planck Institute, Potsdam) C.N. Pope (Mitchell Institute, Texas A&M University),E. Sezgin (Mitshcell Institute, Texas A&M University), A. Strominger (Harvard University)

Black holes can be considered as the most fundamental objects in a theory of gravity, and as such they provide powerful probes for investigating the basic features of the theory.  One of the most important current areas of activity in black hole physics is to try to gain a microscopic understanding of the Bekenstein-Hawking entropy of black holes, and to try to clarify the questions surrounding the information-loss problem.  In particular,studying the microscopic black-hole entropy in the framework of string theory and M-theory has led to many important new insights.
 
One of the most fundamental aspects of string theory is its ability to incorporate gravity in a consistent quantum framework. Understanding the associated geometric and topological structures then becomes one of the principal challenges for the theory, and one of the potentially most fruitful areas for research.  Much current activity is focused in this area, studying the nature of string compactifications to four dimensions; understanding the interplay of the gravitational field and the other fields of the theory, and the way in which the enlarged or “hidden” symmetries emerge; investigating the role of geometry and topology in flux compactifications; and so on.

Since its inception in about 1997, the conjectured gauge/gravity duality has proved to be an extraordinarily rich area for research.  The basic idea, that a theory describing gravity in a (D+1)-dimensional bulk spacetime with a cosmological constant is holographically dual to a D-dimensional flat spacetime field theory on the boundary of  (D+1)-dimensional anti-de Sitter space, has very profound implications.  In the context of string theory, where the (D+1)-dimensional spacetime arises through a compactification from ten dimensions, the gauge/gravity duality relates reasonably well-understood weak-coupling phenomena in string theory to strongly-coupled systems in the dual D-dimensional theory.  This has applications in QCD physics; nuclear physics and the study of the quark-gluon plasma and jet physics; and most recently, in solid-state physics and phenomena such as superconductivity. 

Recently the opposite regime, where the boundary theory is weakly coupled, has also received attention. In this case the bulk theory is strongly coupled, and describes higher-spin gravity, and is expected to arise in the tensionless limit of string theory.

Higher-spin gravity is a remarkable extension of Einstein gravity with a cosmological constant in which massless particles of all spins arise in a manner controlled by an infinite-dimensional extension of the spacetime symmetry group. Their dual CFT description turns out to be remarkably simple and yet their bulk interpretation calls for new concepts in quantum gravity, in the notion of spacetime geometry and black holes. Much is to be uncovered in studying this duality, the physical observables and the connections with string theory.

In recent years there have been significant developments in the study of N=8 supergravity at the quantum level. Powerful new methods have been developed which show that surprisingly, the theory is finite at least up to the four-loop order.  Much remains to be done in order to understand these results, and to uncover their possible relation to string theory.  Three-dimensional massive supergravities, with or without a topological term, are also being actively studied, since they provide rich toy models for exploring questions of renormalisabilty, and exact solubility at both the classical and quantum levels.  Analogous models in higher than three dimensions have also recently been considered. Supergravity models with new couplings, and new exact solutions in various dimensions, are also actively under investigation.

The remarkable discovery that the expansion of the universe has been accelerating in the present epoch, implying the existence of a “dark energy” that is accurately described by a small positive cosmological constant, presents a significant challenge to string theory.  The simplest vacua, or compactifications, of string theory would give rise to four-dimensional supersymmetric models with vanishing cosmological constant. Supersymmetric vacua with negative cosmological constant can also straightforwardly arise, but it is considerably more difficult  to account for a positive cosmological constant.  Such a vacuum could not be supersymmetric, but although one  would certainly eventually want to see supersymmetry spontaneously broken in a model describing the real world, the energy scale associated with supersymmetry breaking would imply a scale for the cosmological constant that was far too large.  Thus, one must find a mechanism for obtaining a cosmological constant that is very small and positive, of a scale size that is far below the supersymmetry breaking scale.  Many ideas in this area are currently being explored, involving flux compactifications and the study of the “string landscape.”  Further topics that are being intensively studied at the moment include inflationary models, which have become the paradigm for understanding the expansion of the universe. Major challenge is to implement the inflationary scenario in a fundamental theory such as string theory.

 Organization Plans

We plan to achieve the goals outlined above by bringing the world’s leading theoretical high-energy physicists and relativists to this program. Participants and short-term visitors will be selected from both inside and outside China. In particular, we aim to make this program a platform for Chinese physicists to exchange and interact directly with their counterparts from all over the world.