String Theory and Gravity
In string theory, the basic objects are one-dimensional strings (closed loops, or open strings-pieces with ends). The different particles seen in experiments today are just different modes of vibration of the string (which we see as pointlike particles because the length of the string is too small to be resolved). Consistent string theories also include Einstein's theory of gravity, General Relativity, as an integral part of the formulation. This a major step in our understanding of how to incorporate the theory of gravity into the same framework as the other interactions. If the effect of interactions is small, they can be described as the splitting and joining of strings.
What happens when the strength of the interactions between individual strings is very strong? This is not just a question of principle; we need the answer to understand, for example, the fundamental description of black holes. Attempts to address this question have led to the discovery of `dualities' relating the five apparently different perturbative theories. Other types of extended objects, known as p-branes, play an important role in these relations, and there are also signs that the fundamental description of string theory may be in terms of a field theory (a theory of pointlike objects) after all.
There is growing evidence that there are dualities relating string theory to field theories. This allows us to deal with non-perturbative issues in string theory by working in field theory. These ideas have been tested using knowledge from non-perturbative field theory. We are studying the encoding of black hole spacetimes in the field theory, and searching for a general framework that unifies these developments.
Two-dimensional conformal field theory is the basis of perturbative string theory. Our research relates to understanding superstring theories outside their critical dimension, and the consistent definition of theories with boundaries, which is relevant to the study of open strings.
Many recent developments in string theory and field theory have highlighted the importance of theories on backgrounds with intrinsically non-commutative geometries. We have played a pioneering role in the study of noncommutativity in string theory, and are actively working on the phenomenological and cosmological implications of these ideas.
We have a particular interest in the study of topological defects in cosmology. As well as being a natural consequence of symmetry breaking in the early universe, the physics of such objects plays an important role in string-inspired models where some fields are confined to a submanifold of spacetime.