String Theory Scotland

There are two types of fundamental forces in Nature: those responsible for particle interactions at subatomic scales and those responsible for the large scale structure of the universe. The latter is described by Einstein's General Theory of Relativity (GR) and the former by Quantum Field Theories (QFTs) such as the Standard Model. Einstein's theory is conceptually simple, but is classical and breaks down when the force of gravity is strong, as it is at very small scales, whereas QFTs are effective theories which ignore the gravitational interactions and which cannot be trusted at very high energies. During the last three decades, String Theory has emerged as a conceptually rich theoretical framework reconciling both GR and QFT. In particular, it is a theory of quantum gravity and our group's research programme is firmly focused on a wide range of quantum gravitational aspects of String Theory. The expansion of the universe is incontrovertible. In String Theory, the universe is described via the notion of a string background. Thus if String Theory is to make contact with cosmology, one has to come to grips with time-dependent backgrounds, which are not directly amenable to investigation using the standard perturbative formulation of string theory. Our group have pioneered the study of time-dependent string backgrounds and an important part of our research activities is centred on this topic. Our goal is to understand the general structure of string perturbation theory in time-dependent backgrounds in order to be able to answer questions such as 'Does string theory resolve Big-Bang-type singularities?'---a natural question in view of the fact that string theory is known to resolve other types of singularities. Another important goal of our research is to derive observable cosmological consequences of string theory in an effort to falsify it or at least to constraint the landscape of possible string backgrounds. String theory challenges the geometrical notions of spacetime on which GR is predicated. At very small ('stringy') scales the nature of spacetime is believed to be fundamentally different from GR---its continuous structures thought to be replaced by discrete, algebraic structures which no longer distinguish individual spacetime events. Noncommutative Geometry (NCG) is a branch of modern mathematics dealing with such discrete structures. There are strong hints that NCG appears naturally in String Theory and a thorough investigation of this expectation is another of the main focal points of our group's research. Perturbative string theory is described in terms of two-dimensional conformal field theory (CFT) and one of our main goals in this area is the emergence of NCG on the important class of quasi-rational conformal field theories. The low-energy limit of String Theory is supergravity (SUGRA), a nontrivial extension of GR, in which the universe is described by a spacetime with additional geometric data / the signature of stringy physics. As the energy increases, these backgrounds are believed to receive corrections, and determining which types of corrections is one of the main questions we will address. As in GR, SUGRA admits solutions describing gravitating objects such as black holes, branes,... We known from Hawking's work that black holes obey the laws of thermodynamics and this prompts the natural question: 'Which microstates are responsible for the entropy of a black hole?' Our group have pioneered the systematic approach to the classification of SUGRA backgrounds and we are well poised to answer some of the more pressing questions in this area, such as 'what are the possible branes in a SUGRA background?', an important question in the gauge/gravity correspondence. In summary, our research encompasses a wide range of gravitational aspects of String Theory, impinging on cosmology, particle physics and on the very nature of String Theory itself.