Search Talks

ABSTRACT: The asymptotic freedom conjecture for gravitation is explored in which strong renormalization effects (as in QCD) may occur at astrophysical distance scales larger than the solar system. Nonperturbative renormalization group trajectories exhibiting such an infrared fixed point describe a theory of gravitation with a running gravitational coupling which grows at large distance. The concept that this extra gravity may provide the answer to the missing mass inherent in the dark matter paradigm is a natural suggestion. We provide the alternative of Modified Gravity to answer the problem of galaxy rotation curves from the smallest dwarf galaxies to the largest giant galaxies and to galaxy clusters including the Bullet Cluster. The theory may also explain the apparent anomalous deceleration of the Pioneer 10 and 11 space probes, within solar system constraints.

The entanglement entropy between quantum fields inside and outside a black hole horizon is a promising candidate for the microscopic origin of black hole entropy. I will explain the motivation behind this interpretation of black hole entropy, and why it requires quantum gravity. I will then apply these ideas to loop quantum gravity and show how to compute the entanglement entropy of spin network states. The result of this calculation agrees asymptotically with results from the isolated horizon framework, and I will give the reason for this agreement. Finally, I will show that the entanglement entropy gives extensive corrections to the area law, suggesting corrections to the gravitational action.

I will discuss recent results in Supersymmetric Large Extra Dimensions (SLED), a scenario which shows promise towards solving both the hierarchy and the cosmological constant problems. One of the issues which arises in this programme is a direct result of the need to use codimension-2 branes, which can only consistently couple to gravity through a tension term in the action. This precludes us from asking certain interesting questions, such as what will happen when a phase transition occurs on the brane. In this talk, I will describe how these codim-2 branes can be modelled as codim-1, thus enabling us to work with more interesting brane actions.

In the sixties, Roger Penrose came up with a radical new idea for a quantum geometry which would be entirely background independent, combinatorial, discrete (countable number of degrees of freedom), and involve only integers and fractions, not complex or real numbers. The basic structures are spin-networks. One reason we might believe that space or space-time might be discrete is that current physique tells us that matter is discrete and that matter and geometry are related through gravity. Once a discrete theory is decided on, it seems awkward that the dynamics would retain "continuous elements" in the form of real numbers (used for the probabilities for example). The great achievement of Penrose's theory is that there is a well defined procedure which gives the semi-classical limit geometry (always of the same dimension) without any input on topology (the fundamental theory does not contain a manifold).

The study of particle-like excitations of quantum gravitational fields in loop quantum gravity is extended to the case of four valent graphs and the corresponding natural evolution moves based on the dual Pachner moves. This makes the results applicable to spin foam models. We find that some braids propagate on the networks and they can interact with each other, by joining and splitting. The chirality of the braid states determines the motion and the interactions, in that left handed states only propagate to the left, and vise versa.

In the late 80s it became clear through notable work by Witten and others that there is a deep connection between (2+1) gravity and Chern-Simons theory making it possible to quantize. In the case where the cosmological constant is negative, spacetime has a boundary and classically there are black holes. We will study the features of this theory as an arena for seeing the simple realization of the Holographic Principle and the duality between quantum gravity theories in asymptotically Anti-deSitter space and conformal field theories. Extensions and open problems with the theory will also be discussed.

We describe how to recover the quantum numbers of extremal black holes from their near horizon geometries. This is achieved by constructing the gravitational Noether-Wald charges which can be used for non-extremal black holes as well. These charges are shown to be equivalent to the U(1) charges of appropriately dimensionally reduced solutions. Explicit derivations are provided for 10 dimensional type IIB supergravity and 5 dimensional minimal gauged supergravity, with illustrative examples for various black hole solutions. We further discuss how to derive the thermodynamic quantities and their relations explicitly in the extremal limit, from the point of view of the near-horizon geometry. We relate our results to the entropy function formalism.

In this talk, I will describe recent work in string phenomenology from the perspective of computational algebraic geometry. I will begin by reviewing some of the long-standing issues in heterotic model building and the goal of producing realistic particle physics from string theory. This goal can be approached by creating a large class of heterotic models which can be algorithmically scanned for physical suitability. I will outline a well-defined set of heterotic compactifications over complete intersection Calabi-Yau manifolds using the monad construction of vector bundles. Further, I will describe how a combination of analytic methods and computer algebra can provide efficient techniques for proving stability and calculating particle spectra.