Triangle Seminars

Abstract:
We briefly review some models of dark energy. We start with the cosmological constant including models arising in string theory, before discussing dynamical models where a scalar field may be responsible for the observed late time acceleration. The possibility that we are not fully in control of the gravity sector is discussed and the possibility that the acceleration may be some manifestation of modified gravity on large scales is shown. All cases require some degree of fine tuning for the models to be compatible with observation. Finally, we briefly discuss some recent fun work involving scalar-tensor theories of gravity in which a self-tuning mechanism exists leading to the same late time cosmology independently of the size of the cosmological constant. Solutions include having radiation and matter dominated like regimes whilst the energy density remains dominated by a cosmological constant.

Abstract:
Instantons and W-bosons in 5d N=2 Yang-Mills theory arise from a circle compactification of the 6d (2,0) theory as Kaluza-Klein modes and winding self-dual strings, respectively. We study an index which counts BPS instantons with electric charges in Coulomb and symmetric phases. We first prove the existence of unique threshold bound state of U(1) instantons for any instanton number. By studying SU(N) self-dual strings in the Coulomb phase, we find novel momentum-carrying degrees on the worldsheet. The total number of these degrees equals the anomaly coefficient of SU(N) (2,0) theory. We finally propose that our index can be used to study the symmetric phase of this theory, and provide an interpretation as the superconformal index of the sigma model on instanton moduli space.