Strings, Conformal Field Theory and Noncommutative Geometry

This thesis describes some aspects of noncommutative geometry and conformal field theory. The motivation for the investigations made comes to a large extent from string theory. This theory is today considered to be the most promising way to find a solution to the problem of unifying the four fundamental interactions in one single theory. The thesis gives a short background presentation of string theory and points out how noncommutative geometry and conformal field theory are of relevance within the string theoretical framework. There is also given some further information on noncommutative geometry and conformal field theory. The results from the three papers on which the thesis is based are presented in the text. It is shown in Paper 1 that, for a gauge theory in a flat noncommutative background only the gauge groups U(N) can be used in a straightforward way. These theories can arise as low energy limits of string theory. Paper 2 concerns boundary conformal field theory, which can be used to describe open strings in various backgrounds. Here different orbifold theories which are described using simple currents of the chiral algebra are investigated. The formalism is applied to ``branes´´ in Z2 orbifolds of the SU(2) WZW-model and to the D-series of unitary minimal models. In Paper 3 two different descriptions of an invariant star-product on S² are compared and the characteristic class that classifies the star-product is calculated. The Fedosov-Nest-Tsygan index theorem is used to compute the characteristic class