Parinya Sirimachan

• Cosmic strings have become interesting over the past years due to the possible connection to the observ- able part of String theory. Solutions to the geodesic equation in cosmic string spacetimes can be used to predict possibilities of the detection of cosmic strings, in particular the gravitational lensing. Therefore, the objectives of this PhD thesis are to find solutions of the geodesic equation in different spacetimes con- taining cosmic strings and make predictions about observables, e.g., light deflection and perihelion shift of planets. In addition, the energy per unit length of the cosmic string can be estimated. In spacetimes containing Schwarzschild and Kerr black holes pierced by an infinitely thin cosmic string, the components of the geodesic equation have been solved numerically and written as a form of the Weierstrass } function. In spacetimes with Abelian-Higgs strings and (p, q) strings with finite width, the metric functions and solu- tions to the geodesic equation have been determined numerically. We find that cosmic strings and cosmic superstrings can be distinguished from the motion of test particles. In addition, the detection of a negative value of the perihelion shift could be a sign of the detection of cosmic strings.