Impact of radiative decay on cosmic string dynamics at small scales

Cosmic strings are topological defects appearing as extended solutions in many high energy physics scenarios. Observation of signatures expected due to the presence of cosmic string networks could provide critical evidence in distinguishing and constraining fundamental cosmological and particle physics theories. Large scale evolution of cosmic string is well understood but the dynamics influenced by small scale structure remains unclear. Radiation back-reaction is expected to smooth strings, setting the scale of small structure and the size of loops produced. We undertake an investigation of cosmic strings numerically simulated from their underlying field theories, in particular we use the U(1) gauge theory of the Abelian-Higgs model which radiates to massive modes and the global U(1) theory of the Goldstone model which additionally radiates into the massless mode of the Goldstone field. By comparison to the emission of Goldstone bosons we can infer the effects of gravitational radiation, a further important energy loss mechanism for cosmological string, but difficult to simulate. We analyse the scaling properties of the string tangent vector correlation function and loop number density distributions which are expected to follow related power law forms and compare the results for gauge and global strings with a view to deciphering the influence of a massless degree of freedom on these attributes of network evolution. We find that the change in correlation function due to a massless mode can be incorporated by an effective value for the exponent of time by which the scale factor evolves whereby the smoothing due to back-reaction behaves like additional causal damping. From long gauge strings we find no evidence for direct ‘core’-sized loop production, finding instead that our simulations favour radiation into the gauge and Higgs modes and fragmentation of horizon-sized loops