Phenomenological implications of cosmic (super)strings with time-varying tension

Cosmic strings and cosmic superstrings could have formed in the early universe. In a particular class of models, the tension associated with these defects is time dependent, either because the strings themselves are composed of evolving moduli fields, or because they are directly coupled to evolving scalar fields leading to an effective time dependence in the string tension. Here we take seriously the possibility of this time dependence and investigate the phenomenological consequences of such an evolution, comparing the results to the more traditional case of a fixed time independent tension. In particular, for five different time varying models, motivated by different possible forms of evolution of the string tension, we evaluate their Gravitational Wave (GW) contributions as well as their impact on the Cosmic Microwave Background (CMB). Performing an MCMC analysis of the models we find there are indeed interesting spectral signatures in the GW emission at frequencies between the current PTA and interferometer data, but the signatures are not unique to these strings, making them hard to pin down using current data. However, we do find cosmic strings with evolving tension fit the GW data as well as other favoured models.