Gravitational Wave Signatures of Hybrid Defects

The breakings of a large gauge symmetry into a smaller gauge symmetry through an intermediate stage with discrete symmetry can lead to the formation of string-domain wall hybrid defects in the early universe. The gravitational wave spectrum from string-domain wall system has significant differences from the normal string spectrum, a steep decay in low frequencies by the destruction of the string network by the domain walls and furthermore an enhancement in the amplitude before the decay if domain wall energy is saturated at the domain wall formation. We show the consistency between the condition for the domain wall saturation and that for the gravitational wave enhancement. We evaluate the degree of the enhancement of the gravitational wave amplitude and the peak frequency in a large parameter space. The saturation does not require fine-tuning in the energy scales of the symmetry breakings.