Metastable cosmic strings are topological defects that can arise from symmetry breaking in the early Universe. Recently, they are gathering attention as a possible source of stochastic gravitational wave (GW) background, whose strong evidence is reported by multiple pulsar timing array experiments. When metastable strings decay, they fragment into segments with monopoles and antimonopoles attached at their endpoints. The monopole and antimonopole are strongly pulled by the string tension. Violent oscillations of these segments have been considered as a potential GW source, in addition to contributions from string loops. We show that, in realistic situations, the monopoles frequently collide with thermal fluctuations on the string segments, which act as a resistance and prevent the oscillation. As a result, we find that the contribution from string segments to the GW background is negligible.
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