Metastable Strings and Grand Unification

The structure of the Standard Model (SM) of particle physics points towardgrand unified theories (GUTs) where strong and electroweak interactions areunified in a non-Abelian GUT group. The spontaneous breaking of the GUTsymmetry to the SM symmetry, together with cosmic inflation, generically leadsto metastable topological defects, the most prominent example being cosmicstrings. The gravitational-wave background (GWB) produced by a cosmic stringnetwork is one of the candidates for an explanation of the GWB recentlyobserved by pulsar timing array (PTA) experiments. We review some properties ofthe predicted GWB with emphasis on potential implications for GUT modelbuilding. The most striking prediction is a GWB in the LIGO-Virgo-KAGRA bandthat could be discovered in the near future.