Inflation, superheavy metastable strings and gravitational waves in non-sup ersymmetric flipped SU(5)

Motivated by the NANOGrav 15 year data and other recent investigations of stochastic gravitational background radiation based on pulsar timing arrays, we show how superheavy strings survive inflation but the slightly heavier monopoles do not in a non -sup ersymmetric hybrid inflation model based on flipped SU(5). With the dimensionless string tension parameter G mu similar to 10-6, the gravitational wave spectrum emitted by the strings, which are metastable due to breaking caused by monopole-antimonopole quantum mechanical tunneling, is compatible with the latest NANOGrav measurement as well as the advanced LIGO-VIRGO third run data. The string network undergoes about 30 e-foldings of inflation which suppresses the spectrum in the LIGO-VIRGO frequency range. With the symmetry breaking chain SU(5)xU(1)X -> SU(3)cxSU(2)LxU(1)ZxU(1)X -> SU(3)cxSU(2)LxU(1)Y, the estimated proton lifetime is of order 1036 - 1037 yrs.