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We explore a variety of composite topological structures that arise from the spontaneous breaking of SO(10) to SU(3)c× U(1)em via one of its maximal subgroups SU(5) × U(1)χ, SU(4)c× SU(2)L× SU(2)R, and SU(5) × U(1)X (also known as flipped SU(5)). They include i) a network of ℤ strings which develop monopoles and turn into necklaces with the structure of ℤ2 strings, ii) dumbbells connecting two different types of monopoles, or monopoles and antimonpoles, iii) starfish-like configurations, iv) polypole configurations, and v) walls bounded by a necklace. We display these structures both before and after the electroweak breaking. The appearance of these composite structures in the early universe and their astrophysical implications including gravitational wave emission would depend on the symmetry breaking patterns and scales, and the nature of the associated phase transitions. [ABSTRACT FROM AUTHOR]

We discuss E 6 based extensions of the Standard Model (SM) containing two varieties of superheavy metastable cosmic strings (CSs) that respectively have neutral and electrically charged current carriers. We employ an extended version of the velocity-dependent one-scale (VOS) model, recently discussed by some authors, to estimate the gravitational wave (GW) spectrum emitted by metastable strings with a dimensionless string tension G μ ≈ 10 − 6 that carry a right-handed neutrino (RHN) current. We find that with a low to moderate amount of current, the spectrum is compatible with the LIGO O3 run and also consistent at the 1 σ level with the recent PTA signals. [ABSTRACT FROM AUTHOR]

We explore the gravitational wave spectrum generated by string-wall structures in an S O (10) (S p i n (10)) based scenario of pseudo-Goldstone boson dark matter (pGDM) particle. This dark matter candidate is a linear combination of the Standard Model (SM) singlets present in the 126 and 16 dimensional Higgs fields. The Higgs 126-plet vacuum expectation value (VEV) 〈 126 H 〉 leaves unbroken the Z 2 subgroup of Z 4 , the center of S O (10). Among other things, this yields topologically stable cosmic strings with a string tension μ ∼ 〈 126 H 〉 2. The subsequent (spontaneous) breaking of Z 2 at a significantly lower scale by the 16-plet VEV 〈 16 H 〉 leads to the appearance of domain walls bounded by the strings produced earlier. We display the gravitational wave spectrum for Gμ values varying between 10 − 15 and 10 − 9 (〈 126 H 〉 ∼ 10 11 - 1014 GeV), and 〈 16 H 〉 ∼ 0.1 - 102 TeV range (G denotes Newton's constant.) These predictions can be tested, as we show, by a variety of (proposed) experiments including LISA, ET, CE and others. [ABSTRACT FROM AUTHOR]