Point like topological defect, called (magnetic) monopoles [464, 465], with a mass of order of the GUT scale are expected to form during the early phase transition from G$_{GUT}$ down to SU (3) × SU (2) × U (1) if G$_{GUT}$ is assumed simple. It was argued that unless their abundance is n$_{M}$/s > 10 − 10 at the time of phase transition, their abundance can not be diluted in an adiabatic expansion of the universe [466], these ideas ultimately propelled the birth of inflation in order to dilute them. energy density of the universe unless they form at an energy lower than ∼ 100 MeV. They cannot even have formed at an energy higher than ∼ 1 MeV without producing temperature fluctuations larger than δT/T > ∼ 10 − 5 in the CMB [463], and without being in contradiction with CMB observations $^{43}$. On the other hand cosmic strings do not suffer from these problems, as a network of cosmic strings can inter commute 44 and lead to the formation of closed loops from long strings. The loops oscillate due to their tension and decay into gravitational (and potentially particle) radiation, leading to a global scaling regime: their relative energy density evolve as, ρ$_{CS}$ ∝ t − $^{1}$, and do not dominate the universe (see [467] for the analytical proof and for example [468–470] for numerical confirmations, see also for more recent discussion [471, 472]. Their observational signatures are numerous (see [473] for a recent discussion); contribution to the CMB temperature fluctuations via the Kaiser-Stebbins effect [474], gravitational lensing [475, 476], generation of gravity wave background [477–480]. The amplitude of these effects are all mainly related to one property - their tension T . If the strings do not carry currents, this tension is equal to their energy per unit length in Planck units: where v is the VEV of the Higgs far away from the string. In the case of non-SUSY strings, or if the strings saturate the Bogomolnyi bound (they are then called “BPS strings” for Bogomolnyi-Prasad-Sommerfeld), ϵ ( β ) = 1. In the context of SUSY/SUGRA, this function becomes dependent on the ratio of the Higgs to the gauge boson mass, β ≡ m$_{φ}$/m$_{A}$ . For 43 The defects in higher dimensions can give rise to inflation, see Sec. VIII B. 44 The probability of inter-commutation, p , that is an exchange of partners when two strings intersect of one another is very close to p = 1 for cosmic strings from 4 D field theory. This probability become much smaller when considering cosmic superstrings appearing from brane inflation. This is the main origin of the differences in the observational signature between cosmic strings and cosmic (super)strings. Gµ = 2 πϵ ( β ) v $^{2}$/M2 P . (158) 74