Cosmic string from hidden-sector flux tubes

The possibilities for obtaining cosmic strings from a underlying E₈ × E'₈ superstring theory are considered. It is shown that the observable sector cannot provide cosmic strings with a tension large enough to give interesting density perturbations: δ ρ{variant}/ρ{variant} ∼ 10^-4. Neither can the hidden sector yield cosmic strings if E'₈ is unbroken, while flux tubes give δ ρ{variant}/ρ{variant} too large if E'₈ is broken to a subgroup bigger than SU(4), and too small if it is broken to a subgroup smaller than SU(4). However, flux tubes in the hidden sector can provide cosmic strings with δ ρ{variant}/ρ{variant} ∼ 10^-4 if E'₈ is broken to an SU(4) subgroup. If there was inflation, this mechanism would only be operative if the universe subsequently reheated to a temperature above 10¹⁶ GeV.