We present results from the first simulations of networks of Type I Abelian Higgs cosmic strings to include both matter and radiation eras and Cosmic Microwave Background ( CMB ) constraints .
In Type I strings , the string tension is a slowly decreasing function of the ratio of the scalar and gauge mass-squared , \beta .
We find that the mean string separation shows no dependence on \beta , and that the energy-momentum tensor correlators decrease approximately in proportion to the square of the string tension , with additional O ( 1 ) correction factors which asymptote to constants below \beta \lesssim 0.01 .
Strings in models with low self-couplings can therefore satisfy current CMB bounds at higher symmetry-breaking scales .
This is particularly relevant for models where the gauge symmetry is broken in a supersymmetric flat direction , for which the effective self-coupling can be extremely small .
If our results can be extrapolated to \beta \simeq 10 ^ { -15 } , even strings formed at 10 ^ { 16 } GeV ( approximately the grand unification scale in supersymmetric extensions of the Standard Model ) can be compatible with CMB constraints .