Microwave anisotropy due to cosmic strings

Abstract

There has been much interest in the cosmological consequences of line-like topological defects (‘cosmic strings’) which may be produced in a phase transition in the early Universe^1–8. Such strings would produce density fluctuations on a broad range of scales and may have been responsible for the formation of galaxies. In view of this possibility, it would be useful to have more direct evidence of the existence of strings than the observed clustering of matter. We show here that cosmic strings would generate anisotropy of the microwave background temperature with a very distinctive feature; the temperature would have steplike discontinuities on curves on the sky. Present anisotropy measurements limit the line density parameter, Gμ to be <10⁻⁵. Popular values of this parameter, for the purpose of galaxy formation, are only an order of magnitude below this. The anisotropy that we discuss will exist in addition to that produced by curvature fluctuations due to strings and matter at the time that the Universe becomes optically thin.