Astronomical constraints on a string-dominated universe

Some theorists have conjectured that strings might make a dominant contribution to the cosmological density parameter Ω. We show that this possibility can be ruled out unless the parameter (Gμ), a measure of the mass per unit length of cosmic strings, is below |10⁻¹⁴-10⁻¹⁵|⁠. Strings heavier than this would produce characteristic effects on observed quasar variability and on pulsar periods. A network of strings whose random motions were sub-relativistic would (unlike more ‘ordinary’ material) make no contribution to the cosmic deceleration. It would, however, contribute to the global curvature, and could, if Ω were high enough, focus light through an ‘antipode’ with an accessible redshift. Observations of gravitationally lensed quasars can thereby set an upper limit [independent of (Gμ)] to the value of Ω contributed by this kind of string network.