Power Spectrum of Cosmic String Perturbations on the Microwave Background

Abstract

I review recent progress towards a detailed understanding of the power spectrum of cosmic string perturbations induced on the Microwave Background (CMB). The use of a simple analytic model allows to include important effects that have not been included in previous studies. These effects are potential fluctuations on the last scattering surface (the Sachs-Wolfe effect) and Doppler CMB perturbations. These previously neglected fluctuations are shown to dominate over post-recombination fluctuations (Gott-Kaiser-Stebbins effect) on angular scales less than about $2^\circ$ assuming that no reionization occurs. The effective power spectrum index $n_{eff}$ is calculated on COBE scales and is shown to be somewhat larger than 1 ($1.35 \lsim n_{eff} \lsim 1.5$). After normalizing the model on COBE data and string simulations, I derive the predicted values of $({{δT}\over T})_{rms}$ for several ongoing CMB experiments on medium and small angular scales. A comparison is also made with current observations and predictions of models based on inflation.