SPECTRAL-ANALYSIS OF MICROWAVE BACKGROUND PERTURBATIONS INDUCED BY COSMIC STRINGS

Using a simple analytic model based on seed superposition we obtain an approximation to the spectrum of microwave background perturbations i nduced by cosmic strings on all angular scales larger than about 2 arc min. We assume standard recombination in an Einstein-de Sitter univers e with h=1/2 and study the fluctuation spectrum along a great circle i n the sky. Doppler and potential perturbations on the last scattering surface (Sachs-Wolfe effect) are shown to dominate over post-recombina tion perturbations on scales smaller than about 2 degrees. Using a fil ter function corresponding to the COBE experiment we obtain a lower bo und to the power spectrum index n greater than or similar to 1.35 in g ood agreement with the recently announced second year COBE data showin g n similar or equal to 1.5. The only free parameter (G mu) of the str ing model is tired by normalizing on the COBE detection leading to G m u=1.6x10(-6). Other parameters (e.g., the rms string velocity) are fix ed by comparing with string simulations. Using these parameter values we compare the rms temperature fluctuations (Delta T/T)(rms) predicted for ongoing experiments (Tenerife, SP91, MAX, OVRO, etc.) with detect ions and with the corresponding predictions of inflationary models.