Spectral Analysis of Microwave Background Perturbations Induced by Cosmic Strings

Using a simple analytic model based on seed superposition we obtain the spectrum of microwave background perturbations induced by cosmic strings on all angular scales larger than about 2 armin. We assume standard recombination in an Einstein de Sitter universe with h = 1/2 and study the fluctuation spectrum along a great circle in the sky. Doppler and potential perturbations on the last scattering surface (Sachs-Wolfe effect) are shown to dominate over post-recombination perturbations on scales smaller than about 2 degrees. Using a filter function corresponding to the COBE experiment we obtain an effective power spectrum index neff ≃ 1.35 in good agreement with the recently announced second year COBE data showing neff ≃ 1.5. The only free parameter (Gμ) of the string model is fixed by normalizing on the COBE detection leading to Gμ = 1.6. Other parameters (e.g. the rms string velocity) are fixed by comparing with string simulations. Using these parameter values we compare the rms temperature fluctuations ( T )rms predicted for ongoing experiments (Tenerife, SP91, MAX, OVRO etc) with detections and with the corresponding predictions of inflationary models. Subject headings: cosmic microwave background cosmic strings Division of Theoretical Astrophysics, Harvard-Smithsonian Center for Astrophysics 60 Garden St. Cambridge, Mass. 02138, USA. also Visiting Scientist, Department of Physics Brown University Providence, R.I. 02912, U.S.A.