Small Scale Microwave Background Fluctuations from Cosmic Strings

The Cosmic Microwave Background (CMB) fluctuations at very small angular scales (less than $10'$) induced by matter sources are computed in a simplified way. The result corrects a previous formula appearing in the literature. The small scale power spectrum from cosmic strings is then calculated by a new analytic method. The result compares extremely well with the spectrum computed by numerical techniques (when the old, incorrect, formula is used). The upper bound on the string parameters derived from OVRO data is re-examined, taking into account the non-Gaussian nature of stringy perturbations on small scales. Assuming a conventional ionization history, the bound is $\gamma G\mu < 11\times 10^{-6}$, where $\gamma^2$ is the number of horizon lengths of string per horizon volume. Current simulations give $\gamma^2 = 31\pm 7$.