The Anisotropy of the Microwave Background tol= 3500: Deep Field Observations with the Cosmic Background Imager

We report measurements of anisotropy in the cosmic microwave background radiation over the multipole range l 200 3500 with the Cosmic Background Imager based on deep observations of three fields. These results confirm the drop in power with increasing l first reported in earlier measurements with this instrument and extend the observations of this decline in power out to l 2000. The decline in power is consistent with the predicted damping of primary anisotropies. At larger multipoles, l 2000 3500, the power is 3. 1 greater than standard models for intrinsic microwave background anisotropy in this multipole range and 3. 5 greater than zero. This excess power is not consistent with expected levels of residual radio source contamination but, for 8 e1, is consistent with predicted levels of a secondary Sunyaev-Zeldovich anisotropy. Further observations are necessary to confirm the level of this excess and, if confirmed, determine its origin.