The Anisotropy of the Microwave Background to l = 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 σ greater than standard models for intrinsic microwave background anisotropy in this multipole range and 3.5 σ greater than zero. This excess power is not consistent with expected levels of residual radio source contamination but, for σ₈>~1, 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.