Observations of structure in the cosmic microwave background (CMB) radiation over many scales, from degrees to arcminutes, yield information on the power spectrum of initial density fluctuations, and are crucial for discriminating between competing theories of structure formation. On scales of , inflationary models predict that increased power should be seen in the CMB sky due to Doppler scattering of photons during acoustic oscillations of the photon-baryon fluid at recombination, and detection of these `Doppler peaks' is one of the primary goals of CMB astronomy. Furthermore, the size and position of these peaks can be used to determine basic cosmological parameters.
Until last year, CMB experiments had found increasing power in CMB spatial fluctuations going from to , but none had measured the expected turnover. In a paper by Scott et al. (1996) (Paper I), a new detection by the Cambridge Cosmic Anisotropy Telescope (CAT) on scales of about provided the first evidence to support the existence of a peak in the power spectrum. This was the first of a series of fields being observed by the CAT; preliminary results for a second field are presented here.