A first determination of the position of the `Doppler' peak

Cosmological theories for the origin and evolution of structure in the Universe are highly predictive of the form of the angular power spectrum of cosmic microwave background fluctuations. We present new results from a comprehensive study of CMB observations which provide the first measurements of the power spectrum all the way down to angular scales of ~10 arcminutes. On large scales a joint likelihood analysis of the COBE and Tenerife data fixes the power spectrum normalisation to be Q_rms = 21.0 +/- 1.6 uK for an initially scale invariant spectrum of fluctuations. The combined data are consistent with this hypothesis, placing a limit of n=1.3 +/- 0.3 on the spectral slope. On intermediate scales we find clear evidence for a `Doppler' peak in the power spectrum localised in both angular scale and amplitude. This first estimate of the angular position of the peak is used to place a new direct limit on the curvature of the Universe, corresponding to a density of Omega=0.7^{+1.0}_{-0.4}. Very low density open Universe models are inconsistent with this limit unless there is a significant contribution from a cosmological constant.