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The angular power spectrum estimator developed by Peebles and Hauser & Peebles has been modified and applied to the 4 yr maps produced by the COBE DMR. The power spectrum of the observed sky has been compared to the power spectra of a large number of simulated random skies produced with noise equal to the observed noise and primordial density fluctuation power spectra of power-law form, with P(k) F k n . The best-fitting value of the spectral index in the range of spatial scales corresponding to spherical harmonic indices 3 = 30 is an apparent spectral index n app 1.13 0.4 0.3 which is consistent with the Harrison-Zeldovich primordial spectral index n pri 1. The best-fitting amplitude for n app 1 is Q rms 2 0.5 18 K.
Wright et al. (Mon,) studied this question.