The roughness of the last scattering surface

We propose an alternative analysis of the microwave background temperature anisotropy maps that is based on the study of the roughness of natural surf aces. We apply it to large-angle anisotropies, such as those measured by th e COBE Differential Microwave Radiometer (DMR). We show that for a large si gnal-to-noise ratio experiment, the spectral index can be: determined indep endently of the normalization. We then analyze the 4 yr COBE map and find t hat for a flat Omega = 1 universe, the best-fitting value for the spectral index is n = 1.15(-0.34)(+0.39) and for the amplitude is Q(rms-PS) = 14.1(- 3.5)(+3.9) mu K. For n = 1, the best-fitting normalization is Q(rms-PS)\(n= 1) = = 16.2(-1.3)(+1.4) mu K.