ar X iv : a st ro - p h / 96 01 07 7 v 3 2 5 Ju l 1 99 6 THE ANGULAR POWER SPECTRUM OF THE 4 YEAR

The angular power spectrum Cl is extracted from the 4 year COBE DMR data with a 20◦ galactic cut, using the narrowest window functions possible. The average power in eight multipole bands is also computed, and plotted together with a compilation of power spectrum measurements from other experiments. The COBE results are found to be consistent with the n = 1 power spectrum with the normalization Qrms,ps = 18μK reported by the COBE DMR team. Certain non-standard cosmologies, such as “small universe” models with nontrivial spatial topology, predict power spectra which are not smooth functions. Rather, they contain bumps and wiggles that may not have been detected by other data analysis techniques such as the HauserPeebles method (Wright et al. 1996), the band-power method (Hinshaw et al. 1996) and orthogonalized spherical harmonics (Górski et al. 1996), since these methods all give broader window functions that can smear such features out beyond recognition. On the large angular scales probed by COBE, the Universe thus appears to be kind to us, presenting a power spectrum that is a simple smooth function. 1 Published in ApJ Lett. 464, L35-L38, June 10, 1996. Submitted Jan. 15, 1996. Available from h t t p://www.sns.ias.edu/ ̃max/cobepow.html (faster from the US) and from h t t p://www.mpa-garching.mpg.de/ ̃max/cobepow.html (faster from Europe). Note that figures 1, 3 and 4 will print in color if your printer supports it.