4-Year COBE DMR Cosmic Microwave Background Observations: Maps and Basic Results

TL;DR

This work analyzes the full 4-year COBE DMR data to characterize the large-scale CMB anisotropy and test inflationary predictions. The analysis uses multi-frequency maps, Galactic foreground modeling, and Gaussian likelihoods across multiple data combinations to infer a power-law power spectrum with spectral index $n=1.2\pm0.3$ and quadrupole normalization $Q_{rms-PS}=15.3^{+3.8}_{-2.8}\,\mu{\rm K}$; for $n=1$ the best-fit normalization is $Q_{rms-PS}|_{n=1}=18\pm1.6\,\mu{\rm K}$. The monopole and dipole are measured as $T_0=2.725\pm0.020$ K and a dipole amplitude of $3.353\pm0.024$ mK, with the quadrupole amplitude compatible with the power-spectrum fit; Gaussian statistics are supported by multiple tests. The 4-year maps achieve an average signal-to-noise of approximately 2 per 10° patch, enabling improved Galactic modeling and foreground constraints, and the full data set is publicly available for community use.

Abstract

The cosmic microwave background radiation provides unique constraints on cosmological models. In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 years of COBE DMR observations, as detailed in a set of companion Letters. The anisotropy is consistent with a scale-invariant power law model and Gaussian statistics. With full use of the multi-frequency 4-year DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of $n=1.2\pm 0.3$ and a quadrupole normalization $Q_{rms-PS}=15.3^{+3.8}_{-2.8}$ $μ$K. For $n=1$ the best-fit normalization is $Q_{rms-PS}\vert_{n=1}=18\pm 1.6$ $μ$K. These values are consistent with both our previous 1-year and 2-year results. The results include use of the $\ell=2$ quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The 4-year sky maps, presented in this Letter, portray an accurate overall visual impression of the anisotropy since the signal-to-noise ratio is ~2 per 10 degree sky map patch. The improved signal-to-noise ratio of the 4-year maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics.