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In 1992 the COBE DMR team, including George F. Smoot and Charles L. Bennett, used radiometer antennas to compare antenna temperatures between sky directions $60^{\circ }$ apart at $31.5$, $53$ and $90\mathrm{GHz}$, with two nearly independent channels per frequency. Repeated sky coverage supported least-squares solution map reconstruction.^[ref]

The analysis fitted magnetic response and removed the assumed kinematic angular dipole and its second-order Doppler effect contribution. Channel sums retained sky structure; channel differences estimated noise. Cross-channel correlations, different frequency maps and Milky Way latitude cuts tested the interpretation. Residual large-angle structure supported Cosmic microwave background anisotropy; Galactic contamination particularly limited the cosmic microwave background quadrupole. Brightness fluctuations alone were not sufficient evidence of primordial structure.^[ref]

The two differences serve different purposes. Comparing two sky directions measures angular variation. Comparing two independently calibrated receiver maps observing the same sky tests noise and consistency. A common sky signal cancels in the latter comparison if their antenna beam patterns and response scales match; a systematic error shared by both channels need not cancel. Separating observer motion, local emission and intrinsic Cosmic microwave background anisotropy requires these additional tests.

Cosmic microwave background anisotropy is the direction dependence of the relic radiation's temperature and polarization. A temperature map can be written $T(\hat{\mathbf{n}})=\bar{T}[1+\Theta (\hat{\mathbf{n}})]$, separating a mean from its fractional angular variation. Comparing directions with a Differential Microwave Radiometer measures these variations without requiring the instrument to determine the absolute mean temperature.^[ref]

Intrinsic plasma fluctuations, motions and gravitational frequency shifts all contribute. The Cosmic microwave background power spectrum organizes the signal by angular scale; the Cosmic microwave background acoustic peaks record the evolution of the photon-baryon fluid. Frequency comparisons and instrumental checks help distinguish the relic signal from galactic foregrounds and noise before the map is used to infer cosmological parameters.

The Differential Microwave Radiometer measured differences in microwave power between pairs of sky directions to map Cosmic microwave background anisotropy. Comparing two directions suppresses a common absolute brightness offset; repeated comparisons during scanning are then combined into a sky map.

The COBE instrument used paired horn antennas at three frequencies, with independent channels that tested detector noise and frequency-dependent galactic foreground contamination. A differencing instrument does not by itself determine the absolute monopole temperature. Its measurement therefore complements the spectrum measured by the Far Infrared Absolute Spectrophotometer.^[ref]

George Smoot led the COBE differential-radiometer analysis that detected angular variations in the cosmic microwave background. The measurement compared different sky directions to separate a nearly uniform background from much smaller temperature fluctuations.

The team had to account for observer motion, Galactic emission and instrumental effects before interpreting the remaining angular structure. These anisotropies supplied evidence about early cosmic inhomogeneity, complementing the distinct measurement of the background's nearly blackbody spectrum.^[ref]

The cosmic microwave background has an almost perfect blackbody spectrum at about $2.725\mathrm{K}$. This is strong evidence for an earlier hot, dense phase in which radiation thermalized, followed by expansion that redshifted the photon spectrum while preserving its Planck form. COBE FIRAS compared the sky with a precision reference blackbody; instrument temperature, gain, foregrounds and calibrator emissivity were crucial controls. Agreement with a blackbody constrains substantial late energy injection or photon-production distortions.^[ref]

After subtracting the dominant dipole associated mainly with observer motion, anisotropies at the level of roughly $10^{-5}$ reveal primordial structure. COBE DMR established large-angular-scale anisotropy; multifrequency measurements and independent maps were needed to distinguish it from Galactic emission and instrumental artifacts.^[ref] Approximate isotropy supports a homogeneous background model under the usual cosmological assumptions, but isotropy at one observing position is not alone a proof of spatial homogeneity everywhere.

The temperature CMB angular power spectrum has multiple acoustic peaks. Their coherent pattern follows oscillations in the pre-recombination photon-baryon fluid. Peak positions compare the sound horizon with angular diameter distance and hence constrain geometry together with expansion parameters. Relative peak heights test baryon inertia, matter–radiation equality and gravitational-potential evolution, motivating both baryonic and nonbaryonic matter. Cosmic microwave background diffusion damping suppresses small-scale power. Cosmic microwave background polarization and temperature–polarization correlations provide further tests of scattering, acoustic phase and reionization; gravitational lensing remaps the observed pattern and probes intervening structure.

By the time of this examination, the Planck 2013 temperature and lensing analysis supported a spatially flat six-parameter model with cold dark matter and a cosmological constant. This is a joint fit within an explicit model, not an inference that each peak uniquely measures one parameter.^[ref] Foreground separation requires frequency coverage, sky masks, beam and noise characterization, and consistency across instruments. The primary microwave background has geometric and other parameter degeneracies, so acoustic-ruler, supernova and structure observations are valuable complements. The spectrum establishes a thermal hot-universe history; anisotropy and polarization constrain its geometry, contents and primordial perturbations. None of these observations by itself demonstrates a unique cosmic inflation mechanism.