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We present an analysis of simultaneous multifrequency measurements of the Galactic emission in the 1-10 GHz range with 18 degrees angular resolution taken from a high-altitude site. Our data yield a determination of the synchrotron spectral index between 1.4 and 7.5 GHz of beta(syn) = 2.81 +/- 0.16. Combining our data with maps made by Haslam et al. and Reich & Reich, we find beta(syn) = 2.76 +/- 0.11 in the 0.4-7.5 GHz range. These results are in agreement with the few previously published measurements. The variation of beta(syn) with frequency based on our results and compared with other data found in the literature suggests a steepening of the synchrotron spectrum toward high frequencies, as expected from theory because of the steepening of the parent cosmic-ray electron energy spectrum. Comparison between the Haslam data and the 19 GHz map of Cottingham also indicates a spatial variation of the spectral index on large angular scales. Additional high-quality data are necessary to provide a serious study of these effects.

The sky rms is the simplest model-independent characterization of a cosmological anisotropy signal. We show that the rms temperature fluctuations determined from the COBE Differential Microwave Radiometer (DMR) 4 yr sky maps are frequency independent, consistent with the Planckian spectrum expected for the cosmic microwave background signal and therefore with the hypothesis that they are cosmological in origin. The typical rms amplitude is ∼35 ± 2 μK at 7° and ∼29 ± 1 μK at 10°. An analysis of the rms anisotropy determined from the data in both Galactic and ecliptic coordinates is used to determine the rms quadrupole normalization, Qrms-PS, for a scale-invariant Harrison-Zeldovich power-law model. Corrections are applied for small biases observed in the likelihood analysis. While there are variations depending on the data selection, all results are consistent with a Qrms-PS normalization of ∼18±2 μK. This is also shown to be true for a "standard" cold dark matter model of cosmological anisotropy. The difference in the normalization amplitudes derived when the quadrupole is either included or excluded from the analysis is attributable to contamination of the observed sky quadrupole by foreground Galactic emission. © 1997. The American Astronomical Society. All rights reserved.

We limit the possible contributions from noncosmological sources to the COBE Differential Microwave Radiometer (DMR) 4 year sky maps. The DMR data are cross-correlated with maps of rich clusters, extragalactic IRAS sources, HEAO 1 A-2 X-ray emission, and 5 GHz radio sources using a Fourier space technique. There is no evidence of significant contamination by such sources at an rms level of ~8 μK [95% confidence level (c.l.) at 7° resolution] in the most sensitive 53 GHz sky map. This level is consistent with previous limits set by analysis of earlier DMR data and by simple extrapolations from existing source models. We place a limit on the rms Comptonization parameter averaged over the high-latitude sky of δy < 1 × 10-6 (95% c.l.). Extragalactic sources have an insignificant effect on the cosmic microwave background power spectrum parameterizations determined from the DMR data.

We use the COBE 8 Differential Microwave Radiometers (DMR) 4 yr sky maps to model Galactic microwave emission at high latitudes (\b\ > 20°). Cross-correlation of the DMR maps with Galactic template maps detects fluctuations in the high-latitude microwave sky brightness with the angular variation of the DIRBE far-infrared dust maps and a frequency dependence consistent with a superposition of dust and free-free emission. We find no significant correlations between the DMR maps and various synchrotron templates. On the largest angular scales (e.g., quadrupole), Galactic emission is comparable in amplitude to the anisotropy in the cosmic microwave background (CMB). The CMB quadrupole amplitude, after correction for Galactic emission, has amplitude Qrms = 10.7 μK with random uncertainty 3.6 μK and systematic uncertainty 7.1 μK from uncertainty in our knowledge of Galactic microwave emission. © 1996. The American Astronomical Society. All rights reserved.

The Differential Microwave Radiometers (DMR) instrument aboard the Cosmic Background Explorer (CO BE) has mapped the full microwave sky to mean sensitivity 26 muK per 7° field of view. The absolute calibration is determined to 0.7% with drifts smaller than 0.2% per year. We have analyzed both the raw differential data and the pixelized sky maps for evidence of contaminating sources such as solar system foregrounds, instrumental susceptibilities, and artifacts from data recovery and processing. Most systematic effects couple only weakly to the sky maps. The largest uncertainties in the maps result from the instrument susceptibility to Earth's magnetic field, microwave emission from Earth, and upper limits to potential effects at the spacecraft spin period. Systematic effects in the maps are small compared to either the noise or the celestial signal: the 95% confidence upper limit for the pixel-pixel rms from all identified systematics is less than 6 muK in the worst channel. A power spectrum analysis of the (A - B)/2 difference maps shows no evidence for additional undetected systematic effects.

The two-point temperature correlation function is evaluated from the 4 yr COBE DMR microwave anisotropy maps. We examine the two-point function, which is the Legendre transform of the angular power spectrum, and show that the data are statistically consistent from channel to channel and frequency to frequency. The most likely quadrupole normalization is computed for a scale-invariant power-law spectrum of CMB anisotropy, using a variety of data combinations. For a given data set, the normalization inferred from the two-point data is consistent with that inferred by other methods. The smallest and largest normalizations deduced from any data combination are 16.4 and 19.6 mu K, respectively, with a value similar to 18 mu K generally preferred.

In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 yr of COBE Differential Microwave Radiometer (DMR) observations, with additional details 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 multifrequency 4 yr DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of n = 1.2 +/- 0.3 and a quadrupole normalization Q(rms-PS) = 15.31(-2.8)(+3.8) mu K. For n = 1 the best-fit normalization is Q(rms-PS/n=1) = 18 +/- 1.6 mu K. These values are consistent with both our previous 1 yr and 2 yr results. The results include use of the l = 2 quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The final DMR 4 yr sky maps, presented in this Letter, portray an accurate overall visual impression of the anisotropy since the signal-to-noise ratio is similar to 2 per 10 degrees sky map patch. The improved signal-to-noise ratio of the 4 yr maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics.

We search the high-latitude portion of the CODE Differential Microwave Radiometer (DMR) 4 yr sky maps for evidence of a non-Gaussian temperature distribution in the cosmic microwave background. The genus, three-point correlation function, and two-point correlation function of temperature maxima and minima are all in excellent agreement with the hypothesis that the CMB anisotropy on angular scales of 7° or larger represents a random-phase Gaussian field. A likelihood comparison of the DMR sky maps to a set of random-phase non-Gaussian toy models selects the exact Gaussian model as most likely. Monte Carlo simulations show that the two-point correlation of the peaks and valleys in the maps provides the greatest discrimination among the class of models tested. © 1996 The American Astronomical Society. All rights reserved.

We employ a pixel-based likelihood technique to estimate the angular power spectrum of the COBE Differential Microwave Radiometer (DMR) 4 yr sky maps. The spectrum is consistent with a scale-invariant power-law form with a normalization, expressed in terms of the expected quadrupole anisotropy, of Qrms-PS\n=1 = 18 ± 1.4 μK, and a best-fit spectral index of 1.2 ± 0.3. The normalization is somewhat smaller than we concluded from the 2 yr data, mainly due to additional Galactic modeling. We extend the analysis to investigate the extent to which the "small" quadrupole observed in our sky is statistically consistent with a power-law spectrum. The most likely quadrupole amplitude ranges between 7 and 10 μK, depending on the details of Galactic modeling and data selection, but in no case is there compelling evidence that the quadrupole is inconsistent with a power-law spectrum. We conclude with a likelihood analysis of the band power amplitude in each of four spectral bands between ℓ = 2 and 40, and find no evidence for deviations from a simple power-law spectrum.

We use the two-point correlation function of the extrema points (peaks and valleys) in the COBE Differential Microwave Radiometers (DMR) 2 year sky maps as a test for non-Gaussian temperature distribution in the cosmic microwave background anisotropy. A maximum-likelihood analysis compares the DMR data to n = 1 toy models whose random-phase spherical harmonic components a(lm) are drawn from either Gaussian, chi(2), or log-normal parent populations. The likelihood of the 53 GHz (A+B)/2 data is greatest for the exact Gaussian model. There is less than 10% chance that the non-Gaussian models tested describe the DMR data, limited primarily by type II errors in the statistical inference. The extrema correlation function is a stronger test for this class of non-Gaussian models than topological statistics such as the genus.

We present an analysis of multifrequency measurements of atmospheric emission in the Rayleigh-jeans portion of the cosmic microwave background spectrum (1-90 GHz) taken since 1986 from White Mountain, CA, and from the South Pole. Correlations of simultaneous data at 10 and 90 GHz and accurate low-frequency measurements show good agreement with model predictions for both sites. Our data from the South Pole 1989 campaign combined with real-time measurements of the local atmospheric profiles provide accurate verification of the expected independent contributions of H2O and O-2 emission. We show that variations on the order of 10% of the oxygen emission (both resonant and nonresonant components) are present on timescales of hours to days, mainly due to the evolution of the atmospheric pressure profile. Oxygen emission fluctuations appear larger than previously expected and may have significant consequences for ground-based cosmic microwave background experiments.

The accuracy of ground-based measurements of the comic microwave background (CMB) in the centimetric wavelength regime (about 1 to 90 GHs) is typically limited by the subtraction of foreground emission from the atmosphere, T_A,atm, and the Galaxy, T_A,Gal. Based on part of the large data set collected by the White Mountain and South Pole measurement campaigns we discuss the effects of these foreground components. We derive independent limits for second-order systematric effects on T_A,atm from measurements at different senith angles.

Ground-based measurements of the Cosmic Background Radiation (CBR) are hampered by emission from atmospheric O2 and water molecules. We performed multifrequency measurements of the atmospheric emission in the 1-90 GHz spectral range from two high-altitude sites (White Mountain, California, and South Pole, Antarctica). A power spectrum analysis of our data shows variations of the atmospheric antenna temperature which exceed, on time scales of a few hours or more, those expected from radiometer noise and changes in water column density. When combined with real-time measurements of the local profiles of atmospheric pressure, temperature and humidity, our results indicate variations of order 10% of the O2 emission (both resonant and non-resonant components) on time scales of hours to days, driven by the evolution of the atmospheric pressure profile. These oxygen emission fluctuations appear significantly larger than previously expected, and may affect ground-based CBR experiments. © 1995.

The cosmic microwave background spectrum measured by the FIRAS instrument on NASA's COBE is indistinguishable from a blackbody, implying stringent limits on energy release in the early universe later than the time t = 1 yr after the big bang. We compare the FIRAS data to previous precise measurements of the cosmic microwave background spectrum and find a reasonable agreement. We discuss the implications of the \y\ < 2.5 × 10-5 and \μ\ 3.3 × 10-4 95% confidence limits found by Mather et al. (1994) on many processes occurring after t = 1 yr, such as explosive structure formation, reionization, and dissipation of small-scale density perturbations. We place limits on models with dust plus Population III stars, or evolving populations of IR galaxies, by directly comparing the Mather et al. spectrum to the model predictions.

The first two years of COBE Differential Microwave Radiometers (DMR) observations of the cosmic microwave background (CMB) anisotropy are analyzed and compared with our previously published first year results. The results are consistent, but the addition of the second year of data increases the precision and accuracy of the detected CMB temperature fluctuations. The 2 yr 53 GHz data are characterized by rms temperature fluctuations of (ΔT)rms(7°) = 44 ± 7 μK and (ΔT)rms(10°) = 30.5 ± 2.7 μK at T and 7° angular resolution, respectively. The 53 × 90 GHz cross-correlation amplitude at zero lag is C(0)1/2 = 36 ± 5 μK(68% CL) for the unsmoothed (7° resolution) DMR data. We perform a likelihood analysis of the cross-correlation function, with Monte Carlo simulations to infer biases of the method, for a power-law model of initial density fluctuations, P(k) ∝ kn. The Monte Carlo simulations indicate that derived estimates of n are biased by + 0.11 ± 0.01, while the subset of simulations with a low quadrupole (as observed) indicate a bias of + 0.31 ± 0.04. Derived values for 68% confidence intervals are given corrected (and not corrected) for our estimated biases. Including the quadrupole anisotropy, the most likely quadrupole-normalized amplitude is Qrms-PS = 14.3 +5.2 -3.3 μK (12.8+5.2 -3.3 μK) with a spectral index n = 1.42+0.49 -0.55 (n = 1.53+0.49 -0.55). With n fixed to 1.0 the most likely amplitude is 18.2 ± 1.5 μK (17.4±1.5 μK). The marginal likelihood of n is 1.42 ± 0.37 (1.53 ± 0.37). Excluding the quadrupole anisotropy, the most likely quadrupole-normalized amplitude is Qrms-PS = 17.4+7.5 -5.2 μK (15.8 +7.5 -5.2 μK) with a spectral index n = 1.11+0.60 -0.55 (n = 1.221+0.60 -0.55). With n fixed to 1.0 the most likely amplitude is 18.6 ± 1.6 μK (18.2 ± 1.6 μK). The marginal likelihood of n is 1.11 ± 0.40 (1.22 ± 0.40). Our best estimate of the dipole from the 2 yr DMR data is 3.363 ± 0.024 mK toward Galactic coordinates (l, b) = (264°.4 ± 0°.2, + 48°.1 ± 0°.4), and

We compute the three-point temperature correlation function of the COBE Differential Microwave Radiometer (DMR) first-year sky maps to search for non-Gaussian temperature fluctuations. The level of fluctuations seen in the computed correlation function are too large to be attributable solely to instrument noise. However the fluctuations are consistent with the level expected to result from a superposition of instrument noise and sky signal arising from a Gaussian power-law model of initial fluctuations, with a quadrupole normalized amplitude of 17 μK and a power-law spectral index n= 1. We place limits on the amplitude of intrinsic three-point correlations with a variety of predicted functional forms.

We have searched the temperature maps from the COBE Differential Microwave Radiometers (DMR) first two years of data for evidence of unresolved sources. The high-latitude sky (\b\ > 30°) contains no sources brighter than 192 iμK thermodynamic temperature (322 Jy at 53 GHz). The cumulative count of sources brighter than threshold T, N(>T), is consistent with a superposition of instrument noise plus scale-invariant spectrum of cosmic temperature fluctuations normalized to Qrms-ps = 17 μK. We examine the temperature maps toward nearby clusters and find no evidence for any Sunyaev-Zel'dovich effect, Δy < 7.3 × 10-6 (95% CL) averaged over the DMR beam. We examine the temperature maps near the brightest expected radio sources and detect no evidence of significant emission. The lack of bright unresolved sources in the DMR maps, taken with anisotropy measurements on smaller angular scales, places a weak constraint on the integral number density of any unresolved Planck-spectrum sources brighter than flux density S, n(>S)<2 × 104 (S/1 Jy)-2 sr-1.

We use statistical and topological quantities to test the COBE DMR first-year sky maps against the hypothesis that the observed temperature fluctuations reflect Gaussian initial density perturbations with random phases. Recent papers discuss specific quantities as discriminators between Gaussian and non-Gaussian behavior, but the treatment of instrumental noise on the data is largely ignored. The presence of noise in the data biases many statistical quantities in a manner dependent on both the noise properties and the unknown cosmic microwave background temperature field. Appropriate weighting schemes can minimize this effect, but it cannot be completely eliminated. Analytic expressions are presented for these biases, and Monte Carlo simulations are used to assess the best strategy for determining cosmologically interesting information from noisy data. The genus is a robust discriminator that can be used to estimate the power-law quadrupole-normalized amplitude, Qrms - PS, independently of the two-point correlation function. The genus of the DMR data is consistent with Gaussian initial fluctuations with Qrms - PS = (15.7 ± 2.2) - (6.6 ± 0.3)(n - 1) μK, where n is the power-law index. Fitting the rms temperature variations at various smoothing angles gives Qrms - PS = 13.2 ± 2.5 μK and n = 1.7- 0.6 + 0.3. While consistent with Gaussian fluctuations, the first year data are only sufficient to rule out strongly non-Gaussian distributions of fluctuations.

A new technique of Fourier analysis on a cut sky has been applied to the two-year Cosmic Background Explorer (COBE) Differential Microwave Radiometer (DMR) 53 and 90 GHz sky maps. The Bayesian power spectrum estimation results are consistent with the Harrison-Zel'dovich n = 1 model. The maximum likelihood estimates of the usual parameters defining the power spectrum of primordial perturbations are n = 1.22 (1.02) and Qrms-PS = 17 (20) microK including (excluding) the quadrupole. A spectral-index-independent normalization is naturally expressed for the two-year maps in terms of the multipole amplitude a9 = 8.2 (8.3) microK (to approximately 12 sigma significance). The marginal likelihood function on n obtained by intergration with respect to a9 renders n = 1.17 +/- 0.31 (0.96 +/- 0.36).

Cosmic anisotropy produces an excess variance sigma(sky)2 in the DELTAT maps produced by the Differential Microwave Radiometer (DMR) on COBE that is over and above the instrument noise. After smoothing to an effective resolution of 10-degrees, this excess, sigma(sky)10-degrees, provides an estimate for the amplitude of the primordial density perturbation power spectrum with a cosmic uncertainty of only 12%. We employ detailed Monte Carlo techniques to express the amplitude derived from this statistic in terms of the universal rms quadrupole amplitude, [Q(RMS)2]0.5. The effects of monopole and dipole subtraction and the non-Gaussian shape of the DMR beam cause the derived [Q(RMS)2]0.5 to be 5%-10% larger than would be derived using simplified analytic approximations. We also investigate the properties of two other map statistics: the actual quadrupole and the Boughn-Cottingham statistic. Both the sigma(sky)(10-degrees) statistic and the Boughn-Cottingham statistic are consistent with the [Q(RMS)2] = 17 +/- 5 muK reported by Smoot et al. (1992) and Wright et al. (1992).

The COBE DMR sky maps contain low-level correlated noise. We obtain estimates of the amplitude and pattern of the correlated noise from three techniques: angular averages of the covariance matrix, Monte Carlo simulations of two-point correlation functions, and direct analysis of the DMR maps. The results from the three methods are mutually consistent. The noise covariance matrix of a DMR sky map is diagonal to an accuracy of better than 1%. For a given sky pixel, the dominant noise covariance occurs with the ring of pixels at an angular separation of 60 degrees due to the 60 degrees separation of the DMR horns. The mean covariance at 60 degrees is 0.45%(+0.18)(-0.14) of the mean variance. Additionally, the variance in a given pixel is 0.7% greater than would be expected from a single beam experiment with the same noise properties. Autocorrelation functions suffer from a similar to 1.5 sigma positive bias at 60 degrees while cross-correlations have no bias. Published COBE DMR results are not significantly affected by correlated noise.

We present a determination of the cosmic microwave background dipole amplitude and direction from the COBE Dierential Microwave Radiometers (DMR) first year of data. Data from the six DMR channels are consistent with a Dopplershifted Planck function of dipole amplitude T = 3:365 0:027 mK toward direction (l I I ; bI I ) = (264:4 0:3 ; 48:4 0:5 ). The implied velocity of the Local Group with respect to the CMB rest frame is ~vLG = 627 22 km s1 toward (l I I ; bI I ) = (276 3; 30 3). DMR has also mapped the dipole anisotropyresulting from the Earth's orbital motion about the Solar system barycenter, yieldinga measurement of the monopole CMB temperature T0 at 31.5, 53, and 90 GHz,T0 = 2:75 0:05 K.

We have used a radio-frequency-gain total-power radiometer to measure the intensity of the cosmic microwave background (CMB) at a frequency of 1.47 GHz (20.4 cm wavelength) from White Mountain, California in 1988 September and from the South Pole, Antarctica in 1989 December. The CMB thermodynamic temperature, T(CMB), is 2.27 +/- 0.25 K (68% confidence limit) measured from White Mountain and 2.26 +/- 0.20 K from the South Pole site. The combined result is 2.26 +/- 0.19 K. The correction for Galactic emission has been derived from scaled low-frequency maps and constitutes the main source of error. The atmospheric signal is extrapolated from our zenith scan measurements at higher frequencies. These results are consistent with our previous measurement at 1.41 GHz (Levin et al. 1988) and approximately 2.5 sigma from the 2.74 +/- 0.01 K global average CMB temperature.

We compare the large-scale cosmic background anisotropy detected by the COBE Differential Microwave Radiometer (DMR) instrument to the sensitive previous measurements on various angular scales, and to the predictions of a wide variety of models of structure formation driven by gravitational instability. The observed anisotropy is consistent with all previously measured upper limits and with a number of dynamical models of structure formation. For example, the data agree with an unbiased cold dark matter (CDM) model with H0 = 50 Km s-1 Mpc-1 and ΔM/M = 1 in a 16 Mpc radius sphere. Other models, such as CDM plus massive neutrinos [hot dark matter (HDM)], or CDM with a nonzero cosmological constant are also consistent with the COBE detection and can provide the extra power seen on 5-10,000 Km s-1 scales.

The first year of data from the Differential Microwave Radiometers (DMR) on the Cosmic Backgoround Explorer (COBE) show statistically significant (>7 σ) structure that is well described as scale-invariant fluctuations with a Gaussian distribution. The major portion of the observed structure cannot be attributed to known systematic errors in the instrument, artifacts generated in the data processing, or known Galactic emission. The structure is consistent with a thermal spectrum at 31, 53, and 90 GHz as expected for cosmic microwave background anisotropy. The rms sky variation, smoothed to a total 10° FWHM Gaussian, is 30 ± 5 μK (ΔT/T = 11 × 10-6) for Galactic latitude \b\ > 20° data with the dipole anisotropy removed. The rms cosmic quadrupole amplitude is 13 ± 4 μK (ΔT/T ≈ 5 × 10-6). The angular autocorrelation of the signal in each radiometer channel and cross-correlation between channels are consistent and give a primordial fluctuation power-law spectrum with index n = 1.1 ± 0.5, and an rms-quadrupole-normalized amplitude of 16 ± 4 μK(ΔT/T ≈ 6 × 10-6). These features are in accord with the Harrison-Zel'dovich (scale-invariant, n = 1) spectrum predicted by models of inflationary cosmology. The low overall fluctuation amplitude is consistent with theoretical predictions of the minimal level gravitational potential variations that would give rise to the observed present day structure.

The COBE spacecraft was launched 1989 November 18 UT carrying three scientific instruments into Earth orbit for studies of cosmology. One of these instruments, the Differential Microwave Radiometer (DMR), is designed to measure the large-angular-scale temperature anisotropy of the cosmic microwave background radiation at three frequencies (31.5, 53, and 90 GHz). In this paper we present three methods used to calibrate the DMR. First, the signal difference between beam-filling hot and cold targets observed on the ground provides a primary calibration that is transferred to space by noise sources internal to the instrument. Second, the Moon is used in flight as an external calibration source. Third, the signal arising from the Doppler effect due to the Earth's motion around the barycenter of the solar system is used as an external calibration source. Preliminary analysis of the external source calibration techniques confirms the accuracy of the currently more precise ground-based calibration. Assuming the noise source behavior did not change from the ground-based calibration to flight we derive a 0.1%-0.4% relative and 0.7%-2.5% absolute calibration uncertainty, depending on radiometer channel.

We have measured the temperature of the cosmic microwave background (CMB) radiation at a frequency of 7.5 GHz (4 cm wavelength), obtaining a brightness temperature of TCMB = 2.70 ± 0.08 K (68% confidence level). The measurement was made from a site near the geographical South Pole during the austral spring of 1989 and was part of an international collaboration to measure the CMB spectrum at low frequencies with a variety of radiometers from several different sites. This recent result is in agreement with our 1988 measurement at the same frequency, which was made from a different site with significantly different systematic errors. The combined result of the 1988 and 1989 measurements is 2.64 ± 0.06 K.

The COBE Differential Microwave Radiometer (DMR) anisotropy experiment is sufficiently sensitive and free from systematic errors that our knowledge of Galactic emission is a limiting factor in interpreting the measurements of the 1 yr DMR maps. In this paper we construct preliminary models of microwave emiSSion from our Galaxy based on COBE and other data for the purpose of distinguishing cosmic and Galactic signals. DMR maps, with the modeled Galactic emission removed, are fitted for a quadrupole distribution. Our best estimate of the cosmic quadrupole is found to be Q(rms) = 13 +/- 4-mu-K, (DELTA-T/T)Q = (4.8 +/- 1.5) x 10(-6). Autocorrelation functions for individual Galactic components are presented. When Galactic emission is removed from the DMR data, the residual fluctuations are virtually unaffected, and therefore they are not dominated by any known Galactic emission component.

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full sky microwave maps at 31.5, 53, and 90 GHz with two independent channels at each frequency. At millimeter wavelengths, the cosmic microwave background (CMB) dominates the sky brightness with an intensity more than a thousand times the foreground Galactic emission. Emission from our Galaxy is seen unambiguously at all three frequencies. The dipole anisotropy, attributed to the motion of our solar system with respect to the CMB reference frame, shows strongly in all six sky maps and is consistent with a Doppler-shifted thermal spectrum. The best fitted dipole has amplitude 3.3 ± 0.2 mK in the direction (α, δ) = (11h2 ± 0.h2, - 7° ± 2°) (J2000) or (l, b) = (265° ± 2°, 48° ± 2°). There is no clear evidence in the maps for any other large angular scale features. We place a limit of ΔT/T0 < 3 ×10-5 (T0 = 2.735 K) for the rms quadrupole amplitude; for monochromatic fluctuations ΔT/T0 < 4 × 10-5; and for Gaussian fluctuations ΔT/T0 < 4 × 10-5. All limits are given at the 95% confidence level and reflect currently estimated limits on systematic effects. These measurements place the most severe constraints to date on many potential physical processes in the early universe, such as anisotropic expansion of the universe, very long wavelength gravitational waves, cosmic strings, and large-scale primordial fluctuations.

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full-sky maps at frequencies 31.5, 53, and 90 GHz. The redundant channels and matched beams at three frequencies distinguish the DMR from previous large-scale surveys. Galactic emission is seen unambiguously at all three frequencies. The only large-scale anisotropy detected in the cosmic microwave background is the dipole anisotropy. There is no clear evidence for any other large-angular-scale feature in the maps. Without correcting for any systematic effects, we are able to place limits ΔT/T0<3x10-5 for the rms quadrupole amplitude, ΔT/T0<4x10-5 for monochromatic fluctuations, and ΔT/T0<4x10-5 for Gaussian fluctuations (all limits are 95% C.L. with T0=2.735 K). The data limit ΔT/T0<10-4 for any feature larger than 7 degree. We briefly review the DMR and discuss some implications of these results for cosmology.

The Rayleigh-Jeans portion of the cosmic microwave background (CMB) was measured over many years, changing major portions of the experimental apparatus and technique to search for unsuspected systematic effects. The resulting data set is consistent with a blackbody CMB spectrum and shows no evidence for significant undiagnosed systematic errors. Comparison with measurements at higher frequencies shows the CMB to be consistent with a blackbody across more than three decades of wavelength.