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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.

Absorption of orthoformaldehyde (H2CO) toward the giant H II region W51A (G49.5 - 0.4) has been used as a remote probe of the cosmic microwave background (CMB) at a distance of 7.5 kpc. VLA observations of the 6 cm and 2 cm wavelength transitions provided sufficient resolution and sensitivity to resolve condensations within the foregound clouds. Solutions to the equations of statistical equilibrium within each condensation in the context of a large velocity gradient model yielded an estimate for the CMB temperature at 2.1 mm wavelength of 3.2 + or - 0.9 K. The uncertainty is dominated by modeling of collisional pumping by neutral hydrogen molecules (H2).

We have measured the temperature of the cosmic background radiation (CBR) at a frequency of 3.8 GHz (7.9 cm wavelength), during two consecutive summers, obtaining a brightness temperature, TCBR, of 2.56 ± 0.08 K in 1987 and 2.71 ± 0.07 K in 1988 (68% confidence level). The new results are in agreement with our previous measurement at 3.7 GHz obtained in 1986 and have smaller error bars. Combining measurements from all 3 years, we obtain TCBR = 2.64 ± 0.07 K.

We have measured the intensity of the cosmic microwave background (CMB) at a frequency of 7.5 GHz (wavelength 4.0 cm) using a ground-based, total power radiometer calibrated at the antenna aperture by an external cryogenic reference target. The radiometer measured the difference in antenna temperature between the reference target and the zenith sky from a dry, high-altitude site. Subtraction of foreground signals (primarily atmospheric and galactic emission) measured with the same instrument leaves the CMB as the residual. The radiometer measured the atmospheric antenna temperature by correlating the signal change with the air mass in the beam during tip scans. The small galactic signal was subtracted based on extrapolation from lower frequencies and was checked by differential drift scans. The limiting uncertainty in the CMB measurement was the effect of ground radiation in the antenna sidelobes during atmospheric measurements. The thermodynamic temperature of the CMB at 7.5 GHz is 2.60 ± 0.07 K (68% confidence level).

The temperature of the cosmic background radiation (CBR) has been measured at a frequency of 3.7 GHz (8.1 cm wavelength), using a total power, direct RF-gain receiver. The results give a brightness temperature, T(CBR), of 2.59 + or - 0.13 K (68 percent confidence level). Details of the instrument and of the experimental procedure are given. This measurement is part of a larger experiment to measure the spectrum of the cosmic background radiation between 0.6 and 90 GHz (50 and 0.33 cm wavelength).

We have measured the fluctuation power spectrum and correlation of fluctuations of atmospheric emission at wavelengths of 3, 0.9, and 0.33 cm. The fluctuations measured at different wavelengths are well correlated with each other and can be described with a simple parameterization. Our results are in agreement with other measurements of phase and absorption variation and with the hypothesis that atmospheric water fluctuations dominate other effects. Our measurements are in good agreement with the predictions of the Kolmogorov turbulence model, which can be used to predict the temporal and spatial spectra of atmospheric scintillation and emission variation