The COBE Differential Microwave Radiometers (DMR)

Photo of DMR receiverThe DMR experiment is designed to search for primeval fluctuations in the brightness of the cosmic microwave background (CMB) radiation, small temperature differences between different parts of the sky.  The instrument consists of six differential microwave radiometers, two nearly independent channels that operate at each of three frequencies: 31.5, 53, and 90 GHz.  These frequencies were chosen to minimize the contamination from Galactic emission.  Each differential radiometer measures the difference in power received from two directions in the sky separated by 60 degrees, using a pair of horn antennas. Each antenna has a 7 degree (FWHM) beam. The COBE satellite rotates at approximately 0.8 rpm and precesses in near-polar orbit, following the day-night terminator.  As a result the DMR makes highly-redundant observations of the entire sky in a six-month period. Observations were made over a four-year period ending in December 1993. Further descriptions of the DMR experiment are published in Bennett et al. (1992a) and Smoot et al. (1990).

link to DMR block
diagram showing signal flow The original DMR results, based on analyses of the first year of data, were published by Bennett et al. (1992b, 1993), Hinshaw et al. (1994), Kogut et al. (1992), Smoot et al. (1992, 1994), and Wright et al. (1992, 1994b).  Bennett et al. (1992b) analyzed the Galactic emission and derived a value for the cosmic quadrupole.  Bennett et al. (1993) searched for evidence of noncosmological source contributions. Hinshaw et al. (1994) analyzed the 3-point correlation function to search for evidence of non-Gaussian fluctuations.  Kogut et al. (1992) analyzed the dipole anisotropy and derived the Solar velocity, relative to the cosmic background radiation, implied by the observed dipole. Smoot et al. (1992) summarized the first year data results and Smoot et al. (1994) searched for evidence of non-Gaussian fluctuations. Wright et al. (1992) interpreted the DMR anisotropy in terms of cold dark matter cosmological models.  Wright et al. (1994b) commented on the power spectrum normalization implied by the excess variance in the first-year maps.

Scientific results of the COBE DMR experiment based on the entire 4-year COBE mission are reported by Bennett et al. (1996), Gorski et al. (1996), Hinshaw et al. (1996a), Wright et al. (1996), Hinshaw et al. (1996b), Kogut et al. (1996a), Banday et al. (1996), Kogut et al. (1996b), Banday et al. (1997), and Fixsen et al. (1997). These papers provide the definitive results of the experiment and update earlier results based on the first two years of data. Following their analysis of the first two years of DMR data Bennett et al. (1994) reported the amplitude of temperature fluctuations is 36 +/- 5 microKelvin at 7 degrees, and 30.5 +/- 2.7 microKelvin when smoothed to 10 degrees.  Several techniques were used to estimate the fluctuation power spectrum amplitude and spectral index (Bennett et al. 1994, Gorski et al. 1994, Wright et al. 1994a). Regardless of the technique, the spectral index was found to be consistent with a scale-invariant power spectrum (n = 1).  Kogut et al. (1994) searched for unresolved point sources in the two-year maps.


Banday, A.J., et al. 1996, "Non-Cosmological Signal Contributions to the COBE-DMR Four-Year Sky Maps," ApJ, 468, L85.

Banday, A.J., et al. 1997, "Root Mean Square Anisotropy in the COBE DMR Four-Year Sky Maps," ApJ, 475, 393.

Bennett, C.L., et al. 1992a, "COBE Differential Microwave Radiometers: Calibration Techniques," ApJ, 391, 466.

Bennett, C.L., et al. 1992b, "Preliminary Separation of Galactic and Cosmic Microwave Emission for the COBE Differential Microwave Radiometer," ApJ, 396, L7.

Bennett, C.L., et al. 1993, "Noncosmological Signal Contributions to the COBE DMR Anisotropy Maps," ApJ, 414, L77.

Bennett, C.L., et al. 1994, "Cosmic Temperature Fluctuations from Two Years of COBE-DMR Observations," ApJ, 436, 423.

Bennett, C.L., et al. 1996, "4-Year COBE DMR Cosmic Microwave Background Observations: Maps and Basic Results," ApJ, 464, L1.

Fixsen, D.J., et al. 1997, "The Spectrum of the Cosmic Microwave Background Anisotropy from the Combined COBE FIRAS and DMR Observations," ApJ, 486, 623.

Gorski, K.M. 1994, "On Determining the Spectrum of Primordial Inhomogeneity from the COBE DMR Sky Maps: II. Results of Two Year Data Analysis," ApJ, 430, L85.

Gorski, K.M., et al. 1996, Power Spectrum of Primordial Inhomogeneity Determined from the 4-Year COBE DMR Sky Maps," ApJ, 464, L11.

Hinshaw, G., et al. 1994, "Limits on Three-Point Correlations in the COBE-DMR First-Year Anisotropy Maps," ApJ, 431, 1.

Hinshaw, G., et al. 1996a, "Band Power Spectra in the COBE DMR 4-Year Anisotropy Maps," ApJ, 464, L17.

Hinshaw, G., et al. 1996b, "2-Point Correlations in the COBE DMR 4-Year Anisotropy Maps," ApJ, 464, L25.

Kogut, A., et al. 1992, "COBE Differential Microwave Radiometers: Preliminary Systematic Error Analysis," ApJ, 401, 1.

Kogut, A., et al. 1994, "Search for Unresolved Sources in the COBE-DMR Two-Year Sky Maps," ApJ, 433, 435.

Kogut, A., et al. 1996a, "Tests for Non-Gaussian Statistics in the DMR Four-Year Sky Maps," ApJ, 464, L29.

Kogut, A., et al. 1996b, "Calibration and Systematic Error Analysis for the COBE DMR 4 Year Sky Maps," ApJ, 470, 653.

Smoot, G., et al. 1990, "COBE Differential Microwave Radiometers: Instrument Design and Implementation," ApJ, 360, 685.

Smoot, G., et al. 1992, "Structure in the COBE Differential Microwave Radiometer First-Year Maps," ApJ, 396, L1.

Smoot, G., et al. 1994, "Statistics and Topology of the COBE-DMR First-Year Sky Maps," ApJ, 437, 1.

Wright, E.L., et al. 1992, "Interpretation of the Cosmic Microwave Background Radiation Anisotropy Detected by the COBE Differential Microwave Radiometer," ApJ, 396, L13.

Wright, E.L., et al. 1994a, "Angular Power Spectrum of the Microwave Background Anisotropy Seen by the COBE Differential Microwave Radiometer," ApJ, 436, 433.

Wright, E.L., et al. 1994b, "Comments on the Statistical Analysis of Excess Variance in the COBE DMR Maps," ApJ, 420, 1.

Wright, E.L., et al. 1996, "Angular Power Spectrum of the Cosmic Microwave Background Anisotropy Seen by the COBE DMR," ApJ, 464, L21.

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