D. T. Wilkinson

The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. I. Limits and Detections

The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) spacecraft was designed primarily to conduct a systematic search for an isotropic cosmic infrared background (CIB) in 10 photometric bands from 1.25 to 240 μm. The results of that search are presented here. Conservative limits on the CIB are obtained from the minimum observed brightness in all-sky maps at each wavelength, with the faintest limits in the DIRBE spectral range being at 3.5 μm (νIν < 64 nW m-2 sr-1, 95% confidence level) and at 240 μm (νIν < 28 nW m-2 sr-1, 95% confidence level). The bright foregrounds from interplanetary dust scattering and emission, stars, and interstellar dust emission are the principal impediments to the DIRBE measurements of the CIB. These foregrounds have been modeled and removed from the sky maps. Assessment of the random and systematic uncertainties in the residuals and tests for isotropy show that only the 140 and 240 μm data provide candidate detections of the CIB. The residuals and their uncertainties provide CIB upper limits more restrictive than the dark sky limits at wavelengths from 1.25 to 100 μm. No plausible solar system or Galactic source of the observed 140 and 240 μm residuals can be identified, leading to the conclusion that the CIB has been detected at levels of νIν = 25 ± 7 and 14 ± 3 nW m-2 sr-1 at 140 and 240 μm, respectively. The integrated energy from 140 to 240 μm, 10.3 nW m-2 sr-1, is about twice the integrated optical light from the galaxies in the Hubble Deep Field, suggesting that star formation might have been heavily enshrouded by dust at high redshift. The detections and upper limits reported here provide new constraints on models of the history of energy-releasing processes and dust production since the decoupling of the cosmic microwave background from matter.

MEASUREMENT OF THE COSMIC MICROWAVE BACKGROUND SPECTRUM BY THE COBE FIRAS INSTRUMENT

The cosmic microwave background radiation (CMBR) has a blackbody spectrum within 3.4 x 10(exp -8) ergs/sq cm/s/sr cm over the frequency range from 2 to 20/cm (5-0.5 mm). These measurements, derived from the Far-Infrared Absolute Spectrophotomer (FIRAS) instrument on the Cosmic Background Explorer (COBE) satellite, imply stringent limits on energy release in the early universe after t approximately 1 year and redshift z approximately 3 x 10(exp 6). The deviations are less than 0.30% of the peak brightness, with an rms value of 0.01%, and the dimensionless cosmological distortion parameters are limited to the absolute value of y is less than 2.5 x 10(exp -5) and the absolute value of mu is less than 3.3 x 10(exp -4) (95% confidence level). The temperature of the CMBR is 2.726 +/- 0.010 K (95% confidence level systematic).

Calibrator Design for the COBE Far Infrared Absolute Spectrophotometer (FIRAS)

The photometric errors of the external calibrator for the Far Infrared Absolute Spectrophotometer (FIRAS) instrument on the COBE are smaller than the measurement errors on the cosmic microwave background radiation (CMBR) spectrum (typically 0.02 MJy sr-1, 1 σ) and smaller than 0.01% of the peak brightness of the CMB. The calibrator is a reentrant cone, shaped like a trumpet mute, made of Eccosorb iron-loaded epoxy. It fills the entire beam of the instrument and is the source of its accuracy. Its known errors are caused by reflections, temperature gradients, and leakage through the material and around the edge. Estimates and limits are given for all known error sources. Improvements in understanding the temperature measurements of the calibrator allow an improved CMB temperature determination of 2.725±0.002 K.

The Microwave Anisotropy Probe Mission

The purpose of the MAP mission is to determine the geometry, content, and evolution of the universe via a 13 arc-min full-width-half-max (FWHM) resolution full sky map of the temperature anisotropy of the cosmic microwave background radiation with uncorrelated pixel noise, minimal systematic errors, multifrequency observations, and accurate calibration. These attributes were key factors in the success of NASA’s Cosmic Background Explorer (COBE) mission, which made a 7Æ FWHM resolution full sky map, discovered temperature anisotropy, and characterized the fluctuations with two parameters, a power spectral index and a primordial amplitude. Following COBE considerable progress has been made in higher resolution measurements of the temperature anisotropy. With 45 times the sensitivity and 33 times the angular resolution of the COBE mission, MAP will vastly extend our knowledge of cosmology. MAP will measure the physics of the photon-baryon fluid at recombination. From this, MAP measurements will constrain models of structure formation, the geometry of the universe, and inflation. In this paper we present a pre-launch overview of the design and characteristics of the MAP mission. This information will be necessary for a full understanding of the MAP data and results, and will also be of interest to scientists involved in the design of future cosmic microwave background experiments and/or space science missions.

Four year COBE DMR cosmic microwave background observations: Maps and basic results

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 Qrms-PS = 15.3−2.8+3.8 μK. For n = 1 the best-fit normalization is Qrms-PS|n=1 = 18 ± 1.6 μK. 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 ~2 per 10° 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.

Dipole anisotropy in the COBE DMR first year sky maps

We present a determination of the cosmic microwave background dipole amplitude and direction from the COBE Differential Microwave Radiometers (DMR) first year of data. Data from the six DMR channels are consistent with a Doppler-shifted Planck function of dipole amplitude ΔT=3.365±0.027 mK toward direction (l II , b II )=(264°.4±0°.3, 48°.4±0°.5). The implied velocity of the Local Group with respect to the CMB rest frame is v LG =627±22 km s −1 toward (l II , b II )=(276°±3°, 30°±3°). DMR has also mapped the dipole anisotropy resulting from the Earths orbital motion about the Solar system barycenter, yielding a measurement of the monopole CMB temperature T 0 at 31.5, 53, and 90 GHz, T 0 =2.75±0.05 KWe present a determination of the cosmic microwave background dipole amplitude and direction from the COBE Differential Microwave Radiometers (DMR) first year of data. Data from the six DMR channels are consistent with a Doppler-shifted Planck function of dipole amplitude Delta T = 3.365 +/-0.027 mK toward direction (l,b) = (264.4 +/- 0.3 deg, 48.4 +/- 0.5 deg). The implied velocity of the Local Group with respect to the CMB rest frame is 627 +/- 22 km/s toward (l,b) = (276 +/- 3 deg, 30 +/- 3 deg). DMR has also mapped the dipole anisotropy resulting from the Earths orbital motion about the Solar system barycenter, yielding a measurement of the monopole CMB temperature at 31.5, 53, and 90 GHz, to be 2.75 +/- 0.05 K.

The COBE mission - its design and performance two years after launch

COBE, NASAs first space mission devoted primarily to cosmology, carries three scientific instruments to make precise measurements of the spectrum and anisotropy of the cosmic microwave background radiation on angular scales greater than 7° and to conduct a search for a diffuse cosmic infrared background radiation with 0°.7 angular resolution. The mission goal is to make these measurements to the limit imposed by the local astrophysical foregrounds. The COBE instruments cover the wavelength range from 1.2 μm to 1 cm. The instruments are calibrated periodically in orbit using internal calibrators and celestial standards

The Lunar Laser Ranging Experiment: Accurate ranges have given a large improvement in the lunar orbit and new selenophysical information

The lunar ranging measurements now being made at the McDonald Observatory have an accuracy of 1 nsec in round-trip travel time. This corresponds to 15 cm in the one-way distance. The use of lasers with pulse-lengths of less than 1 nsec is expected to give an accuracy of 2 to 3 cm in the next few years. A new station is under construction in Hawaii, and additional stations in other countries are either in operation or under development. It is hoped that these stations will form the basis for a worldwide network to determine polar motion and earth rotation on a regular basis, and will assist in providing information about movement of the tectonic plates making up the earths surface. Several mobile lunar ranging stations with telescopes having diameters of 1.0 m or less could, in the future, greatly extend the information obtainable about motions within and between the tectonic plates. The data obtained so far by the McDonald Observatory have been used to generate a new lunar ephemeris based on direct numerical integration of the equations of motion for the moon and planets. With this ephemeris, the range to the three Apollo retro-reflectors can be fit to an accuracy of 5 m by adjusting the differences in moments of inertia of the moon about its principal axes, the selenocentric coordinates of the reflectors, and the McDonald longitude. The accuracy of fitting the results is limited currently by errors of the order of an arc second in the angular orientation of the moon, as derived from the best available theory of how the moon rotates in response to the torques acting on it. Both a new calculation of the moons orientation as a function of time based on direct numerical integration of the torque equations and a new analytic theory of the moons orientation are expected to be available soon, and to improve considerably the accuracy of fitting the data. The accuracy already achieved routinely in lunar laser ranging represents a hundredfold improvement over any previously available knowledge of the distance to points on the lunar surface. Already, extremely complex structure has been observed in the lunar rotation and significant improvement has been achieved in our knowledge of lunar orbit. The selenocentric coordinates of the retroreflectors give improved reference points for use in lunar mapping, and new information on the lunar mass distribution has been obtained. Beyond the applications discussed in this article, however, the history of science shows many cases of previously unknown, phenomena discovered as a consequence of major improvements in the accuracy of measurements. It will be interesting to see whether this once again proves the case as we acquire an extended series of lunar distance observations with decimetric and then centimetric accuracy.

Cosmic temperature fluctuations from two years of COBE differential microwave radiometers observations

The first two years of COBE DMR observations of the 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 two-year 53 GHz data are characterized by RMS temperature fluctuations of DT=44+/-7 uK at 7 degrees and DT=30.5+/-2.7 uK at 10 degrees angular resolution. The 53X90 GHz cross-correlation amplitude at zero lag is C(0)^{1/2}=36+/-5 uK (68%CL) for the unsmoothed 7 degree DMR data. A likelihood analysis of the cross correlation function, including the quadrupole anisotropy, gives a most likely quadrupole-normalized amplitude Q_{rms-PS}=12.4^{+5.2}_{-3.3} uK (68% CL) and a spectral index n=1.59^{+0.49}_{-0.55} for a power law model of initial density fluctuations, P(k)~k^n. With n fixed to 1.0 the most likely amplitude is 17.4 +/-1.5 uK (68% CL). Excluding the quadrupole anisotropy we find Q_{rms-PS}= 16.0^{+7.5}_{-5.2} uK (68% CL), n=1.21^{+0.60}_{-0.55}, and, with n fixed to 1.0 the most likely amplitude is 18.2+/-1.6 uK (68% CL). Monte Carlo simulations indicate that these derived estimates of n may be biased by ~+0.3 (with the observed low value of the quadrupole included in the analysis) and {}~+0.1 (with the quadrupole excluded). Thus the most likely bias-corrected estimate of n is between 1.1 and 1.3. Our best estimate of the dipole from the two-year DMR data is 3.363+/-0.024 mK towards Galactic coordinates (l,b)= (264.4+/-0.2 degrees, +48.1+/-0.4 degrees), and our best estimate of the RMS quadrupole amplitude in our sky is 6+/-3 uK.The first two years of COBE DMR observations of the 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 two-year 53 GHz data are characterized by RMS temperature fluctuations of DT=44+/-7 uK at 7 degrees and DT=30.5+/-2.7 uK at 10 degrees angular resolution. The 53X90 GHz cross-correlation amplitude at zero lag is C(0)^{1/2}=36+/-5 uK (68%CL) for the unsmoothed 7 degree DMR data. A likelihood analysis of the cross correlation function, including the quadrupole anisotropy, gives a most likely quadrupole-normalized amplitude Q_{rms-PS}=12.4^{+5.2}_{-3.3} uK (68% CL) and a spectral index n=1.59^{+0.49}_{-0.55} for a power law model of initial density fluctuations, P(k)~k^n. With n fixed to 1.0 the most likely amplitude is 17.4 +/-1.5 uK (68% CL). Excluding the quadrupole anisotropy we find Q_{rms-PS}= 16.0^{+7.5}_{-5.2} uK (68% CL), n=1.21^{+0.60}_{-0.55}, and, with n fixed to 1.0 the most likely amplitude is 18.2+/-1.6 uK (68% CL). Monte Carlo simulations indicate that these derived estimates of n may be biased by ~+0.3 (with the observed low value of the quadrupole included in the analysis) and {}~+0.1 (with the quadrupole excluded). Thus the most likely bias-corrected estimate of n is between 1.1 and 1.3. Our best estimate of the dipole from the two-year DMR data is 3.363+/-0.024 mK towards Galactic coordinates (l,b)= (264.4+/-0.2 degrees, +48.1+/-0.4 degrees), and our best estimate of the RMS quadrupole amplitude in our sky is 6+/-3 uK.

Preliminary separation of galactic and cosmic microwave emission for the COBE Differential Microwave Radiometer

Preliminary models of microwave emission from the Milky Way Galaxy based on COBE and other data are constructed for the purpose of distinguishing cosmic and Galactic signals. Differential Microwave Radiometer (DMR) maps, with the modeled Galactic emission removed, are fitted for a quadrupole distribution. 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. 42 refs.