Showing papers by "Edward L. Wright published in 1996"

The Cosmic Microwave Background spectrum from the full COBE FIRAS data set

Abstract: We have refined the analysis of the data from the FIRAS (Far-InfraRed Absolute Spectrophotometer) on board the COBE (COsmic Background Explorer). The FIRAS measures the difference between the cosmic microwave background and a precise blackbody spectrum. We find new, tighter upper limits on general deviations from a blackbody spectrum. The rms deviations are less than 50 parts per million of the peak of the cosmic microwave background radiation. For the Comptonization and chemical potential, we find |y| < 15 × 10–6 and |μ| < 9 × 10–5 (95% confidence level [CL]). There are also refinements in the absolute temperature, 2.728 ± 0.004 K (95% CL), the dipole direction, (1, b)/(26414 ± 0.30, 4826 ± 0.30) (95% CL), and the amplitude, 3.372 ± 0.014 mK (95% CL). All of these results agree with our previous publications.

Four-Year COBE* DMR Cosmic Microwave Background Observations: Maps and Basic Results

Abstract: 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.

4-Year COBE DMR Cosmic Microwave Background Observations: Maps and Basic Results

Abstract: The cosmic microwave background radiation provides unique constraints on cosmological models. In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 years of COBE DMR observations, as detailed 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 multi-frequency 4-year DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of $n=1.2\pm 0.3$ and a quadrupole normalization $Q_{rms-PS}=15.3^{+3.8}_{-2.8}$ $\mu$K. For $n=1$ the best-fit normalization is $Q_{rms-PS}\vert_{n=1}=18\pm 1.6$ $\mu$K. These values are consistent with both our previous 1-year and 2-year results. The results include use of the $\ell=2$ quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The 4-year 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 degree sky map patch. The improved signal-to-noise ratio of the 4-year maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics.

Power spectrum of primordial inhomogeneity determined from the four year COBE DMR sky maps

Abstract: Fourier analysis and power spectrum estimation of the cosmic microwave background anisotropy on an incompletely sampled sky developed by Gorski has been applied to the 4 yr COBE DMR 31.5, 53, and 90 GHz sky maps. Likelihood analysis using newly constructed Galaxy cuts (extended beyond |b| = 20° to excise the known foreground emission) and simultaneously correcting for the faint high-latitude Galactic foreground emission is conducted on the DMR sky maps pixelized in both ecliptic and Galactic coordinates. The Bayesian power spectrum estimation from the foreground-corrected 4 yr COBE DMR data renders n ~ 1.2 ± 0.3 and Qrms-PS ~ 15.3 -->−2.8+3.7 μK (projections of the two-parameter likelihood). The results are consistent with the Harrison-Zeldovich n = 1 model of amplitude Qrms-PS ~ 18 μK detected with significance exceeding 14 σ (δQ/Q 0.07). (A small power spectrum amplitude drop below the published 2 yr results is predominantly due to the application of the new, extended Galaxy cuts.)

Microwave Emission at High Galactic Latitudes in the Four-Year DMR Sky Maps

Abstract: We use the COBE 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.

Microwave Emission at High Galactic Latitudes

Abstract: We use the COBE Differential Microwave Radiometers (DMR) 4-year sky maps to model Galactic microwave emission at high latitudes (|b| > 20 deg). 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 $Q_{rms}$ = 10.7 uK with random uncertainty 3.6 uK and systematic uncertainty 7.1 uK from uncertainty in our knowledge of Galactic microwave emission.

Power Spectrum of Primordial Inhomogeneity Determined from the 4-Year COBE DMR Sky Maps

Abstract: Fourier analysis and power spectrum estimation of the cosmic microwave background anisotropy on an incompletely sampled sky developed by Gorski (1994) has been applied to the high-latitude portion of the 4-year COBE DMR 31.5, 53 and 90 GHz sky maps. Likelihood analysis using newly constructed Galaxy cuts (extended beyond |b| = 20deg to excise the known foreground emission) and simultaneously correcting for the faint high latitude galactic foreground emission is conducted on the DMR sky maps pixelized in both ecliptic and galactic coordinates. The Bayesian power spectrum estimation from the foreground corrected 4-year COBE DMR data renders n ~ 1.2 +/- 0.3, and Q_{rms-PS} ~ 15.3^{+3.7}_{-2.8} microK (projections of the two-parameter likelihood). These results are consistent with the Harrison-Zel'dovich n=1 model of amplitude Q_{rms-PS} ~ 18 microK detected with significance exceeding 14sigma (dQ/Q < 0.07). (A small power spectrum amplitude drop below the published 2-year results is predominantly due to the application of the new, extended Galaxy cuts.)

Two-Point Correlations in the COBE DMR Four-Year Anisotropy Maps

Abstract: 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 μK, respectively, with a value ~18 μK generally preferred.

2-Point Correlations in the COBE DMR 4-Year Anisotropy Maps

Abstract: The 2-point temperature correlation function is evaluated from the 4-year COBE DMR microwave anisotropy maps. We examine the 2-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 2-point data is consistent with that inferred by other methods. The smallest and largest normalization deduced from any data combination are 16.4 and 19.6 uK respectively, with a value ~18 uK generally preferred.

Tests for Non-Gaussian Statistics in the DMR Four-Year Sky Maps

Abstract: We search the high-latitude portion of the COBE 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.

Angular Power Spectrum of the Microwave Background Anisotropy seen by the COBE Differential Microwave Radiometer

Abstract: The angular power spectrum estimator developed by Peebles (1973) and Hauser & Peebles (1973) has been modified and applied to the 4 year maps produced by the COBE DMR The power spectrum of the observed sky has been compared to the power spectra of a large number of simulated random skies produced with noise equal to the observed noise and primordial density fluctuation power spectra of power law form, with $P(k) \propto k^n$ The best fitting value of the spectral index in the range of spatial scales corresponding to spherical harmonic indices $3 \leq \ell \lesssim 30$ is an apparent spectral index $n_{app}$ = 113 (+03) (-04) which is consistent with the Harrison-Zel'dovich primordial spectral index $n_{pri} = 1$ The best fitting amplitude for $n_{app} = 1$ is $\langle Q_{RMS}^2\rangle^{05}$ = 18 uK

Band Power Spectra in the COBE DMR 4-Year Anisotropy Maps

Abstract: We employ a pixel-based likelihood technique to estimate the angular power spectrum of the COBE Differential Microwave Radiometer (DMR) 4-year 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 Q_{rms-PS|n=1} = 18 +/- 1.4 uK, and a best-fit spectral index of 1.2 +/- 0.3. The normalization is somewhat smaller than we concluded from the 2-year 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 is somewhat dependent on the details of Galactic foreground subtraction and data selection, ranging between 7 and 10 uK, but in no case is there compelling evidence that the quadrupole is too small to be consistent 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.

Band Power Spectra in the COBE* DMR Four-Year Anisotropy Maps

Abstract: 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.

The Cosmic Microwave Background Spectrum from the Full COBE/FIRAS Data Set

Abstract: We have refined the analysis of the data from the FIRAS (Far InfraRed Absolute Spectrophotometer) on board the COBE (COsmic Background Explorer). The FIRAS measures the difference between the cosmic microwave background and a precise blackbody spectrum. We find new tighter upper limits on general deviations from a blackbody spectrum. The RMS deviations are less than 50 parts per million of the peak of the CMBR. For the Comptonization and chemical potential we find $|y| < 15\times10^{-6}$ and $|\mu| < 9\times10^{-5}$ (95\% CL). There are also refinements in the absolute temperature, 2.728 $\pm$ 0.004 K (95\% CL), and dipole direction, $(\ell,b)=(264.14^\circ\pm0.30, 48.26^\circ\pm0.30)$ (95\% CL), and amplitude, $3.372 \pm 0.007$ mK (95\% CL). All of these results agree with our previous publications.

Angular Power Spectrum of the Cosmic Microwave Background Anisotropy Seen by the COBE* DMR

Abstract: The angular power spectrum estimator developed by Peebles and Hauser & Peebles has been modified and applied to the 4 yr maps produced by the COBE DMR. The power spectrum of the observed sky has been compared to the power spectra of a large number of simulated random skies produced with noise equal to the observed noise and primordial density fluctuation power spectra of power-law form, with P(k) ∝ kn. The best-fitting value of the spectral index in the range of spatial scales corresponding to spherical harmonic indices 3 ≤ l 30 is an apparent spectral index napp = 1.13−0.4+0.3 which is consistent with the Harrison-Zeldovich primordial spectral index npri = 1. The best-fitting amplitude for napp = 1 is Qrms20.5 = 18 μK.

Calibration and systematic error analysis for the COBE-DMR four-year sky maps

Abstract: The Differential Microwave Radiometers (DMR) instrument aboard the Cosmic Background Explorer (COBE) has mapped the full microwave sky to mean sensitivity 26 mu K per 7 degrees held of view. The absolute calibration is determined to 0.7 percent with drifts smaller than 0.2 percent 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 percent confidence upper limit for the pixel-pixel rms from all identified systematics is less than 6 mu K in the worst channel. A power spectrum analysis of the (A-B)/2 difference maps shows no evidence for additional undetected systematic effects.

Producing Megapixel Cosmic Microwave Background Maps from Differential Radiometer Data

Abstract: A major goal of cosmology is to obtain sensitive, high-resolution maps of the cosmic microwave background anisotropy. Such maps, as would be produced by the recently proposed Microwave Anisotropy Probe (MAP), will contain a wealth of primary information about conditions in the early universe. To mitigate systematic effects when observing the microwave background, it is desirable for the raw data to be collected in differential form: as a set of temperature differences between points in the sky. However, the production of large (megapixel) maps from a set of temperature differences is a potentially severe computational challenge. We present a new technique for producing maps from differential radiometer data that has a computational cost that grows in the slowest possible way with increasing angular resolution and number of map pixels. The required CPU time is proportional to the number of differential data points, and the required random-access memory is proportional to the number of map pixels. We test our technique, and demonstrate its feasibility, by simulating 1 yr of a spaceborne anisotropy mission.

Scanning and Mapping Strategies for CMB Experiments

Abstract: CMB anisotropy experiments seeking to make maps with more pixels than the 6144 pixels used by the COBE DMR need to address the practical issues of the computer time and storage required to make maps. A simple, repetitive scan pattern reduces these requirements but leaves the experiment vulnerable to systematic errors and striping in the maps. In this paper I give a time-ordered method for map-making with one-horned experiments that has reasonable memory and CPU needs but can handle complex COBE-like scans paths and 1/f noise.

Tests for Non-Gaussian Statistics in the DMR Four-Year Sky Maps

Abstract: We search the high-latitude portion of the COBE Differential Microwave Radiometers (DMR) 4-year sky maps for evidence of a non-Gaussian temperature distribution in the cosmic microwave background. The genus, 3-point correlation function, and 2-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 degrees 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 2-point correlation of the peaks and valleys in the maps provides the greatest discrimination among the class of models tested.

Latest COBE results

Abstract: The latest results on the spectrum and anisotropy of the microwave background from the COsmic Background Explorer (COBE) are discussed. The RMS deviation of the spectrum from a 2.728 .004 K [95% CL] Planck function is less than 50 parts per million of the peak intensity. Energy added to the CMBR by hot electrons (y and μ distortions) is less than 60 parts per million [95% CL] of the total CMBR energy for all times later than 1 year after the Big Bang. The anisotropy of the CMBR is consistent with a power law power spectrum of primordial density perturbations with P(k) σ kn, and the spectral index n = 1.2 .3 (1σ), consistent with the n≈ 1 predicted by inflation. The amplitude of power spectrum gives an expected RMS quadrupole of 〈QRMS2〉0.5 = 18 μK if n is forced to be 1.