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C. L. Bennett

Publications and source records attributed to C. L. Bennett.

At least 73 records · Page 4Linked to original sources
arXiv1996

Microwave Emission at High Galactic Latitudes

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.

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arXiv1996

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

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 $n_{app}$ = 1.13 (+0.3) (-0.4) which is consistent with the Harrison-Zel'dovich primordial spectral index $n_{pri} = 1$ The best fitting amplitude for $n_{app} = 1$ is $\langle Q_{RMS}^2\rangle^{0.5}$ = 18 uK.

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arXiv1996

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

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.

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arXiv1996

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

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 $Q_{rms-PS}=15.3^{+3.8}_{-2.8}$ $μ$K. For $n=1$ the best-fit normalization is $Q_{rms-PS}\vert_{n=1}=18\pm 1.6$ $μ$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.

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arXiv1996

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

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.

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arXiv1996

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

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.)

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arXiv1996

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

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.

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arXiv1996

Non-cosmological Signal Contributions to the COBE-DMR Four-Year Sky Maps

We limit the possible contributions from non-cosmological sources to the COBE-DMR four-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 uK (95% confidence level at 7 degree 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 95% confidence level rms limit on the Comptonization parameter averaged over the high-latitude sky of $δy\ <\ 1\ \times\ 10^{-6}$. Extragalactic sources have an insignificant effect on CMB power spectrum parameterizations determined from the DMR data.

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arXiv1995

High-Latitude Galactic Emission in the COBE DMR Two-Year Sky Maps

We cross-correlate the COBE-DMR 2-year sky maps with spatial templates from long-wavelength radio surveys and the far-infrared COBE DIRBE maps. We place an upper limit on the spectral index of synchrotron radiation beta_{synch} < -2.9 between 408 MHz and 31.5 GHz. We obtain a statistically significant cross-correlation with the DIRBE maps whose dependence on the DMR frequencies indicates a superposition of dust and free-free emission. The high-latitude dust emission (|b| > 30 deg) is well fitted by a single dust component with temperature T = 18 (+3, -7) K and emissivity epsilon ~ nu^beta with beta = 1.9 (+3.0, -0.5). The free-free emission is spatially correlated with the dust on angular scales larger than the 7 degree DMR beam, with rms variations 5.3 +/- 1.8 uK at 53 GHz and angular power spectrum P ~ ell^{-3}. If this correlation persists to smaller angular scales, free-free emission should not be a significant contaminant to measurements of the cosmic microwave anisotropy at degree angular scales for frequencies above 20 GHz.

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arXiv1995

FAR-INFRARED SPECTRAL OBSERVATIONS OF THE GALAXY BY COBE

We derive Galactic continuum spectra from 5-96/cm from COBE/FIRAS observations. The spectra are dominated by warm dust emission, which may be fit with a single temperature in the range 16-21 K (for nu^2 emissivity) along each line of sight. Dust heated by the attenuated radiation field in molecular clouds gives rise to intermediate temperature (10-14 K) emission in the inner Galaxy only. A widespread, very cold component (4-7 K) with optical depth that is spatially correlated with the warm component is also detected. The cold component is unlikely to be due to very cold dust shielded from starlight, because it is present at high latitude. We consider hypotheses that the cold component is due to enhanced submillimeter emissivity of the dust that gives rise to the warm component, or that it may be due to very small, large, or fractal particles. Lack of substantial power above the emission from warm dust places strong constraints on the amount of cold gas in the Galaxy. The microwave sky brightness due to interstellar dust is dominated by the cold component, and its angular variation could limit our ability to discern primordial fluctuations in the cosmic microwave background radiation.

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arXiv1995

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

We compute the three-point temperature correlation function of the COBE Differential Microwave Radiometer (DMR) two-year sky maps to search for evidence of non-Gaussian temperature fluctuations. We detect three-point correlations in our sky with a substantially higher signal-to-noise ratio than from the first year data. However, the magnitude of the signal is consistent with the level of cosmic variance expected from Gaussian fluctuations, even when the low order multipole moments, up to l = 9, are filtered from the data. These results do not strongly constrain most existing models of structure formation, but the absence of intrinsic three-point correlations on large angular scales is an important consistency test for such models.

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arXiv1994

On the RMS Anisotropy at 7 degrees and 10 degrees Observed in the COBE-DMR Two Year Sky Maps

The frequency-independent RMS temperature fluctuations determined from the COBE-DMR two year sky maps are used to infer the parameter Q_{rms-PS}, which characterizes the normalization of power law models of primordial cosmological temperature anisotropy. In particular, a 'cross'-RMS statistic is used to determine Q_{rms-PS} for a forced fit to a scale-invariant Harrison-Zel'dovich (n = 1) spectral model. Using a joint analysis of the 7 degree and 10 degree RMS temperature derived from both the 53 and 90 GHz sky maps, we find Q_{rms-PS} = 17.0^{+2.5}_{-2.1} uK when the low quadrupole is included, and Q_{rms-PS} = 19.4^{+2.3}_{-2.1} uK excluding the quadrupole. These results are consistent with the n = 1 fits from more sensitive methods (e.g. power spectrum, correlation function). The effect of the low quadrupole derived from the COBE-DMR data on the inferred Q_{rms-PS} normalization is investigated. A bias to lower Q_{rms-PS} is found when the quadrupole is included. The higher normalization for a forced n = 1 fit is then favored by the cross-RMS technique. As initially pointed out in Wright et al. (1994a) and further discussed here, analytic formulae for the RMS sky temperature fluctuations will NOT provide the correct normalization amplitude.

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arXiv1994

Gaussian Statistics of the Cosmic Microwave Background: Correlation of Temperature Extrema In the COBE DMR Two-Year Sky Maps

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 $a_{\ell m}$ are drawn from either Gaussian, $χ^2$, or log-normal parent populations. The likelihood of the 53 GHz (A+B)/2 data is greatest for the exact Gaussian model. All non-Gaussian models tested are ruled out at 90\% confidence, limited 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.

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arXiv1994

CMB Maps at 0.5 degree Resolution I: Full-Sky Simulations and Basic Results

We have simulated full-sky maps of the Cosmic Microwave Background (CMB) anisotropy expected from Cold Dark Matter (CDM) models at 0.5 and 1.0 degree angular resolution. Statistical properties of the maps are presented as a function of sky coverage, angular resolution, and instrument noise, and the implications of these results for observability of the Doppler peak are discussed. The rms fluctuations in a map are not a particularly robust probe of the existence of a Doppler peak, however, a full correlation analysis can provide reasonable sensitivity. We find that sensitivity to the Doppler peak depends primarily on the fraction of sky covered, and only secondarily on the angular resolution and noise level. Color plates and one-dimensional scans of the maps are presented to visually illustrate the anisotropies.

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arXiv1994

CMB Maps at 0.5 Degree Resolution II: Unresolved Features

High-contrast peaks in the cosmic microwave background (CMB) anisotropy can appear as unresolved sources to observers. We fit simulated CMB maps generated with a cold dark matter model to a set of unresolved features at instrumental resolution 0.5 degrees to 1.5 degrees and present the integral density per steradian n(>|T|) of unresolved features brighter than threshold temperature |T|. A typical medium-scale experiment observing 0.001 sr at 0.5 degree resolution would expect to observe one unresolved feature brighter than 85 \muK after convolution with the beam profile, with less than 5% probability to observe a source brighter than 150 \muK. Increasing the power-law index of primordial density perturbations n from 1 to 1.5 raises these temperature limits |T| by a factor of 2. The results are only weakly dependent on the assumed values of the Hubble constant and baryon density.

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arXiv1994

Observations of the Cosmic Microwave Background

The properties of the cosmic microwave background radiation provide unique constraints on cosmological models, i.e. on the content, history, and evolution of the Universe. I discuss the latest measurements of the spectral and spatial properties of the cosmic microwave background radiation. Recent measurements from NASA's Cosmic Background Explorer (COBE) satellite, and from balloon-borne and ground-based platforms, are summarized and their cosmological implications are discussed.

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arXiv1994

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

The angular power spectrum estimator developed by Peebles (1973) and Hauser & Peebles (1973) has been modified and applied to the 2 year maps produced by the COBE DMR. The power spectrum of the real 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 $P(k) \propto k^n$. Within the limited range of spatial scales covered by the COBE DMR, corresponding to spherical harmonic indices $3 \leq \ell \lsim 30$, the best fitting value of the spectral index is $n = 1.25^{+0.4}_{-0.45}$ with the Harrison-Zeldovich value $n = 1$ approximately 0.5$σ$ below the best fit. For $3 \leq \ell \lsim 19$, the best fit is $n = 1.46^{+0.39}_{-0.44}$. Comparing the COBE DMR $ΔT/T$ at small $\ell$ to the $ΔT/T$ at $\ell \approx 50$ from degree scale anisotropy experiments gives a smaller range of acceptable spectral indices which includes $n = 1$.

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