Cosmic Background Explorer Research Articles

Balloon Flight Brings Cosmic Glow Into Focus

Seven years ago, the Cosmic Background Explorer (COBE) satellite thrilled cosmologists by revealing the first glimpse of imprints left on the young universe during its birth in the big bang. Now, a telescope carried aloft by a balloon has scrutinized the microwave glow much more closely--giving cosmologists who have seen the early results another thrill. The data, which come from BOOMERANG, a joint U.S. and Italian mission that flew over Antarctica, could offer insights into the proportions of matter and energy in the cosmos.

What Have We Already Learned from the Cosmic Microwave Background?

ABSTRACTThe Cosmic Background Explorer (COBE) satellite, and the Differential Microwave Radiometer (DMR) experiment in particular, was extraordinarily successful. However, the DMR results were announced about 7 years ago, during which time a great deal more has been learned about anisotropies in the cosmic microwave background (CMB). We assess the current state of knowledge and discuss where we might be going. The CMB experiments currently being designed and built, including long‐duration balloons, interferometers, and two space missions, promise to address several fundamental cosmological issues. We present our evaluation of what we already know, what we are beginning to learn now, and what the future may bring.

First Detection of Visible Zodiacal Dust Bands from Ground‐based Observations

We present the first ground-based observational evidence of the zodiacal dust bands originally discovered by the Infrared Astronomical Satellite (IRAS) and confirmed by the Cosmic Background Explorer (COBE). Our photometric observations have been performed on Mauna Kea, Hawaii, between 1997 October 29 and November 2, using a 24 mm wide-angle lens attached to a cooled CCD camera, and a blue filter centered at 440 nm. Photometric data of the morning zodiacal light have revealed the presence of zodiacal dust bands at ecliptic latitudes β=0°, 3°, and ±10°, as well as additional faint structures at approximately β=±5°, between solar elongations =75° and 90°. The brightness of dust bands is approximately 2%-3% of the background zodiacal light. Moreover, our observation of the Gegenschein has discovered dust bands at β=+2°, -4°, and -9° at 165°≤≤185°. Using the separation of the inner dust band pair observed in two different regions of , we estimate the parallactic distance of this band pair to be about 1.6 AU from the Sun.

Evidence for Scale-Scale Correlations in the Cosmic Microwave Background Radiation

We perform a discrete wavelet analysis of the Cosmic Background Explorer differential microwave radiometer (DMR) 4-yr sky maps and find a significant scale-scale correlation on angular scales from about 11\ifmmode^\circ\else\textdegree\fi{} to 22\ifmmode^\circ\else\textdegree\fi{}, only in the DMR face centered on the north galactic pole. This non-Gaussian signature does not arise either from the known foregrounds or the correlated noise maps, nor is it consistent with upper limits on the residual systematic errors in the DMR maps. Either the scale-scale correlations are caused by an unknown foreground contaminate or systematic errors on angular scales as large as 22\ifmmode^\circ\else\textdegree\fi{}, or the standard inflation plus cold dark matter paradigm is ruled out at the $>99%$ confidence level.

TheCOBEDiffuse Infrared Background Experiment Search for the Cosmic Infrared Background. IV. Cosmological Implications

A direct measurement of the extragalactic background light (EBL) can provide important constraints on the integrated cosmological history of star formation, metal and dust production, and the conversion of starlight into infrared emission by dust. In this paper we examine the cosmological implications of the recent detection of the EBL in the 125 to 5000 ?m wavelength region by the Diffuse Infrared Background Experiment (DIRBE) and Far Infrared Absolute Spectrophotometer (FIRAS) on board the Cosmic Background Explorer (COBE). We first show that the 140 and 240 ?m isotropic residual emission found in the DIRBE data cannot be produced by foreground emission sources in the solar system or the Galaxy. The DIRBE 140 and 240 ?m isotropic residuals, and by inference the FIRAS residuals as well, are therefore extragalactic. Assuming that most of the 140 and 240 ?m emission is from dust yields a 2 ? lower limit of ?I(?) ? 5 nW m-2 sr-1 for the EBL at 100 ?m. The integrated EBL detected by the COBE between 140 and 5000 ?m is ~16 nW m-2 sr-1, roughly 20%-50% of the integrated EBL intensity expected from energy release by nucleosynthesis throughout cosmic history. This also implies that at least ~5%-15% of the baryonic mass density implied by big bang nucleosynthesis has been processed through stars. The COBE observations provide important constraints on the cosmic star formation rate, and we calculate the EBL spectrum for various star formation histories. The results show that the UV and optically determined cosmic star formation rates fall short in producing the observed 140 to 5000 ?m background. The COBE observations require the star formation rate at redshifts of z ? 1.5 to be larger than that inferred from UV-optical observations by at least a factor of 2. This excess stellar energy must be mainly generated by massive stars, since it otherwise would result in a local K-band luminosity density that is larger than observed. The energy sources could either be yet undetected dust-enshrouded galaxies, or extremely dusty star-forming regions in observed galaxies, and they may be responsible for the observed iron enrichment in the intracluster medium. The exact star formation history or scenarios required to produce the EBL at far-IR wavelengths cannot be unambiguously resolved by the COBE observations and must await future observations.

A Tentative Detection of the Cosmic Infrared Background at 3.5 μ[CLC]m[/CLC] from [ITAL]COBE[/ITAL]/DIRBE Observations

Foreground emission and scattered light from interplanetary dust (IPD) particles and emission from Galactic stellar sources are the greatest obstacles to determining the cosmic infrared background (CIB) from diffuse sky measurements in the ~1-5 μm range. We use ground-based observational limits on the K-band intensity of the CIB in conjunction with sky maps obtained by the Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer satellite to reexamine the limits on the CIB at 1.25, 3.5, and 4.9 μm. Adopting a CIB intensity of 7.4 nW m−2 sr−1 at 2.2 μm, and using the 2.2 μm DIRBE sky map from which the emission from the IPD cloud has been subtracted, we create a spatial template of the Galactic stellar contribution to the diffuse infrared sky. This template is then used to subtract the contribution of the diffuse Galactic stellar emission from the IPD emission-subtracted DIRBE sky maps at 1.25, 3.5, and 4.9 μm. The DIRBE 100 μm data are used to estimate the small contribution of emission from interstellar dust at 3.5 and 4.9 μm. Our method significantly reduces the errors associated with the subtraction of Galactic starlight, leaving only the IPD emission component as the primary obstacle to the detection of the CIB at these wavelengths. The analysis leads to a tentative detection of the CIB at 3.5 μm with an intensity of νIν={9.9+0.312[ν0ICIB(ν0)-7.4]} ± 2.9 nW m−2 sr−1, where ν0ICIB(ν0) is the CIB intensity at 2.2 μm in units of nW m−2 sr−1. The analysis also yields new upper limits (95% confidence limit) on the CIB at 1.25 and 4.9 μm of 68 and 36 nW m−2 sr−1, respectively. The cosmological implications of these results are discussed in this Letter.

Estimating non-Gaussianity in the microwave background

The bispectrum of the microwave background sky is a possible discriminator between inflationary and defect models of structure formation in the Universe. The bispectrum, which is the analogue of the temperature three-point correlation function in harmonic space, is zero for most inflationary models, but non-zero for non-Gaussian models. The expected departures from zero are small, and easily masked by noise, so it is important to be able to estimate the bispectrum coefficients as accurately as possible, and to know the errors and correlations between the estimates so that they may be used in combination as a diagnostic to rule out non-Gaussian models. This paper presents a method for estimating in an unbiased way the bispectrum from a microwave background map in the near-Gaussian limit. The method is optimal, in the sense that no other method can have smaller error bars, and, in addition, the covariances between the bispectrum estimates are calculated explicitly. The method deals automatically with partial sky coverage and arbitrary noise correlations without modification. A preliminary application to the Cosmic Background Explorer 4-yr data set shows no evidence for non-Gaussian behaviour.

Nonextensive thermostatistics and deformed structures

Abstract We obtain a generalized Planck law within the framework of nonextensive statistics making use of a deformed oscillator system. Our results are used to fit the data from the COBE (Cosmic Background Explorer) satellite. Best fit values for the entropy parameter q, the deformation parameter r and for the temperature are found.

Estimating the power spectrum of the cosmic microwave background

We develop two methods for estimating the power spectrum, ${C}_{\mathcal{l}},$ of the cosmic microwave background from data and apply them to the Cosmic Background Explorer Differential Microwave Radiometer and Saskatoon datasets. One method involves a direct evaluation of the likelihood function, and the other is an estimator that is a minimum-variance weighted quadratic function of the data. Applied iteratively, the quadratic estimator is not distinct from likelihood analysis, but is rather a rapid means of finding the power spectrum that maximizes the likelihood function. Our results bear this out: direct evaluation and quadratic estimation converge to the same ${C}_{\mathcal{l}}$s. The quadratic estimator can also be used to determine directly cosmological parameters and their uncertainties. While the two methods both require ${O(N}^{3})$ operations, the quadratic is much faster, and both are applicable to datasets with arbitrary chopping patterns and noise correlations. We also discuss approximations that may reduce it to ${O(N}^{2})$ thus making it practical for forthcoming megapixel datasets.

ACOBEModel of the Galactic Bar and Disk

A model of the bar and old stellar disk of the Galaxy has been derived from the survey of the Diffuse Infrared Background Experiment (DIRBE) of the Cosmic Background Explorer at wavelengths of 1.25, 2.2, 3.5, and 4.9 μm. It agrees very well with the data, except in directions in which the near-infrared optical depth is high. Among the conclusions drawn from the model is that the Sun is located approximately 16.5 pc above the midpoint of the Galactic plane. The disk has an outer edge 4 kpc from the Sun and is warped like the H I layer. It has a central hole roughly the diameter of the inner edge of the 3 kpc molecular cloud ring, and within that hole lies a bright, strong, early-type bar, tilted approximately 14° from the Sun-Galactic center line. The model has 47 free parameters. The model is discussed in detail, and contour plots and images of the residuals are presented.

The microwave background anisotropies: observations.

Most cosmologists now believe that we live in an evolving universe that has been expanding and cooling since its origin about 15 billion years ago. Strong evidence for this standard cosmological model comes from studies of the cosmic microwave background radiation (CMBR), the remnant heat from the initial fireball. The CMBR spectrum is blackbody, as predicted from the hot Big Bang model before the discovery of the remnant radiation in 1964. In 1992 the cosmic background explorer (COBE) satellite finally detected the anisotropy of the radiation-fingerprints left by tiny temperature fluctuations in the initial bang. Careful design of the COBE satellite, and a bit of luck, allowed the 30 microK fluctuations in the CMBR temperature (2.73 K) to be pulled out of instrument noise and spurious foreground emissions. Further advances in detector technology and experiment design are allowing current CMBR experiments to search for predicted features in the anisotropy power spectrum at angular scales of 1 degrees and smaller. If they exist, these features were formed at an important epoch in the evolution of the universe--the decoupling of matter and radiation at a temperature of about 4,000 K and a time about 300,000 years after the bang. CMBR anisotropy measurements probe directly some detailed physics of the early universe. Also, parameters of the cosmological model can be measured because the anisotropy power spectrum depends on constituent densities and the horizon scale at a known cosmological epoch. As sophisticated experiments on the ground and on balloons pursue these measurements, two CMBR anisotropy satellite missions are being prepared for launch early in the next century.

Cosmic microwave background theory

A long-standing goal of theorists has been to constrain cosmological parameters that define the structure formation theory from cosmic microwave background (CMB) anisotropy experiments and large-scale structure (LSS) observations. The status and future promise of this enterprise is described. Current band-powers in -space are consistent with a DeltaT flat in frequency and broadly follow inflation-based expectations. That the levels are approximately (10(-5))2 provides strong support for the gravitational instability theory, while the Far Infrared Absolute Spectrophotometer (FIRAS) constraints on energy injection rule out cosmic explosions as a dominant source of LSS. Band-powers at 100 suggest that the universe could not have re-ionized too early. To get the LSS of Cosmic Background Explorer (COBE)-normalized fluctuations right provides encouraging support that the initial fluctuation spectrum was not far off the scale invariant form that inflation models prefer: e.g., for tilted Lambda cold dark matter sequences of fixed 13-Gyr age (with the Hubble constant H0 marginalized), ns = 1.17 +/- 0.3 for Differential Microwave Radiometer (DMR) only; 1.15 +/- 0.08 for DMR plus the SK95 experiment; 1.00 +/- 0.04 for DMR plus all smaller angle experiments; 1.00 +/- 0.05 when LSS constraints are included as well. The CMB alone currently gives weak constraints on Lambda and moderate constraints on Omegatot, but theoretical forecasts of future long duration balloon and satellite experiments are shown which predict percent-level accuracy among a large fraction of the 10+ parameters characterizing the cosmic structure formation theory, at least if it is an inflation variant.

A Search for Millimetric Emission from Gamma‐Ray Bursts

We have used the 2- year Differential Microwave Radiometer data from the COsmic Background Explorer (COBE) satellite to systematically search for millimetric (31 - 90 GHz) emission from the Gamma Ray Bursts (GRBs) in the Burst And Transient Source Experiment (BATSE) GRB 3B catalog. The large beamsize of the COBE instrument (7 degs FWHM) allows for an efficient search of the large GRB positional error boxes, although it also means that fluxes from (point source) GRB objects will be somewhat diluted. A likelihood analysis has been used to look for a change in the level of millimetric emission from the locations of 81 GRB events during the first two years (1990 & 1991) of the COBE mission. The likelihood analysis determined that we did not find any significant millimetric signal before or after the occurance of the GRB. We find 95% confidence level upper limits of 175, 192 and 645 Jy or, in terms of fluxes, of 9.6, 16.3 and 54.8 10^{-13} erg/cm^2/s, respectively at 31, 53 and 90 GHz. We also look separately at different classes of GRBs, including a study of the top ten (in peak flux) GRBs, the short burst and long burst subsets, finding similar upper limits. While these limits may be somewhat higher than one would like, we estimate that using this technique with future planned missions could push these limits down to \sim 1 mJy.

Age of the universe: Influence of the inhomogeneities on the global expansion factor

For the first time we calculate quantitatively the influence of inhomogeneities on the global expansion factor by averaging the Friedmann equation. In the framework of the relativistic second-order Zel'dovich-approximation scheme for irrotational dust we use observational results in the form of the normalization constant fixed by the Cosmic Background Explorer results and we check different power spectra, namely, for adiabatic cold dark matter (CDM), isocurvature CDM, hot dark matter, warm dark matter, strings, and textures. We find that the influence of the inhomogeneities on the global expansion factor is very small. So the error in determining the age of the universe using the Hubble constant in the usual way is negligible. This does not imply that the effect is negligible for local astronomical measurements of the Hubble constant. Locally the determination of the redshift-distance relation can be strongly influenced by the peculiar velocity fields due to inhomogeneities. Our calculation does not consider such effects, but is constrained to comparing globally homogeneous and averaged inhomogeneous matter distributions. In addition we relate our work to previous treatments.

Flat Spots: Topological Signatures of an Open Universe in Cosmic Background Explorer Sky Maps

We investigate the behavior of light rays in an open universe with a partly periodic horn topology. The geodesics can be solved exactly and the periodic topology creates characteristic new effects in temperature maps of the microwave background. Large flat regions appear in Cosmic Background Explorer sky maps even when multiple images of astronomical sources are unobservable.

The very first light: the true inside story of the scientific journey back to the dawn of the universe

In the early 1990s, a NASA-led team of scientists changed the way we view the universe. With the COBE (Cosmic Background Explorer) project, they showed that the microwave radiation that fills the universe must have come from the Big Bang-effectively proving the Big Bang theory beyond any doubt. It was one of the greatest scientific findings of our generation, perhaps of all time. In The Very First Light, John Mather, one of COBEs leaders, and science writer John Boslough tell the story of how it was achieved. A gripping tale of big money, bigger egos, tense politics, and cutting-edge engineering, The Very First Light offers a rare insiders account of the world of big science.

The Three-Dimensional Structure of the Zodiacal Dust Bands

Using observations of the infrared sky brightness by the Cosmic Background Explorer (COBE)11The National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) is responsible for the design, development, and operation of the Cosmic Background Explorer (COBE). Scientific guidance is provided by the COBE Science Working Group. GSFC is also responsible for the development of the analysis software and for the production of the mission data sets.Diffuse Infrared Background Experiment (DIRBE) and Infrared Astronomical Satellite (IRAS), we have created maps of the surface brightness Fourier-filtered to suppress the smallest (< 1°) structures and the large-scale background (>15°). Dust bands associated with the Themis, Koronis, and Eos families are readily evident. A dust band associated with the Maria family is also present. The parallactic distances to the emitting regions of the Koronis, Eos, and Maria dust bands were found to be 1.4 to 2.5 AU. A weak dust band associated with the Eunomia/Io family is evident, together with another weak and previously unattributed dust band, which may split further into two band pairs, potentially associated with the Hygiea or Vesta family. The brightnesses of the blended Themis/Koronis bands and the Eos dust band vary with ecliptic longitude, such that the northern or southern component of the band pair becomes brighter when its tilt brings it into the ecliptic plane. We attribute the brightness variations to the motion of the Earth within the emitting region, and conclude that at least some dust-band particles are on Earth-crossing orbits. For the Themis and Koronis families, the tilt is sufficient that the Earth may pass to the edges of the emitting region, where the density is highest, leading to “glints” two or four times a year. We compared the observed distributions to theoretically motivated, empirical models for the three-dimensional distribution of asteroid family dust. In the torus model, the dust is distributed among the asteroid family members with the same distributions of proper orbital inclination and semimajor axis but a random ascending node. In the migrating model, particles are presumed to be under the influence of Poynting–Robertson drag, so that they are distributed throughout the inner Solar System. The migrating model is better able to match the parallactic variation of dust-band latitude as well as the 12- to 60-μm spectrum of the dust bands. The annual brightness variations can be explained only by the migrating model. Upper limits are placed on the dust density associated with the Nysa and Flora families—both of the large, inner-belt families with wide inclination dispersions. The association of five (and potentially seven) dust bands with the largest asteroid families suggests that dust bands are an integral part of asteroid families. If nonfamily asteroids produce dust at a rate similar to that of the families with the lowest dust density, then they can account for the brightness of the zodiacal light in the ecliptic.

The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe

In the early 1990s, a NASA-led team of scientists changed the way we view the universe. With the COBE (Cosmic Background Explorer) project, they showed that the microwave radiation that fills the universe must have come from the Big Bang-effectively proving the Big Bang theory beyond any doubt. It was one of the greatest scientific findings of our generation, perhaps of all time. In The Very First Light, John Mather, one of COBEs leaders, and science writer John Boslough tell the story of how it was achieved. A gripping tale of big money, bigger egos, tense politics, and cutting-edge engineering, The Very First Light offers a rare insiders account of the world of big science.

Constraints on a Primordial Magnetic Field

We derive an upper limit of ${B}_{0}&lt;3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}({\ensuremath{\Omega}}_{0}{h}_{50}^{2}{)}^{1/2}$ G on the present strength of any primordial homogeneous magnetic field. The microwave background anisotropy created by cosmological magnetic fields is calculated in the most general flat and open anisotropic cosmologies containing expansion-rate and 3-curvature anisotropies. Our limit is derived from a statistical analysis of the 4-year Cosmic Background Explorer (COBE) data for anisotropy patterns characteristic of homogeneous anisotropy averaged over all possible sky orientations with respect to the COBE receiver. The limits we obtain on homogeneous magnetic fields are stronger than those imposed by nucleosynthesis.

Limits to global rotation and shear from the COBE DMR four-year sky maps

Small departures from a homogeneous isotropic spacetime create observable features in the large-scale anisotropy of the cosmic microwave background. We cross correlate the maps of the cosmic microwave background anisotropy from the Cosmic Background Explorer Differential Microwave Radiometers four-year data set with template maps from Bianchi type VII${}_{h}$ cosmological models to limit global rotation or shear in the early universe. On the largest scales, spacetime is well described by the Friedmann-Robertson-Walker metric, with departures from isotropy about each spatial point limited to shear $\ensuremath{\sigma}{/H}_{0}&lt;{10}^{\ensuremath{-}9}$ and rotation $\ensuremath{\omega}{/H}_{0}&lt;6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ for $0.1&lt;~{\ensuremath{\Omega}}_{0}&lt;~1$.