George F Smoot

The Wavelength-Oriented Microwave Background Analysis Team (WOMBAT) is constructing microwave skymaps which will be more realistic than previous simulations. Our foreground models represent a considerable improvement: where spatial templates are available for a given foreground, we predict the flux and spectral index of that component at each place on the sky and estimate the uncertainties in these quantities. We will produce maps containing simulated Cosmic Microwave Background anisotropies combined with all major expected foreground components. The simulated maps will be provided to the cosmology community as the WOMBAT Challenge, a "hounds and hares" exercise where such maps can be analyzed to extract cosmological parameters by scientists who are unaware of their input values. This exercise will test the efficacy of current foreground subtraction, power spectrum analysis, and parameter estimation techniques and will help identify the areas most in need of progress.

Data from a series of balloon flights covering both the Northern and Southern Hemispheres, measuring the large angular scale anisotropy in the cosmic background radiation at 3.3 mm wavelength are presented. The data cover 85 percent of the sky to a limiting sensitivity of 0.7 mK per 7-deg field of view. The data show a 50-sigma (statistical error only) dipole anisotropy with an amplitude of 3.44 +/- 0.17 mK and a direction of alpha= 11.2 h +/- 0.1 h, and delta = -6.0 deg +/- 1.5 deg. A 90 percent confidence level upper limit of 0.00007 is obtained for the rms quadrupole amplitude. Flights separated by six months show the motion of Earth around the Sun. Galactic contamination is very small, with less than 0.1 mK contribution to the dipole quadrupole terms. A map of the sky has been generated from the data.

The measurements of fluctuations in the cosmic microwave background are shown to be consistent with the first order anisotropy at the level of about one part in a thousand and the limits of about one part in three thousand for fluctuations on any other scale. It is suggested that the first order anisotropy is due to the motion of the solar system relative to the radiation. This interpretation is evidence supporting the Cosmological Principle. The observed isotropy of the cosmic microwave background contradicts the concept of an indefinite hierarchy of clustering in the universe.

Measurements of the large-angular-scale anisotropy of the cosmic background radiation made from the northern hemisphere are in essential agreement with each other and indicate a first order spherical harmonic component with an amplitude of approximately 3 mK. New data from the southern hemisphere support these previous results. This first order anisotropy is interpreted as resulting from the motion of the solar system relative to the cosmic background radiation. There is no evidence of any higher order anisotropy to the level of 1 mK.

Observations of the Cosmic Microwave Background Radiation (CM BR) have put the standard model of cosmology, the Big Bang, on firm footing and provide tests of various ideas of large scale structure formation and the origin of the universe. Ground-based measurements of the cosmic background radiation are hampered by emission from atmospheric O 2 and H 2O molecules. This has forced precision experiments to high altitudes and balloon-borne instrumentation. Ultimately, space has proven to be the most reliable venue for these observations. However, the foreground galactic emissions. in particular dust thermoral emission as well as emission from energetic electrons, provide a serious limit to observations.

Inflation creates both scalar (density) and tensor (gravity wave) metric perturbations. We find that the tensor-mode contribution to the cosmic microwave background anisotropy on large-angular scales can only exceed that of the scalar mode in models where the spectrum of perturbations deviates significantly from scale invariance. If the tensor mode dominates at large-angular scales, then the value of DeltaT/T predicted on 1 deg is less than if the scalar mode dominates, and, for cold-dark-matter models, bias factors greater than 1 can be made consistent with Cosmic Background Explorer (COBE) DMR results.

We estimate the level of confusion to cosmic microwave background (CMB) anisotropy measurements caused by extragalactic infrared sources. CMB anisotropy observations at high resolution and high frequencies are especially sensitive to this foreground. We use data from the COBE satellite to generate a Galactic emission spectrum covering millimeter and submillimeter wavelengths. Using this spectrum as a template, we predict the microwave emission of the 5319 brightest infrared galaxies seen by IRAS. We simulate sky maps of extragalactic infrared sources over the relevant range of frequencies (30-900 GHz) and instrument resolutions (10'-10° FWHM). An analysis of the temperature anisotropy of these sky maps shows that a reasonable observational window is available for CMB anisotropy measurements.

The observed energy spectrum of ultra high energy cosmic rays (UHECR) is distorted by errors in the energy reconstruction. We estimate the UHECR spectrum at the Earth assuming it is originated from a cosmological flux. We then convolute this flux assuming a lognormal error in the energy. We show that if the standard deviation of the lognormal error distribution is equal or larger than 0.25, not only the shape but also the normalization of the measured energy spectra will be modified. As a consequence the GZK cutoff might be sufficiently smeared and as not to be seen. This result is independent of the power law of the cosmological flux. As a conclusion we show that in order to establish the presence or not of the GZK feature, not only more data is needed but also that the shape of the energy error distribution has to be known well.

This paper discusses the three instruments aboard NASA's Cosmic Background Explorer (COBE) satellite and presents early results obtained from the first six months of observations. The three instruments (FIRAS, DMR, and DIRBE) have operated well and produced significant new results. The FIRAS measurement of the CMB spectrum supports the standard Big Bang model. The maps made from the DMR instrument measurements show a spatially smooth early universe. The maps of galactic and zodiacal emission produced by the DIRBE instrument are needed to identify the foreground emissions from extragalactic and thus to interpret its and the other COBE results in terms of events in the early universe.

The Cosmic Background Explorer (COBE) satellite was developed to measure the diffuse infrared and microwave radiation from the early universe, to the limits set by the astrophysical foregrounds. COBE has three instruments: The Differential Microwave Radiometer (DMR) maps the cosmic radiation precisely. The Far Infrared Absolute Spectrophotometer (FIRAS) compares the cosmic background radiation spectrum with that of a precise blackbody. The Diffuse Infrared Background Experiment (DIRBE) searches for the accumulated light of primeval galaxies and stars. FIRAS has measured the cosmic background radiation spectrum nearly 1000 times more precisely than previous observations. The DMR discovered the cosmic background to be anisotropic at a level of a part in 105. The results strongly constrain comological models and the formation of structure in the present universe. The results from COBE are in good quantitative agreement with the calculations of the hot Big Bang model. The DMR data are in agreement with inflationary model predictions.

Observations of the cosmic microwave background play a central role in modern cosmology. The existence of the CMB as a remanent of the early Universe has constituted a pillar for the Big Bang scenario. The recent cosmic background explorer differential microwave radiometer results have provided further support to the generally accepted standard model by detecting for the first time primordial