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temperature on the Celestial Sphere as determined with the COBE satellite, (top) uncorrected, (middle) corrected for the dipole term due to our peculiar velocity, (bottom) corrected for contributions from the dipole term and from our galaxy.]] Observation of the Cosmic Microwave Background (CMB) were first made by Arno Penzias and Robert Woodrow Wilson at Bell Telephone Laboratories in 1964 . Subsequently, hundreds of cosmic microwave background experiments have been conducted to measure and characterize the signatures of the radiation. The most famous experiment is probably the NASA Cosmic Background Explorer ( COBE ) satellite that orbited in 1989 – 1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out Cosmic Strings as the leading theory of cosmic structure formation, and suggested Cosmic Inflation was the right theory. During the 1990's, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG Experiment reported that the highest power fluctuations occur at scales of apporoximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is Flat . A number of ground-based Interferometer s provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array , Degree Angular Scale Interferometer (DASI) and the Cosmic Background Imager . In fact, DASI made the first detection of the polarization of the CMB. In June 2001 , NASA launched a second CMB space mission, WMAP , to make much more precise measurements of the large scale anisotropies over the full sky. The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum to below degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from Cosmic Inflation as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (see links below). Although WMAP provided very accurate measurements of the large angular-scale fluctuations in the CMB (structures about as large in the sky as the moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed using previous ground-based Interferometer s. A third space mission, the Planck Surveyor , is to be launched in 2007. Planck employs both HEMT radiometers as well as Bolometer technology and will measure the CMB on smaller scales than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency). Its detectors got a trial run at the Antarctic Viper Telescope as ACBAR ( Arcminute Cosmology Bolometer Array Receiver ) experiment – which has produced the most precise measurements at small angular scales to date – and at the Archeops balloon telescope. Additional ground-based instruments such as the South Pole Telescope in Antarctica and the proposed Clover Project and Atacama Cosmology Telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization. DESIGN The design of cosmic microwave background experiments is a very challenging task. The greatest problems are:
ANALYSES The analysis of cosmic microwave background data to produce maps, an angular power spectrum and ultimately cosmological parameters is a complicated, computationally difficult problem. Although computing a power spectrum from a map is in principle a simple Fourier Transform , decomposing the map of the sky into Spherical Harmonics , in practice it is hard to take the effects of noise and foregrounds into account. Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using Markov Chain Monte Carlo sampling techniques. Low multipoles With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB suffers from anomalies, such as , dust and Free-free emission, and from experimental uncertainty in the monopole and dipole. A full Bayesian analysis of the WMAP power spectrum demonstrates that the quadrupole prediction of Lambda-CDM Cosmology is consistent with the data at the 10% level and that the octupole is not remarkable 7 . Carefully accounting for the procedure used to remove the foregrounds from the full sky map further reduces the significance of the alignment by ~5%.8 . 9 . 10 . 11 . LIST OF EXPERIMENTS IN APPROXIMATE CHRONOLOGICAL ORDER ). The data shown come from the WMAP (2006), Acbar (2004) Boomerang (2005), CBI (2004) and VSA (2004) instruments. Also shown is a theoretical model (solid line).]] Each experiment provided improved data quality when compared with previous experiments.
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