Launched on November 18, 1989, Cosmic Background Explorer (COBE) was a satellite sent into orbit to observe the cosmic background. Created through the effort of over one thousand people, the satellite carried three separate instruments for observation: DMR (Differential Microwave Radiometer), DIRBE (Diffuse Infrared Background Experiment), and FIRAS (Far Infrared Absolute Spectrophotometer). COBE is most famous for the data produced by FIRAS and DMR regarding the cosmic microwave background.
The primary goal of FIRAS was to compare the spectrum of the cosmic microwave background to that of an ideal blackbody. The instrument collected data by comparing the intensity of the cosmic microwave background to the intensity of a calibrated blackbody for a given wavelength of radiation. FIRAS covered the range of the spectrum from 0.1 mm to 10 mm in wavelength. After ten months of operation, results showed that the spectrum of the cosmic microwave background was nearly identical to that of an ideal blackbody of temperature 2.728K.
DMR was designed to measure the uniformity of the cosmic microwave background and to search for fluctuations in intensity across the sky. This instrument searched for intensity differences at frequencies of 31.5 GHz, 53 GHz, and 90 GHz, frequencies at which little galactic emission is assumed to interfere with results. A full sky survey conducted over four years provided conclusive evidence that the cosmic microwave background was uniform at 2.728K to better than one part in a thousand. Results also demonstrated nonuniformities that appeared as splotches across the sky. These splotches differ from the otherwise uniform background by only one part in 100,000. The experiment was designed to measure to a precision of seven angular degrees (the moon is half a degree in diameter). So when compared to a clear part of the sky, the typical fluctuation had a size of seven angular degrees and an intensity of 35 microkelvin. There are of course smaller ripples in the fabric of the cosmic microwave background that are currently under study with ground-based and balloon-based instruments.
The results of the COBE experiment support several predictions of the big bang theory. First of all, theorists had predicted that a big bang implied a cosmic microwave background that cooled with expansion exactly as a blackbody. The FIRAS results proved that the spectrum of the cosmic background radiation is nearly identical to that of a 2.728K blackbody. Secondly, theory predicted that the universe should expand evenly in all directions, therefore the cosmic microwave background should appear fairly uniform across the sky. The first result of the DMR experiment was to show that the cosmic microwave background is indeed constant in all directions. Finally, and possibly most important, the DMR experiment provided an explanation for large scale structure. The splotches that appear in the cosmic microwave background provide information about the distribution of matter in the early universe. The radiation that comprises the cosmic microwave background last scattered from matter when the universe was only a few hundred thousand years old (the present age of the universe is thought to be near 15 billion years). The ripples observed by the DMR experiment result from primordial pockets of matter that later evolved into galactic clusters, superclusters and possibly larger structures. In fact, many of the anisotropies in the cosmic microwave background are larger than any structures known today.
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