Launched on November 18, 1989, Cosmic Background Explorer (COBE) was a satellite sent into orbit to observe the cosmic microwave background radiation. 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 2.728 K.
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 2.728 K 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 7 angular degrees (in comparison, 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 7 angular degrees and an intensity of 35 microkelvin.
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 2.728 K blackbody. Additionally, 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 the large-scale structure of the universe. The splotches that appear in the cosmic microwave background provide information about the distribution of matter in the early universe. However, the image of the cosmic microwave background produced by the COBE mission was analogous to that of taking a fuzzy snapshot of a car. The fuzzy picture reveals that it is the photograph of a car, but fine detail is blurred and information such as the year, make, and model of the car can not be determined from the picture.
In order to improve on COBE's image, the Wilkinson Microwave Anistropy Probe (WMAP) was launched on June 30, 2001. WMAP was designed to map the cosmic background radiation to an angular resolution of 0.3 degrees with a sensitivity of 20 microkelvins per 0.3 degree square pixel. Stationed at the second Lagrangian point (L2) in the Earth-Sun system at a distance of 1.5 million kilometers, WMAP completed the initial phase of its survey of the distribution of the cosmic background radiation by 2003. The cosmic background radiation image map produced showed much finer details than the previous COBE image. The temperature differences in the WMAP represent very small fluctuations in the cosmic background radiation on the order of 200 microkelvins. This image shows the afterglow of the Big Bang at a time when the universe was around 400,000 years old. The anisotropies in the WMAP image show where matter in the early universe began to congeal, setting the stage for the formation of galaxies.
In 2006, the WMAP team released an improved image with detail that allowed researchers to set the age of the universe at 13.7 billion years old. The data obtained from WMAP also reveals that the universe is composed of 4% baryonic matter, 22% dark matter, and 74% dark energy.
In May 2009, the European Space Agency (ESA) launched an even more sophisticated space observatory with the code name of Planck. Planck’s detection system was 2.5 times as sensitive as that of WMAP and thus produced even more detailed information about anisotropies in space. Among Planck’s discoveries was an updated age of the universe (13.819 billion years) and a distribution of composition of 4.9 percent ordinary matter, 26.8 percent dark matter, and 68.3 percent dark energy. Planck’s instruments were turned off on October 23, 2013, after having functioned about twice as long as originally planned.