One of the predictions of the Big Bang model for the origin of the Universe is that the initial explosion was extremely hot and that the remnants of the initial fireball might still be detected at the edges of the Universe. Support for this hypothesis came from the discovery in the 1960s by Arno Penzias and Robert Wilson of the Bell telephone Laboratories, of what came to be known as the Cosmic Microwave Background (http://www.pbs.org/wgbh/aso/databank/entries/dp65co.html). The discovery of the Cosmic Microwave Background coincided with the work of some theoretical physicists who showed that if the Universe began with a hot Big Bang, then the Universe should be filled with electromagnetic radiation cooled from the early fireball to a temperature of around 10 degrees above absolute zero (~10°K). In subsequent years a large number of measurements of the Cosmic Microwave Background at different wavelengths yielded an intensity-wavelength plot which had the characteristics of black body radiation at 2.73°K (2.73 degrees above absolute zero). This is the remnant of the initial fireball of the Big Bang.
A difficulty with our measurements of the Cosmic Microwave Background is that they are all derived from our local region of the Universe. Further confirmation of the Big Bang Hypothesis has to come from the measurement of the Cosmic Microwave Background from a more distant part of the Universe. This has now been achieved by a very clever experiment reported in the journal Nature in December 2000. Srianand and colleagues analysed light from a distant quasar, one of the most luminous objects in the Universe. [Quasars are very bright objects which only exist at immense distances. They are thought to be the cores of extremely distant, young galaxies, that pump out huge amounts of energy. There are no nearby quasars. Objects seen at immense distances are seen as they were billions of years ago, because of the time it takes for their light to reach us. The presence of quasars implies that the Universe was different in the past.] Their observations showed that atoms of carbon, in the quasar were in an 'excited' fine-structure state, that is they showed more energy than might be normally expected. The extra energy imparted to these carbon atoms is thought to be due to the presence of the Cosmic Microwave Background in the vicinity of the quasar. Calculations suggest that at the distance of the measured quasar the Cosmic Microwave Background should be about 9°K. Measurements reported in the study of Srianand and colleagues show that the Cosmic Microwave Background is between 6°K and 14°K and in accordance with the predictions of Big Bang theory. Reporting in the news and views section of Nature John Bahall writes of this experiment 'the Big Bang theory has survived a crucial test, … for … the theory would have been abandoned if astronomers had found that clouds at earlier times had lower temperatures than predicted'.
More recently the quest has been for ever-precise measurements of the Cosmic Microwave Background for these hold vital clues to the very early history of the Universe. These measurements, however, have turned out to extremely difficult, principally because of the very large amount of microwave radiation from other sources, particularly from human activity, which has to be filtered out. Thus, in order to avoid Earth-generated microwave radiation, a satellite was launched in 1989 carrying data to detect the cosmic microwave background. This experiment, known as the COBE experiment, the COsmic Background Explorer satellite (http://space.gsfc.nasa.gov/astro/cobe/ and http://space.gsfc.nasa.gov/astro/cobe/ed_resources.html), produced an extremely important result. High resolution temperature measurements with a sensitivity of a few millionths of a degree were mapped over the entire sky and showed that the Cosmic Microwave Background is variable on the scale of ~30 millionths of a degree K (see Figure 1). This observation is very important for it is this small variation to which we owe our origins. A perfectly uniform Big Bang would have been unacceptable because only heterogeneities, of the type now discovered by the COBE experiment, are capable of permitting the formation of regions of matter, as we now have in galaxies.
The most recent studies of the Cosmic Microwave Background have been based upon balloon-borne microwave telescopes. The results from two such studies were reported in 2000. In August 1998 the Maxima telescope (http://cfpa.berkeley.edu/group/cmb/index.html) spent one night at 40 km above Texas and later that year the Boomerang experiment was launched from Antarctica and spent 10 days circumnavigating Antarctica. The results of these recent studies have allowed an in-depth study of competing Big Bang hypotheses, and provide confirmation that the Universe is 'flat', i.e. finely balanced between expanding for ever or collapsing back into a 'big crunch'.