Once Before Time - Martin Bojowald [71]
In the very early universe, the distribution of matter as well as radiation was almost homogeneous: nearly equal at all places. Slight fluctuations existed nonetheless, seeds that at later times, after the radiation pressure decreased, led to the buildup of much denser regions, eventually culminating in galaxies. Traces of these small inhomogeneities are still discernible from the directional distribution of cosmic background radiation: Its intensity varies slightly from different parts of the sky. Such variations can be computed by means of theoretical models for the evolution of the universe at the big bang and then compared with measurement data to give information about the validity of the theory.
As early as 1948, the existence of cosmic background radiation was predicted by Ralph Alpher and Robert Herman, who estimated the temperature, surprisingly precisely, to be about 5 Kelvin, or –268 degrees Celsius. At that low temperature, the radiation was considered undetectable by most physicists, and so the prediction had little influence. It took twenty more years for cosmic microwave radiation to be detected, first only by chance in 1965 by Arno Penzias and Robert Wilson, as a result of new developments in cooled detectors.2 Penzias and Wilson were awarded the Nobel Prize in Physics in 1978 (together with Pyotr Kapitsa for his work in low-temperature physics).
Owing to the limited measurement precision available at that time, the background radiation appeared to be very homogeneously distributed over the sky. Contributions to the intensity distribution from different microwave frequencies were, as expected, in close agreement with Planck’s formula for black body radiation, already encountered in quantum mechanics. In fact, the universe itself can be considered a perfectly enclosed box: There is clearly nothing outside it. Nowadays, the microwave background is the most precisely measured example of black body radiation. Since the distribution depends on the temperature of the radiation, one can use it to measure the average temperature of the universe at those times. For the microwave background, this yields 2.7 Kelvin, about –270 degrees Celsius, very close to the prediction by Alpher and Herman. This sounds cold, even though the radiation is supposed to have come from the hot early universe. But it has only cooled down during subsequent expansion of the universe and was much hotter at the time of its release: about 4,000 degrees Celsius.
In 1992, the satellite COBE (Cosmic Background Explorer, launched in 1989) took another look at the microwave sky and provided the first measurements of the cosmic background radiation to show irregularities in its directional distribution, or anisotropies. Variations are very small, about a millionth of the total intensity, but measurable nonetheless. For this finding, together with the confirmation of Planck’s formula for the background radiation, the principal investigators of the measurement apparati, John Mather and George Smoot, were awarded the Nobel Prize in Physics in 2006. In the meantime, numerous further experiments have been undertaken, initially on balloons in Antarctica. Today, satellites such as WMAP (the Wilkinson Microwave Anisotropy Probe) are providing spectacular data. A new satellite, called Planck and prepared by the European Space Agency (ESA), was launched in May 2009. It should drive the data to unprecedented levels of detail, and for several years will probably be the ultimate measure of all cosmological observations. (Given the time required to collect and evaluate data, first results are not expected before 2012.)
Anisotropies in the heavenly distribution of microwave