Cosmic background radiation: electromagnetic radiation in the microwave region that comes from all directions in the sky.
The cosmic background radiation was accidentally discovered in 1964 during the testing of a satellite antenna by physicists Robert Wilson and Arno Penzias of Bell Telephone Company. What they initially attributed to bird droppings on the antenna wound up earning them the Nobel Prize in 1978. The radiation turned out to be the first direct piece of evidence for the ►big bang theory developed in the 1940s.
According to this theory, the recombination of protons and electrons into neutral hydrogen atoms caused the ►universe to became transparent to light about 380,000 years after its birth. As a result, radiation could now travel freely in all directions and evenly fill the universe. At that time the universe — and its radiation — had a temperature of 3000 Kelvin (2700 degrees Celsius, 4892 degrees Fahrenheit). This temperature corresponds to a yellow-reddish light color. However, because space continuously expanded, the wavelength of the background radiation increased as well, resulting in its shifting further into the red region and decreasing in temperature. In the 13.7 billion years since the big bang, the background radiation's temperature has decreased to -270 degrees Celsius and is now at just about 2.7 degrees above the absolute zero point. The expansion of its wavelength led to an extreme ►redshift by a factor of 1089, so that the radiation is now no longer visible but can be measured only with radio antennas.
The background radiation's wavelength is almost equal in all directions of the universe. The greatest fluctuation is approximately 0.1% and is caused by a Doppler effect originating from the movement of our Milky Way toward the ►Great Attractor. Other than that, the background radiation has only very minor fluctuations of about 0.001%. This uniformity is indicative of an extremely rapid "inflationary" expansion of space in the early phase of the big bang.
Because the background radiation can tell us so much about the structure and early phase of the universe, it is of the utmost importance for physics that we obtain measurements of it that are as precise as possible. The NASA probes COBE and WMAP have been measuring the radiation since 1990 and 2001, respectively. The minor spatial fluctuations in the radiation temperature, which are represented as spots in the above image, are of special importance, for they enable us to determine whether the universe is curved or flat (see ►Parallels). Analysis of the data delivered by the space probes confirmed the flatness of the universe, thus also indirectly confirming its infinitude. At the same time, it yielded a puzzling result that cannot be satisfactorily explained by any of the existing theories.
The Puzzle of the Missing Overtones
The spatial fluctuations in the temperature are caused by plasma fluctuations that occurred about 380,000 years after the ►big bang, when matter and radiation became separated. Now, just like a plucked guitar string, temperature can undergo different types of spatial fluctuation. The guitar string can vibrate either at its key tone or at its overtones (see image). The key tone corresponds to one "antinode" (or point of greatest vibration) over the length of the string (as in the top figure), the first overtone to two antinodes (as in the middle figure), the second one to three antinodes (bottom figure), and so forth.
The actual appearance of the vibrating string results from the combination of its key tone and its various overtones. Much the same can be said of the fluctuating plasma* of protons and electrons in the early period of the universe. Gravity and radiation pressure here acted much like the tension in a guitar string. The resulting vibrations were impressed on the density of the plasma and thereby on the radiation energy. We can therefore still see them as temperature or wavelength fluctuations of the background radiation. Furthermore, just as the relative strengths of the guitar string's various overtones is determined by the length, tension, and material of the string, the types of fluctuation in the primordial plasma are determined by the ►metric of the universe, that is, by its curvature and expansion speed. The more than thousand "overtones" measured by the WMAP probe almost exactly match what the big bang theory would predict for a flat, or Euclidian, universe.
However, the correspondence is not complete. Two of the overtones, namely the second and third, do produce a dissonance in the cosmic orchestra. The values measured for the strength of these overtones are somewhat lower than they should be.** At present, this cannot be satisfactorily explained. Though we could conceive of a certain metric of the universe that would conform to the measurements, scientists see such an adjustment of the theory as merely ad hoc and are not tempted by it. Other possibilities include an error in the measurements and a hitherto unknown source of radiation in outer space that affects the measurement results. These, too, are regarded as unlikely. Perhaps the European space probe Planck, launched in May 2010 and supposed to measure cosmic background radiation pattern once again at a three-times-higher resolution, can contribute to a solution of the puzzle.
* Plasma is a high-temperature gas in which electrons are no longer bound to atomic nuclei.
** Starkman, Schwarz, Missklänge im Universum ("Dissonances in the Universe"), Spektrum 12/2005