According to the current Big-Bang Model, our Universe was born from an extremely hot fireball. Its temperature gradually dropped with its continual expansion. In the early Universe, the space was full of high-energy photons which kept interacting with electrons and protons. The Universe was not transparent at this stage. About 400,000 years after its birth, the average temperature dropped to about 3,000 degrees Centigrade. Electrons and nuclei¹
began to form atoms for the first time and caused a drastic drop in the number of free charged particles. Photons from different parts of the Universe could then roam freely in the vast reaches of space, and became the first light of the Universe. Today, that first light is still extant, but the expansion of the Universe through billions of years has caused its wavelength to increase by a thousand-fold. This radiation has come to be known as the Cosmic Microwave Background - CMB.

Back in 1948, the existence of CMB was first theoretically predicted by physicists George Gamow, Ralph Alpher and Robert Herman, though they held that this radiation was not detectable. In 1965, astronomers Arno Penzias and Robert Wilson at the Bell Labs in the United States pointed an antenna towards the sky and found noises that they failed to eradicate from the signals received. While the noise was bothering them, physicist Robert Dicke was devising an experiment to find CMB with his research team at the Princeton University not faraway. Penzias and Wilson contacted Dicke and confirmed that the noise was actually the CMB which astronomers had long been expecting. They published respectively
results of the experiment and their findings. As a result of this accidental discovery of CMB, Penzias and Wilson were awarded the 1978 Nobel Prize in Physics.²

In 1989, NASA launched the Cosmic Background Explorer - COBE³ to commence a precise and detailed study on CMB. Minute variations of CMB gradually emerged (Fig 1) under the sensitive detectors of COBE. Though the variations were miniscule, they represented firsthand information concerning the structure of the Universe and provided important clues for understanding the early Universe, as well as verifying and developing the Big Bang Model.

Fig 1 (Top) CMB appears uniform in different directions under a sensitivity of 1/1,000 K 4, indicative of a near-constant temperature at different parts of the early Universe. (Bottom) At a resolution level of 1/100,000 K, CMB anisotropy appears (noises), reflecting the slight variations in temperature in the early Universe.  (credit: NASA/COBE Science Team)

The success of COBE not only made CMB an important subject in astronomy in the new century, but also brought the 2006 Nobel Prize in Physics to John Mather, an astronomer who coordinated the COBE project, and physicist George Smoot, who specialized in measuring the minute temperature variations in CMB.⁵

In the 21st century, new space probes to explore CMB will be put into operation one after the other. In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe - WMAP⁶, which is not only more sensitive and accurate than COBE in terms of temperature measurements, but also boasts a much higher spatial resolution. The probe is expected to
shed new light on the Big Bang Model, the structure of our Universe, dark matter, dark energy and the formation of galaxies and stars. The European Space Agency is also planning to launch the Planck Surveyor⁷ in 2008, which may take the study on CMB to a new level.

Fig 2 The latest, most detailed map of CMB distribution produced by WMAP. Red represents areas of higher temperature, blue for lower temperature and white for the direction of polarization in CMB. This new data is useful for determining when the first generation of stars was formed. It also provides new clues for the subsequent evolution of the Universe that took place 1/1,000,000,000,000 second after its birth.  (credit: NASA/WMAP Science Team)

¹ In the newly born Universe, light elements like hydrogen, deuterium and helium were produced.
² http://nobelprize.org/nobel_prizes/physics/laureates/1978/index.html
³ For information on COBE, please refer to http://lambda.gsfc.nasa.gov/product/cobe/
⁴ K (Kelvin) is the absolute temperature. 0 K is equivalent to -273.15°C.
⁵ http://nobelprize.org/nobel_prizes/physics/laureates/2006/index.html
⁶ For information on WMAP, please refer to http://map.gsfc.nasa.gov/
⁷ For information on Planck Surveyor, please refer to http://www.esa.int/esaSC/120398_index_0_m.html