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
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
(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 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
(credit: NASA/WMAP Science Team)
1 In the newly born Universe, light elements like hydrogen,
deuterium and helium were produced.
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¢XC.
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