Ever wondered how cool the universe is? How much has it cooled since it was born with the Bang! How the temperature of the universe is measured? How big a thermometer do we need? What do we do with that temperature? Well, read on.
The cosmic microwave background radiation (abbreviated CMB or CMBR), considered a relic of the explosion at the beginning of the universe some 13.5 billion years ago, is one of the most powerful aids in determining the structure and dynamics of the universe. Microwaves are a part of the electromagnetic spectrum, which have wavelengths slightly less than radio waves, of the order of 1mm to 1m in length. It was discovered in 1965 that this radiation fills the entire universe. The phenomenon is very important for cosmology since every cosmological theory must explain this radiation as it is observed. This is one of the best pieces of evidence for the Big Bang theory, as we will see in the later part of this article. There are also slight anisotropies (property of non-homogeneity with respect to direction, unlike isotropy) in the radiation that can be mapped to minute thermal fluctuations at the beginning of the universe.
History of CMBR
CMBR was predicted by George Gamow and Ralph Alpher, and also by Alpher and Robert Herman in 1948. In 1964, A. G. Doroshkevich and Igor Novikov published a brief paper describing CMBR as an observable phenomenon. The problems with the early predictions were that they did not mention the isotropic nature of the radiation. They were calculated assuming that observations were made from the edge of the Milky Way. The Planckian  temperature of the radiation was also estimated by Alpher and Gamow. In 1964, Robert Woodrow Wilson and Arno Penzias built a Dicke radiometer  (used to measure the intensity of radio waves) for use in radio astronomy. They detected an antenna temperature, that is, the radiation they received followed the Planck’s black body curve for specific temperature and it was finally confirmed that this was due to microwave background radiation. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery. Anisotropies in the CMBR were predicted long ago but they could not be observed with ground based equipment due to low resolution. NASA's Cosmic Background Explorer (COBE ) can be credited with the first observation of the anisotropy in CMBR. However, the first peak in the anisotropy could not be detected. It was later observed by the Toco Experiment. These results were confirmed by the BOOMERanG and MAXIMA experiments. NASA's Wilkinson Microwave Anisotropy Probe (WMAP ), also detected a second peak in the anisotropies.
Nature of the radiation
The radiations overlap typically a Planck blackbody radiation curve with temperature of 2.725 K. This temperature is continuously decreasing with the expansion of the universe. The anisotropies in the radiation are 1 part in 100,000, that is, root mean square variations of only 18 µK. Both, the radiation and the anisotropies have been predicted by the Big Bang theory. At the beginning of the universe, the universe was just too hot and dense for photons to escape. Matter and radiation were indistinguishable. Any photon that would have been created at that time would immediately have been reabsorbed. As the universe expanded and cooled, it became feasible for electrons and protons to combine and form neutral hydrogen. The photons then scattered off these neutral atoms and were free to move in space. This stage is called decoupling of matter and radiation. The photons we receive now are basically from this spherical surface of the universe called the surface of last scattering. The universe temperature of 2.725 K gives us a fair idea of what the temperature would have been then and also how fast the universe is expanding.
However, this does not explain the anisotropies inherent in the CMBR. This is now the major part of research and study in this field. The anisotropies give us a flavour of the disorder in the universe at the time of decoupling. These are called primary anisotropies. The secondary anisotropies refer to galactic or other distortions between the the surface of last scattering and the observer. The inflation of these anisotropies is thought to be the signs of formation of galaxies and galaxy clusters. This very well matches the mass distribution in our universe. This has led to the prediction of the picture of the universe as we see it. The following picture on the left was released by NASA as a representation of anisotropy in the universe.
Below: The Planck's curve obtained by COBE for the CMBR. (http://en.wikipedia.org/wiki/File:Firas_spectrum.jpg)
The Nobel prize for the year 2006 was awarded to John Mather and George Smoot for work that looks back into the infancy of the universe and attempts to gain some understanding of the origin of galaxies and stars using data from the COBE satellite. There have been other efforts to observe furthur anisotropies of lower order in the radiation pattern by various satellites. The continuing research holds promising results to help us understand the origin of the universe in a better way.