THE Cosmic Microwave Background (CMB) is the leftover radiation from the Big Bang. According to the Big Bang model, the universe began about 14 billion years ago as an extremely hot, dense plasma of particles (mostly protons, neutrons and electrons) and photons (light) occupying a space of a few millimetres across. The theory also tells us that the universe has been expanding from the instant it was born. From this extraordinarily dense state it has expanded into the vast and much cooler cosmos that we see today.
Since photons are scattered by electrons, for the first 380,000 years or so after the Big Bang, light remained trapped inside the hot, dense particle cloud. This particle cloud was opaque, like a thick fog. As the universe expanded, it cooled and when the temperature dropped to around 3,000 Kelvin (0C corresponds to 273 K and 0 K corresponds to absolute zero or total absence of heat), which is the threshold temperature for protons and electrons to combine, hydrogen atoms formed and the temperature was too low to ionise them again.
Neutral hydrogen is almost transparent to light, and, in the absence of free electrons, photons were able to travel unhindered through the universe. That is, at this epoch of recombination (of electrons and nuclei), light decoupled from matter. During the intervening billions of years, the universe has expanded and cooled greatly. This afterglow of the Big Bang has been expanding with the universe and freely propagating and spreading through the cosmos. Owing to the expansion of space, the wavelengths of this decoupled light from the baby universe have also got stretched (that is, they have been “red-shifted”) to millimetre wavelengths (or gigahertz frequencies, the microwave region) at the present. These decoupled photons now fill the universe as background radiation almost uniformly in all directions with about 412 photons in every cubic centimetre of space.
Multiple scattering within enclosed space, with little leakage of energy, produces what is known in physics as “thermal” or “black body” radiation. The characteristic of black body radiation is that its intensity and spectrum are determined by temperature alone and are independent of the composition of the source.
The spectral distribution of this “red-shifted” light peaks at wavelengths around 1-5 mm and corresponds to a thermal or black body radiation corresponding to the chilly temperature of about 2.7K. This is the CMB radiation. It is the oldest light that can be detected. But to “see” it one needs antennae tuned to these wavelengths, like the far-infrared and radio telescopes on board Planck.
The distribution characteristics of the CMB contain information about the “surface of last scatter”, before light freed itself from matter to fill the universe. When we observe the CMB, we effectively “see” the epoch of recombination. The CMB, therefore, carries the image of the early universe, and therefore, scientists believe that the traces of the seeds of the formation of stars and galaxies, and clusters of galaxies that eventually formed, should be finely imprinted on the image of the present cold 2.7 K afterglow. Thus, the pattern of the detected CMB is a treasure trove of information for cosmologists to unravel and understand how this elaborate structure of the universe, which appears the same in all directions, came to be. Therein lies the importance of the high quality of Planck’s measurements.