The Big Bang and the origin of the Cosmic Microwave Background

In 1923, Hubble discovered that the universe was dotted with galaxies, being the Milky Way one of those. It was an incredible discovery as until that time the Milky Way was thought to be the only galaxy making up our universe.

Right after, he made an even more sensational discovery, he realized that all (or almost) the galaxies are moving away from us. He observed that the electromagnetic emissions of the galaxies were redder than they were supposed to be. This phenomenon, called red shift, is explained by the Doppler effect: the radiation emitted by a body shifts to lower frequency (for the light, more red) if the source moves away from the observer, higher if the source get closer. We experience the Doppler effect in everyday life as this applies for any type of wave phenomena, including acoustic waves. The experience of the train whistle is a valid example. When the train comes towards us, its whistle appears to be of higher frequency, while when it moves away from us, it appears of a lower frequency.

The Hubble discovery can be explained with the hypothesis that the universe is expanding. Therefore, there must probably be a time when the entire mass and energy making up the universe were concentrated in a very small region of space.

In 1948, Gamow proposed the theory that the universe originated from an immense explosion, the Big Bang, and that traces of that explosion should still be around today. He came up with the idea of a "fossil" radiation (i.e. Cosmic Microwave Background) permeating the universe and ultimate sign of that immense explosion. A clear trace of the birth of the universe. The calculations led Gamow to hypothesize an isotropic radiation with a certain frequency corresponding to the emission of a body at a temperature of about 3 degrees Kelvin.

And guess what? In 1965 Penzias and Wilson, working on a microwave telecommunications antenna, accidentally detected a background noise corresponding to isotropic radiation of 2.7 degrees Kelvin with a frequency of 150 GHz. The radiation accidentally found by Penzias and Wilson was the fossil radiation, what remains of the primordial "fiat lux" !!!

Some theory: the General Relativity

According to Einstein's General Relativity theory (1916), space and time are one single entity part of the four-dimensional space-time whose characteristic are regulated by the matter.

A fundamental consequence of general relativity is that space-time does not respond to the Euclidean geometry rules, but it is an entity curved by the presence of mass. Only locally or in case of small masses, it can be approximately treated with the Euclidean rules (in our solar system, space-time is almost everywhere Euclidean, apart from the immediate vicinity of the Sun).

If the matter is concentrated in a small volume, the space-time curves so much to trap everything in that region, even light. This is what occurs in a black hole, appearing to be completely black.

According to the Big Bang theory, the entire mass (and energy) of the universe was initially condensed into a very small region of space, trapping everything there. When after the Big Bang the expansion of the universe began, the electromagnetic radiation produced by the initial explosion began to expand in the space-time, gradually filling the entire universe.

The 2.7 Kelvin radiation is what is left of that big and amazing event, last witness of one of the most important secret of moder Physics.

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