Introduction

The Cosmic Microwave Background (CMB) was discovered by Penzias and Wilson [1] in 1965, and interpreted by Dicke et al. [2]. This radiation comes from the first moments of our Universe, and was first predicted by Gamow, Alpher and Hermann [3] in 1948, in the context of the Big Bang theory. The Big Bang theory predicted that our Universe was in expansion, which was first observed by Hubble [4] in 1929, via the redshift of galaxies. As a consequence of the expansion, the Universe also cools down, meaning that at earlier times it was hotter. Going back in the past, our Universe was so hot that matter and photons were tightly coupled with each other. They formed a plasma in thermal equilibrium. As the Universe cooled down, the photon energy became too small to ionise the matter, below 0.1 eV (below 13.6 eV due to the high photon to electron ratio), the mean free path of the photons became larger than the horizon. The photons were free to cross the Universe towards our detectors. The blackbody dsitribution and spatial properties of these photons remain unchanged due to their negligeable cross-section with matter, the blackbody temperature is just cooled to 2.7 K due to the Universe expansion. Through this radiation, we obtain a picture of our Universe 300 000 years after the Big Bang.