Abstract:
Cosmology is the study of the universe as a whole, in particular its physical origin and
evolution. According to the current understanding, the universe came into existence some
13.7 billion years ago in a Big Bang (a very hot and very dense state) and has expanded
and cooled since then. After about 380,000 years, ions and electrons combined to form the
first atoms. At the end of this recombination epoch, the universe became transparent to the
photons that could then travel freely. The photons released after recombination formed a
thermal radiation that is still observed today in the Cosmic Microwave Background (CMB),
with a current temperature of 2.73 K and discovered by Penzias & Wilson (1965) . Matter
then slowly assembled in the expanding universe. The first stars, galaxies and active
galactic nuclei were formed and started to radiate photons, and thereby re-ionizing the
universe. The large structures observed today were finally progressively created. There
are three major observations that support the Big Bang theory, namely, CMB, primordial
nucleosynthesis and the expansion of the universe.
