What is the big bang?

The Big Bang is a cosmological model, a theory used to describe the beginning and the evolution of our universe. It says that the universe was in a very hot and dense state before it started to expand 13,7 billion years ago. This theory is based on fundamental observations:

  • In 1920, Hubble observed that the distance between galaxies was increasing everywhere in the universe. This means that galaxies had to be closer to each other in the past.
  • In 1964 Wilson and Penzias discovered the cosmic background radiation, which is a like a fossil of radiation emitted during the beginning of the universe, when it was hot and dense. The cosmic background radiation is observable everywhere in the universe (see image).
  • The composition of the universe (i.e. the number of atoms of different elements) is consistent with the Big Bang Theory. So far, this theory is the only one that can explain why we observe an abundance of primordial elements in the universe.

What is Big Bang

Cosmic background radiation spectrum as determined with the COBE satellite: uncorrected (top), corrected for the dipole term due to our peculiar velocity (middle), and corrected for contributions from the dipole term and from our galaxy (bottom). The plane of the Milky Way Galaxy is horizontal across the middle of each picture. Courtesy of the NASA.