Two great discoveries of this century allow us to study the origin of the Universe:

• The expansion of the Universe (Hubble's Law)
• The Cosmic Microwave Background Radiation

Hubble's Law shows that all the galaxies in the visible universe are moving away from us at a speed proportional to their distance. We think there is nothing special about our perspective: observers in all galaxies would observe the same expansion of the universe.

If we extrapolate backwards in time, then all the galaxies must have started their expansion together from a state of huge density: we call this beginning state of the Universe the

Following the Big Bang, the Universe was filled with radiation associated with the extremely hot conditions. As the Universe expanded, it cooled, and the radiation color distribution changed in turn (with the peak of the distribution shifting to redder colors).

In 1964 Penzias and Wilson discovered (by accident) the relic radiation from the heat of the Big Bang which now appears like the energy emitted by a source just 3 degrees above absolute zero. This radiation has recently been studied by the
Cosmic Background Explorer Satellite (COBE).