Most astronomers believe that the Universe began with an unimaginably huge
explosion which happened about 13 700 million years ago. We call this the Big Bang.
The age of the universe is therefore 13 700 million years.
We have no idea what happened
before the Big Bang because astronomers think that time and space began at that moment.
The temperature in that
explosion was unbelievably large – astrophysicists think it may have been as high as 1000 million
million oC!
Moments after the Big Bang some of the radiation began to 'turn into' matter – the first particles were
formed. The universe began to cool. After about 300 000 years the temperature had fallen to about 6000 oC.
Some 500 000 years after the Big Bang the Universe had cooled
so much that it became dark. The radiation emitted had passed over the barrier between visible and
the infrared. It was still hot (about 700 oC) but the radiation
coming from it was too long to see. After
about a million years atoms began to form and these had slowly grouped together under gravitational
attraction to make the embryo of a star.
Eventually the temperature in the centre of these
stars had become high enough for nuclear fusion to take place and about a billion years after
the Big Bang the first star was born and blazed out into the darkness of space - there was
light!