The Big Bang

Big Bang Timeline of the universe - big bang expansion over 13.7 billion years. © NASA / WMAP Science Team

By Marcus Chown, cosmology consultant for New Scientist

What was the Big Bang?

About 13.7 billion years ago, the Universe burst into being in a titanic explosion called the Big Bang. Out of the expanding and cooling debris eventually congealed the galaxies, great islands of stars of which our own Milky Way is one.

How do we know there was a Big Bang?

The universe is expanding - its 100 billion galaxies or so flying apart from each other like pieces of cosmic shrapnel. Imagine the expansion running backwards like a movie in reverse, we come to a moment when everything was squeezed into a tiny volume. This was the moment of the Universe’s birth - the Big Bang. When the universe was small, it was hot, for the same reason air in a bicycle pump is hot when it is squeezed. But the big bang heat had nowhere to go. It was bottled up in the universe - which, by definition, is all there is. Consequently, it is still around today, cooled by the expansion of the universe. This "afterglow of creation" is powerful evidence of the Big Bang.

What happened before the Big Bang?

In the standard picture (known as "inflation") the vacuum that existed in the beginning was in a weird, high-energy state. Bizarrely, it had repulsive gravity, which made it expand ever faster. At random locations bubbles formed and the vacuum “decayed” into a normal vacuum. The surplus energy went into creating matter and heating it. In short it created a hot Big Bang. We live in one of the bubble universes in the vast, ever-expanding vacuum.

Who was the first to propose the Big Bang theory?

The was proposed independently by Aleksandr Friedmann and Georges Lemaître, who called it 'hypothesis of the primeval atom’. British cosmologist Sir Fred Hoyle, who ironically never believed in the idea, coined the phrase ‘the Big Bang’ in a 1949 BBC radio broadcast.


Marcus Chown's We Need to Talk About Kelvin was short-listed for the 2010 Royal Society Prize for Science Books. Visit: or