The Big Bang sounded more like a deep hum than a bang, according to an analysis of the radiation left over from the cataclysm.

Giant sound waves propagated through the blazing hot matter that filled the Universe shortly after the Big Bang. These squeezed and stretched matter.

Even though the Universe has been expanding and cooling ever since, the sound waves have left their imprint as temperature variations on the afterglow of the big bang fireball, the so-called cosmic microwave background.