evidence: Before stars or planets, before black holes and white dwarfs, before even atoms or rays of light, the Universe reverberated with something surprising – sound. This primordial hum moved at more than half the speed of light through a superheated plasma of baryons, photons, and dark matter. [...] The Planck Space Telescope was able to pick up echos of BAOs from the early Universe and scientists have been able to translate them into audible frequencies, in the example below. The hum is composed of a low tone with higher overtones. The whoosing sound that can be heard is an artefact of the processing used to make the sound file. [...] The blast of light energy that now spread through the Universe was so powerful that it still jangles radio telescopes and tantalises physicists more than 13 billion years later as a signal known as cosmic microwave background radiation. The "CMB" is the oldest and most detailed visual record of the early Universe. Here too scientists can see a "fossil record" of the Universe's first sounds.