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.
It arose from a tug of war between ancient and powerful fundamental forces generating soundwaves in this electrically charged soup of particles. Then, just a few hundred thousand years into its existence, the plasma disappeared like a morning fog. The Universe fell suddenly, and profoundly silent.
Yet, it is still possible to pick up echos of these first soundwaves that spread out across our early Universe – if you know where to look. The ripples they created in the plasma have left a permanent imprint on the distribution of matter around the Universe. And they are also providing astronomers with clues about one of the deepest mysteries of our Universe today, the mysterious force known as dark energy.
The primordial soundwaves – also known as baryon acoustic oscillations (BAOs) – formed as the particles in the early Universe began to be pulled together by gravity.
“The gravitational pull of dark matter in the early Universe created ‘potential wells,’ pulling plasma inward,” says Larissa Santos, a professor at the Center for Gravitation and Cosmology at the University of Yangzhou, China. The plasma, however, was so hot that it also created an opposing outward force. “Photons created radiation pressure that fought gravity, and pushed everything back out again. This fight created acoustic oscillations – sound waves.”
BAOs burst outward from uncountable potential wells, forming expanding, concentric spheres of sound energy. They crisscrossed each other, sculpting the plasma into dazzlingly complex three-dimensional interference patterns.
Had a human somehow existed in the epoch of “baryon acoustic oscillations” (BAOs), they would have heard nothing. The sounds were around 47 octaves lower than the bottom note on a piano with enormous wavelengths of about 450,000 light years.
This incredibly deep, inaudible rumble travelled through a medium that even our most powerful telescopes is unable to penetrate. The deeper we look into the Universe, the further back into its history we see due to the time it takes for light to reach us. We can only see so far, however, as the electrical charges from unattached protons and electrons in these early stages of the Universe continuously scattered and diffused light, creating an impenetrably random glow. But BAOs created patterns in this medium which rippled outwards, and we can see evidence of these in the Universe today.
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.
