Sound waves from the nascent universe, called baryon acoustic oscillations (BAOs), left their imprint on the cosmos by influencing galaxy distribution. Researchers have explored this imprint back to when the universe was three billion years old, or roughly 20% of its current age of 13.8 billion years.
For most of its first half-million years, the universe looked extremely different than it does today. Instead of being speckled with stars and galaxies, the cosmos was filled with a sea of plasma – charged particles – that formed a dense, almost uniform fluid.
There were tiny fluctuations of about one part in 100,000. What few variations there were took the form of slightly denser kernels of matter, like a single ounce of cinnamon sprinkled into about 13,000 cups of cookie dough. Since the clumps had more mass, their gravity attracted additional material.
It was so hot that particles couldn’t stick together when they collided – they just bounced off each other. Alternating between the pull of gravity and this repelling effect created waves of pressure – sound – that propagated through the plasma.
Over time, the universe cooled and particles combined to form neutral atoms. Because ...