Acoustic Oscillations

The first four frequency modes for a closed vibrating string.

Before decoupling, the Universe consisted of a fluid-like plasma of photons and free baryons. Gravity caused pockets of high matter density to contract while the constant interaction between photons and electrons created pressure forces which resisted the pull of gravity. The competition between these forces caused oscillating regions of compression and rarefaction in the cosmic fluid, sending acoustic waves coursing through the early Universe. We can see evidence of these density waves in the characteristic pattern of anisotropies in the the CMB.

Like the acoustic waves produced by a musical instrument, the oscillations in the cosmic fluid consisted of multiple frequency modes superimposed over one another. When an instrument plays a single note, there is a core frequency which we hear as the central pitch, along with secondary frequencies called overtones or harmonics. These overtones are what make each instrument sound unique. Similarly, the pattern of frequency modes in the cosmic acoustic oscillations reveal details about the unique structure of the Universe.