acoustic peaks
Strike a bell and it rings at certain favored notes, set by its size and shape. Blow across a bottle and you get a particular pitch. The infant universe did something surprisingly similar: it rang. Before recombination, the hot soup of matter and light sloshed back and forth as sound waves, and those waves left a fingerprint of preferred sizes in the cosmic microwave background. The acoustic peaks are that fingerprint — a pattern revealing which spot sizes are most common on the microwave sky, and from it we read off the universe's deepest properties.
Here is the physics. In the early plasma, gravity pulled matter into the denser primordial spots while the pressure of trapped light pushed back out — a tug-of-war that set the soup oscillating, compressing and rarefying like genuine sound waves traveling at over half the speed of light. When recombination froze everything at 380,000 years, each oscillation was caught at a particular phase, and certain wavelengths were caught at maximum compression or rarefaction. Plotting how strong the temperature ripples are as a function of their angular size on the sky produces a curve with a series of bumps — the acoustic peaks. The first and tallest peak corresponds to the wave that had just reached maximum compression once; the others are its harmonics.
These peaks are a goldmine. The angular size of the first peak (spots about one degree across, twice the width of the full Moon) reveals the geometry of space — and its position tells us the universe is flat. The relative heights of the peaks weigh the ingredients: the ratio of the first to the second peak measures the amount of ordinary (baryonic) matter, while the third peak helps pin down dark matter. So a single curve, painstakingly measured by the WMAP and Planck satellites, simultaneously tells us the universe is geometrically flat, that ordinary matter is about 5 percent of the total, dark matter about 27 percent, and dark energy the rest. Few measurements in all of science constrain so much from so little.
The first acoustic peak sits at an angular scale of about one degree — meaning the most common hot or cold spots in the CMB are roughly twice the apparent width of the full Moon. The fact that they come out at exactly one degree, rather than larger or smaller, is the measurement that tells us space is flat: in a curved universe those spots would appear a different size.
The size of the first peak's spots — about one degree — reveals that space is flat.
These were real pressure (sound) waves in the pre-recombination plasma, not sound you could hear and not light waves. After recombination the sloshing stopped, freezing the pattern we now read in the peaks.