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Imagine a fantastically large orchestra playing expansively for 14 billion years. At first, the strains sound harmonious. But listen more carefully: something is off key. Puzzlingly, the tuba and bass are softly playing a different song.
So it is when scientists "listen" to the music of the cosmos played in the cosmic microwave background (CMB) radiation, our largest-scale window into the conditions of the early universe. Shortly after the big bang, random fluctuations--probably thanks to the actions of quantum mechanics--apparently arose in the energy density of the universe. They ballooned in size and ultimately became the galaxy clusters of today. The fluctuations were a lot like sound waves (ordinary sound waves are oscillations in the density of air), and the "sound" ringing throughout the cosmos 14 billion years ago was imprinted on the CMB. Now we see a map of that sound drawn on the sky in the form of CMB temperature variations.