E4 Cosmic Background Radiation (CMB)

Topic

The central element of this model is the cosmic microwave background—the universe's relic thermal radiation—which exhibits a nearly perfect blackbody spectrum with deviations of less than one part in ten thousand.

Temperature anisotropies in this background, amounting to just one part in one hundred thousand, reflect the universe's primordial density fluctuations; acoustic modes within the baryon-photon plasma produce a series of peaks in the angular spectrum of these anisotropies.

The position of the first of these peaks alone determines the spatial curvature of the entire universe. Odd-numbered peaks correspond to compressions of the plasma, while even-numbered peaks correspond to rarefactions—an alternation that directly reflects the oscillation phase in which the plasma was "frozen" at the precise moment of recombination.

Polarization of the cosmic background—in its various forms—also enables us to measure the universe's subsequent reionization and search for the signature of primordial gravitational waves. Thus, from the cosmic microwave background that opens this document to the sought-after primordial signature that concludes it—taking in temperature anisotropies and acoustic peaks along the way—cosmic background radiation demonstrates how a single, nearly perfectly uniform relic radiation encodes the entire physical history of the early universe within its minute imperfections.

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