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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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CMB Spectrum

The cosmic microwave background is a thermal relic of a hot, dense phase in the early universe. For the first year after the Big Bang, the temperature and density remained high enough for photon-creating processes (pair creation and double Compton scattering) to proceed rapidly compared to the overall Hubble expansion. The matter and radiation in the early universe were thus in thermal equilibrium, characterized only by the temperature plus any conserved quantum numbers. The subsequent expansion of the universe shifts the radiation to colder temperatures but does not otherwise change the spectrum: in the absence of later non-equilibrium interactions, the cosmic microwave background will follow a blackbody spectrum.
CMB intensity vs frequency
The plot above shows measurements of the intensity of the cosmic microwave background as a function of observing frequency (or wavelength). The CMB follows the expected blackbody curve over more than 5 orders of magnitude in intensity.
CMB intensity vs frequency
A more compact way to plot these data is to show the thermodynamic temperature corresponding to the measured intensity of each data point. The second plot shows the result. Although the data still cluster around a temperature of 2.725 K, in agreement with the intensity data above, it is apparent that the experimental uncertainies become large at long wavelengths. Deviations from a perfect blackbody curve as large as several percent could exist at wavelengths longer than 1 cm and would have escaped detection.

ARCADE is designed to measure the CMB spectrum at centimeter wavelengths a decade below FIRAS. It will measure the path length of ionized gas in the early universe to determine the epoch of reionization and subsequent structure formation, and will detect or limit spectral distortions from dark matter iteractions in the early universe. Both of these processes are likely under current cosmological theory and allowed by current measurement limits. Even a null detection will place important new constraints on the matter content, structure, and evolution of the universe.
CMB Data vs expected signals

The plot above compares the COBE/FIRAS data set to current upper limits on the spectral distortions from reionization and structure formation (purple) or the decay of massive relic particles (green). Both signals become unobservably small at millimeter wavelengths. The gray box shows the ARCADE frequency range and the anticipated 1 mK error budget. ARCADE will measure the CMB spectrum at wavelengths 15 cm to 3 mm (3 GHz to 90 GHz frequency) where the cosmological signals are largest.