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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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Relic Particle Decay

The cosmic microwave background is the primary reservoir of photons in the universe, and serves as a heat sink for energetic processes. Energy released in the form of photons or charged particles will interact with the CMB and distort the spectrum away from the equilibrium blackbody form.
Schematic: Compton scattering
Consider a photon gas interacting with a (hotter) electron gas. The photons scatter off the electrons (Compton scattering) and will, on average, gain energy. Since the photon number is conserved (no new photons are created), the effect is to shift the original photon spectrum (solid curve) to higher frequencies or higher energies (dashed curve). If scattering is rare, the gas is optically thin and we obtain the Sunyaev-Zeldovich effect: compared to the original spectrum, the shifted spectrum has too few photons in the long-wavelength Rayleigh-Jeans portion of the spectrum and too many photons in the short-wavelength Wein tail. This effect is commonly observed in the CMB observed along the line of sight through large clusters of galaxies.

At very early times (redshift between 10^4 and 10^7), each CMB photon can scatter multiple times and the gas becomes optically thick. Since the photon number is still conserved, the system approaches statistical equilibrium but not thermal equilibrium. The resulting photon spectrum is characterized by the chemical potential, which in turn is proportional to the ratio of the electron energy to the CMB energy.

CMB Data vs expected signals

Measurements of the CMB spectrum are thus sensitive probes for energetic processes in the early universe. Consider the decay (or annihilation) of dark-matter particles in the early universe. Any charged or photonic decay products will heat the electron gas, producing a chemical potential distortion with amplitude proportional to the integrated energy release. The plot above shows the resulting spectrum (green curve), characterized by a sharp drop in temperature at wavelengths longer than a few cm. Recent analyses of gamma ray emission hint at a signal from annihilating dark matter toward the Galactic center. Since the annihilation rate varies as z^6, annihilation in the early universe could produce a measurable energy input, resulting in a detectable distortion. Measurements of the CMB spectrum can provide important constraints on dark matter properties and high-energy physics.

ARCADE will measure the chemical potential of the CMB to sensitivity mu < 2 x 10^{-5}, a factor of 5 more sensitive than existing limits.