The COSmic Monopole Observer (COSMO)

The COSmic Monopole Observer (COSMO) is an experiment to measure low-level spec-tral distortions in the isotropic component of the Cosmic Microwave Background (CMB).Deviations from a pure blackbody spectrum are expected at low level (<1 ppm) dueto several astrophysical and cosmological phenomena, and promise to provide importantindependent information on the early and late phases of the universe. They have notbeen detected yet, due to the extreme accuracy required, the best upper limits beingstill those from the COBE-FIRAS mission. COSMO is based on a cryogenic differentialFourier Transform Spectrometer, measuring the spectral brightness difference betweenthe sky and an accurate cryogenic blackbody. The first implementation of COSMO,funded by the Italian PRIN and PNRA programs, will operate from the Concordia sta-tion at Dome-C, in Antarctica, and will take advantage of a fast sky-dip technique toget rid of atmospheric emission and its fluctuations, separating them from the monopolecomponent of the sky brightness. Here we describe the instrument design, its capabili-ties, the current status. We also discuss its subsequent implementation in a balloon-flight,which has been studied within the COSMOS program of the Italian Space Agency.