  The COSmic Monopole Observer (COSMO) is an experiment to measure low-level
spectral distortions in the isotropic component of the Cosmic Microwave
Background (CMB). Deviations from a pure blackbody spectrum are expected at low
level ($<$ 1 ppm) due to several astrophysical and cosmological phenomena, and
promise to provide important independent information on the early and late
phases of the universe. They have not been detected yet, due to the extreme
accuracy required, the best upper limits being still those from the COBE-FIRAS
mission. COSMO is based on a cryogenic differential Fourier Transform
Spectrometer, measuring the spectral brightness difference between the sky and
an accurate cryogenic blackbody. The first implementation of COSMO, funded by
the Italian PRIN and PNRA programs, will operate from the Concordia station at
Dome-C, in Antarctica, and will take advantage of a fast sky-dip technique to
get rid of atmospheric emission and its fluctuations, separating them from the
monopole component of the sky brightness. Here we describe the instrument
design, its capabilities, 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.
