logo     The COSMO (COSmic Monopole Observer) experiment : Science
The Cosmic Microwave Background radiation was released from matter in the early universe, around 380000 years after the big bang, and is still detectable today, as a faint background of microwaves. The distribution of this radiation at different frequencies follows very closely a black-body, the radiation produced in all the environments where there is thermal equilibrium between matter and photons.

This means that the hot, early universe (the primeval fireball) was in good thermal equilibrium, due to the continuous scattering of photons against free electrons. The temperature of the CMB black-body is 2.725K, as measured by the FIRAS instrument on the COBE satellite.

 		Figure 2: The 2.725K blackbody spectrum of the Cosmic Microwave Background.
 
FIRAS demonstrated that any deviation from a blackbody must be smaller than 0.01% of the peak brightness of the CMB (Mather et al. (1990) Ap.J.L. 354 37) : a remarkably perfect blackbody. However, very small deviations from the blackbody curve are expected, due non-thermal processes happening in the primeval fireball or later.

The target of COSMO is the detection of these deviations.  These are very small (< 0.01%) but include important information on the very early, early and late universe (depending on which physical phenomenon  affected the distribution of photons, and when): a wealth of cosmological information still to be harvested (see e.g. J. Chluba, R. A. Sunyaev, MNRAS (2012) 419 1294 for complete information, and figure 4).

Figure 3: Expected deviation from a perfect blackbody: y distortion, due to all the ionized matter present on the line of sight, from here to recombination (y=1.8x10-6):
y distortion
The science of spectral distortions has a huge discovery potential, and stimulated several proposals for dedicated satellite missions (see e.g. PIXIE, PRISM, Voyage2050).  COSMO represents a propaedeutic experiment, with a phased program of observations from the ground (Dome-C, in Antarctica) and from a stratospheric balloon, targeting at the largest spectral distortion, the so-called y-distortion, visible in figure 3.  history of spectral distortionsFigure 4: reproduced from Jens Chluba. Science with CMB spectral distortions. In Proceedings, 49th Rencontres de Moriond on Cosmology: La Thuile, Italy, March 15-22, 2014, pages 327–334, 2014

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