The cosmic microwave background

Fig. 2:The power spectrum of CMB anisotropies as seen by BOOMEranG.The first peak corresponds to an angular scale of about 1degree as predicted for it "flat Universe" (i.e., a Universe where the spatial part of the metric is Euclidean). ';2 4000 .3 Fig. 1: Map of the Cosmic Microwave Background obtained by the BOOMERanG experiment.The one-degree-Iike bumps ofthe CMS emission are tiny local fluctuations of the brightness temperature ofthe CMB. The Cosmic Microwave Background (CMB) is the most distant, and therefore the most ancient source of electromagnetic radiation that can be directly observed from Earth in any frequency range. The Cosmic Background Explorer satellite (COBE) has measured its sub-millimeter emission, which is that of a nearly perfect blackbody at 2.73 K. The relative deviation from a pure Planck spectrum is very small (typically less than 10-5). This emission is attributed to the primordial Universe when it was about 300 000 years old and warm enough (3000K) to ionise the hydrogen gas that constitutes most of its mass. Owing to the expansion of the Universe, this radiation was red-shifted by the Doppler effect by a factor of about 1000, and thanks to the cooling due to the expansion, it couId travel and reach us through the very transparent neutral hydrogen. The discovery of the CMB and even more the measurement of its Planck spectrum by the COBE-FIRASl experiment is the most compelling evidence for a hot big bang model. The existence of the CMB was predicted by Alpher, Bethe and Gamow as a very unique feature of a hot big bang in which the nudeosynthesis of most of the He, D and 7Li seen in the Universe (but which cannot be produced in stars) is produced in the early phases of a hot big bang.