After the Big Bang, and the Universe first upheavals, the Universe reached its baby period: it was then three minutes old. At this time, the formation of the light elements took place. From this moment on, the Universe evolved with physics we fully understand: electromagnetism and low energy general relativity.

    At 100,000 to 300,000 years of age, photons streamed freely in all directions. This is the celebrated cosmic background microwave radiation (CMBR), that we observe today. Detecting the CMBR today gives invaluable and accurate information about the past.  It is the most perfect black body known. The small deviations from a perfect Planck distribution encode information about the parameters of the Universe and its geometry.

    In particular, the latest experiments have established that the Universe is practically flat.  We will discuss the underlying theory, recent experiments, and the forthcoming new observations.