The era of the reionization of the Universe

The Universe begins with a gigantic explosion, the Big Bang. Energy density and temperature are enormous, and in the rapid expansion that follows, energy dilutes and temperature decreases. At first, the Universe is a plasma of charged particles, essentially electrons and protons. 380 000 years later, the temperature is 3 000 Kelvin, and the gas recombines into hydrogen atoms. The photons are then no longer scattered by the plasma and travel in a straight line to us : after 13.8 billion years, they correspond to a temperature of 2.73 K. The period just after the last diffusion is called " dark age " ; at this time, structures are only dark matter sketches, there are no stars to illuminate. Little by little, hydrogen gas falls into these dark galaxies, forming the first stars that reionize the intergalactic gas. After several percolations from galaxy to galaxy, all of space will be reionized : this is the epoch of reionization. It's an era that's still little-known, but rich in information about the formation of the structures of the Universe. Large metric-wave telescopes, such as SKA and its precursors, will be able to detect the dark-age hydrogen line globally, in emission or absorption, and watch it gradually disappear during the reionization epoch.