Physics of Baryons

International audienceIn this lecture, different milestones in the cosmological history of baryons are reviewed. First, the appearance of hadrons as a consequence of the confinement of quarks is discussed. Then the era in which nuclei interact to produce light elements like deuterium, helium and lithium is described. The third relevant episode is the decoupling between matter-radiation and the properties of the “last scattering” surface. The next covered aspect refers to the value of the residual ionization fraction when “freezing” occurs and the thermal decoupling of matter from CMB photons. As stars appear in the Universe, their UV radiation begins to reionize the intergalactic medium and such a process is also discussed in this lecture. Finally, results from cosmological simulations are presented, permitting to describe where baryons can be found either in the form of stars or in the form of cold, warm and hot gas