Gaia, the Oldest Stars and the Early Universe

The Extremely Metal-Poor (EMP) stars with [Fe/H]< -3 are the witnesses of the earliest phases of the evolution of our Galaxy; they were likely formed at an early cosmological epoch which corresponds to about z=5. At the present time, only hundreds of EMP stars are known but the Gaia mission is expected to detect thousands of them and will give very accurate distances and space velocities for these very peculiar objects. After a brief summary of what is known about these very old stars and what can be derived (like the nature and ejecta of the first supernovae, the age of the Galaxy etc) a short presentation is made of the improvements which can be expected in this field of research from the Gaia mission.