Magnetic Moment of Leptons in a Medium

We show that the magnetic moment of leptons is significantly modified at finite temperature and density as compared to the corresponding vacuum value. We compare the magnetic~moment of all different leptons near nucleosynthesis temperature to show the relevance of the calculations with the early universe. It is shown that the significance of~thermal corrections depends on the temperature of the universe and the respective lepton~mass. In the early universe, particle mass was growing quadratically with temperature which~affects the corresponding value of magnetic moment.