The lepton asymmetry of the universe is just weakly constraint and might

be orders of magnitude larger than the baryon asymmetry. In this thesis, we

investigate how a lepton asymmetry influences the cosmic trajectory through

the phase diagram of Quantum Chromodynamics (QCD). Therefore, we develop

a technique to determine the temperature evolution of chemical potentials

during the QCD epoch of the early universe at arbitrary lepton flavor

asymmetries. We will rely on an ideal quark gas approximation at high temperatures,

T > 150 MeV, and a hadron resonance gas model at low temperatures,

T < 250 MeV. Higher-order perturbative corrections are (partially)

included in the ideal quark gas approximation. To interpolate between these

approximations, we will for the first time use lattice QCD susceptibilities

to properly account for strong interaction effects close to the QCD transition

temperature TQCD. We therefore use a Taylor series ansatz of the QCD

pressure up to second order in the chemical potentials. With this technique

we investigate the impact of equally and unequally distributed lepton flavor

asymmetries on the cosmic trajectory. We conclude with an estimate on the

reliability of our technique via using a Taylor series of the QCD pressure.