Cosmic Trajectories calculation with state of the art lattice QCD equation of state

Abstract

We compute the full cosmic trajectories of the early Universe across the QCD phase diagram as the plasma cools from $T\simeq500\,$MeV to $30\,$MeV, assuming $\beta$-equilibrated matter. The trajectories are obtained by simultaneously solving baryon-number, electric-charge, and lepton-asymmetry conservation, closed by a state-of-the-art lattice-QCD equation of state: a fourth-order Taylor expansion in the chemical potentials that merges the latest $(2\!+\!1)$-flavor susceptibilities with charm-quark contributions, thus delivering a consistent $(2\!+\!1\!+\!1)$-flavor equation of state. Results are compared with an ideal quark-gluon plasma and with a hadron-resonance gas to highlight interaction effects. Two cases of primordial lepton asymmetries are analyzed: a symmetric configuration $(\ell_e=\ell_\mu=\ell_\tau=\ell/3)$ and an asymmetric one $(\ell_e=0,\;\ell_\mu=-\ell_\tau)$. Increasing $|\ell|$ systematically drives the trajectories toward larger values of $\mu_B$ and more negative $\mu_Q$. In the asymmetric case, a non-monotonic bounce develops when the $\tau$ chemical potential reaches $m_\tau$, generating a maximum in $\mu_B(T)$, the position of which depends on $\ell_\tau$. Assuming a modest $\mu_{Q}$-dependence of the lattice-QCD critical end point estimates (obtained at $\mu_{Q} = 0$), the trajectories for all lepton asymmetries explored ($|\ell|\lesssim 0.1$) lie to their left, implying that in a standard cosmological scenario the QCD transition is almost certainly a smooth crossover. Nevertheless, we estimate the magnitude of baryon and lepton asymmetries needed to obtain a cosmic trajectory closer to the QCD critical point, providing inputs for future studies of the strong-interaction epoch.