Thermal RG Flow of AS Quantum Gravity
We perform the thermal Renormalization Group (RG) study of the Asymptotically Safe (AS) quantum gravity in the Einstein-Hilbert truncation by relating the temperature parameter to the running RG scale as $T \equiv k_T = τk$ (in natural units) in order to determine its thermal evolution in terms of the dimensionless temperature $τ$ which is associated with the temperature of the expanding Universe. Thus, $k_T$ and $k$ are understood as running cutoffs for thermal and quantum fluctuations, respectively. Quantum effects are taken into account by moving along the thermal RG trajectory with fixed value of $τ$ producing the quantum effective action at a given dimensionless temperature. The $τ$-evolution of the dimensionless Newton coupling $g(τ)$ and the dimensionless cosmological constant $λ(τ)$ results in a vanishing $g$-coordinate of the Reuter (i.e., non-Gaussian UV) fixed point in the high temperature limit ($τ\to \infty$) which means that only the symmetric phase of AS gravity survives at $τ= \infty$. Thus, in case of large temperatures the cosmological constant takes on a negative value in the limit $k\to 0$ which was also initially predicted by certain string theories, however, in our approach this is not in disagreement with observations, since during the thermal evolution of the Universe a phase transition occurs and the cosmological constant runs to the expected positive value at low temperatures.