Free energy fluctuations of the $2$-spin spherical SK model at critical temperature

Abstract: We investigate the fluctuations of the free energy of the $2$-spin spherical Sherrington-Kirkpatrick model at critical temperature $\beta_c = 1$. When $\beta = 1$ we find asymptotic Gaussian fluctuations with variance $\frac{1}{6N^2} \log(N)$, confirming in the spherical case a physics prediction for the SK model with Ising spins. We furthermore prove the existence of a critical window on the scale $\beta = 1 +\alpha \sqrt{ \log(N) } N^{-1/3}$. For any $\alpha \in \mathbb{R}$ we show that the fluctuations are at most order $\sqrt{ \log(N) } / N$, in the sense of tightness. If $ \alpha \to \infty$ at any rate as $N \to \infty$ then, properly normalized, the fluctuations converge to the Tracy-Widom$_1$ distribution. If $ \alpha \to 0$ at any rate as $N \to \infty$ or $ \alpha <0$ is fixed, the fluctuations are asymptotically Gaussian as in the $\alpha=0$ case. In determining the fluctuations, we apply a recent result of Lambert and Paquette on the behavior of the Gaussian-$\beta$-ensemble at the spectral edge.