Dynamical ultrametricity in the critical trap model

We show that the trap model at its critical temperature presents dynamical ultrametricity in the sense of Cugliandolo and Kurchan (Cugliandolo L F and Kurchan J 1994 J. Phys. A: Math. Gen. 27 5749). We use the explicit analytical solution of this model to discuss several issues that arise in the context of mean-field glassy dynamics, such as the scaling form of the correlation function, and the finite time (or finite forcing) corrections to ultrametricity, that are found to decay only logarithmically with the associated timescale, as well as the fluctuation-dissipation ratio. We also argue that in the multi-level trap model, the short-time dynamics is dominated by the level which is at its critical temperature, so that dynamical ultrametricity should hold in the whole glassy temperature range. We revisit some experimental data on spin glasses in the light of these results.