Central density of the simulated halo (upper panel) and the total energy relative to its initial value (lower panel) are shown as a function of time. We display the results for our simulation with collisionless DM (black) and SIDM (blue). In the upper panels, the error on the central densities is indicated by the bands. The vertical red line indicates t = 9.6 Gyr, to roughly mark the point in time when the total energy starts to increase. The simulations were run with a variable time step and our wider opening criterion (ErrTolForceAcc = 5 × 10⁻⁴).