  The dynamics of galaxies in an expanding universe is often determined for
gravitational and dark matter in an Einstein-de Sitter universe, or
alternatively by modifying the gravitational long-range attractions in the
Newtonian dynamics (MOND). Here the time evolution of galaxies is determined by
simulations of systems with pure gravitational forces by classical Molecular
Dynamic simulations. A time reversible algorithm for formation and aging of
gravitational systems by self-assembly of baryonic objects, recently derived
(Eur. Phys. J. Plus 2022, 137:99), is extended to include the Hubble expansion
of the space. The algorithm is stable for billions of time steps without any
adjustments. The algorithm is used to simulate simple models of the Milky Way
with the Hubble expansion of the universe, and the galaxies are simulated for
times which corresponds to more than 25 Gyr. The rotating galaxies lose bound
objects from time to time, but they are still stable at the end of the
simulations. The simulations indicate that the explanation for the dynamics of
galaxies may be that the universe is very young in cosmological times. Although
the models of the Milky Way are rather stable at 13-14 Gyr, which corresponds
to the cosmological time of the universe, the Hubble expansion will sooner or
later release the objects in the galaxies. But the simulations indicate that
this will first happen in a far away future.
