  We start from the author's metric for a rotating and accelerating Universe
and we calculate the force acting on a particle in it. This force consists of
two parts: one is derived from a potential, and the other describes a Coriolis
force coming from the overall rotation of the Universe. We then write down the
equations of motion, and we solve them analytically in the Newtonian
approximation. The result has to describe in particular a spiral arm. It gives
right a logarithmic helix, as it has been found empirically to be the case for
our Galaxy (in particular).
