Peculiar motions develop, in addition to the cosmic expansion.
In this section, we look at how denser regions of the Universe have become denser and voids more rarified, and how the galaxies acquired the motions we observe today. Peculiar velocities tend to die away as the Universe expands, because a moving galaxy keeps overtaking others, until it reaches the region where its motion matches that of the cosmic expansion. V pec falls in exactly the same way as the momentum of a photon is reduced, according to Equation 7.11. But we will see in Section 7.4 that the sound speed was then only a few kilometers per second. Even if they had been accelerated to relativistic speeds, the fast-moving clouds would have lost their energy in huge shock waves.