If a force is exercised on an object the velocity will change. Force = mass x acceleration. Formula: F = m x a. An object is in the space. If a forewarddriving force is exercised the velocity will increase. The object will have at the front more collisions with gravityparticles than at the backside. It generates a force that counteracts the forewarddriving force. F = F1 – F2 = m x a. If the forewarddriving force F1 doesn’t change, then the acceleration decreases. With the first formula it seems as if the mass of the object increases.
At the border of the universe almost all gravityparticles move away from the center of the universe. The gravityparticles collide at one side against the stars. Therefore these stars move away with great velocity from the center. The velocity of the stars cann’t become greater than the velocity of the gravityparticles. If there was a rocket it would move with the same velocity as the stars away from the center. If the engine is switched on the rocket would move faster than the stars. The velocity can become greater than the velocity of light.