Save guides, add subjects and pick up where you left off with your BBC account. A horizontal line means the object is moving at a constant speed. The gradient of the line = the rate of change of ...

Velocity-time graphs show how the velocity (or speed) of a moving object changes with time. These graphs also show if the object is moving at a constant speed or accelerating, decelerating, or ...

In reality, the Hubble parameter changes over time, and it's only a constant everywhere in space. Yet if we lived far enough in the future, we'd see that H stops changing entirely.