In an expanding universe, the distance between galaxies can change over time, while the distance between stars and planets within a galaxy remains relatively constant. This difference arises due to the nature of cosmic expansion and the gravitational interactions at different scales.
The concept of an expanding universe is based on observations that indicate galaxies are moving away from each other. This observation led to the development of the Big Bang theory, which suggests that the universe originated from an incredibly dense and hot state and has been expanding ever since. As space itself expands, it carries galaxies along with it, causing the distances between them to increase.
On the other hand, within a galaxy, gravitational forces dominate the interactions between stars and planets. These gravitational forces hold galaxies together and prevent them from being significantly affected by the overall expansion of the universe. While individual galaxies may experience some internal motions and interactions, the gravitational forces within a galaxy are much stronger than the effects of cosmic expansion on smaller scales.
To summarize, the distance between galaxies can change due to the expansion of the universe, while the distance between stars and planets within a galaxy remains relatively constant due to the strong gravitational forces at play within the galaxy.