The expansion of the universe is a fundamental concept in cosmology. It refers to the observation that the space between galaxies and other cosmic structures is expanding over time. However, it's important to note that this expansion does not mean that individual objects within the universe are moving away from each other like objects on a cosmic conveyor belt. Instead, it is the fabric of space itself that is stretching, causing the apparent motion of objects.
The expansion of the universe can be explained using the concept of the metric expansion of space. It means that the scale of space itself is changing. As the universe expands, the distances between objects, such as stars and galaxies, also increase. It's similar to how dots on the surface of a balloon move away from each other as the balloon is inflated.
On small scales, such as within galaxies or galaxy clusters, gravity dominates over the expansion, so the gravitational forces can hold objects together despite the overall expansion of the universe. For example, the gravitational forces between stars in a galaxy are strong enough to counteract the expansion, so the distances between stars within a galaxy generally do not increase due to the expansion of the universe.
However, on larger scales, such as between galaxy clusters or superclusters, the expansion of the universe becomes more evident. The overall motion of galaxies within these structures is influenced by the expansion, causing them to move away from each other. The farther apart two galaxies are, the faster they appear to be receding from each other due to the expansion of space.
It's worth noting that the expansion of the universe does not occur because galaxies or other objects are moving through space or away from a central point. Instead, it is the space itself that is expanding. The metric expansion of space is a property of the universe as a whole, and it is observed on the largest scales.