Final answer:
The expansion of the universe refers to the increase in average distances between galaxies over time, not the growth of individual objects or the distances within galaxies. This cosmological phenomenon is described by Hubble's law and involves the stretching of the fabric of space itself.
Step-by-step explanation:
When we say that the universe is expanding, we mean that the average distances between galaxies are increasing with time. This phenomenon is supported by observations showing that the spectral lines of distant galaxies are redshifted, which indicates that they are moving away from us. This expansion, known as Hubble's law, reveals that the recession velocities of these galaxies are proportional to their distances from our viewpoint.
It's important to note that within a galaxy like our Milky Way, the distances between star systems do not increase as part of this cosmic expansion. Instead, the expansion is occurring on a large scale between galaxies and galactic clusters. The Hubble constant gives us a rate of this expansion, approximately 22 kilometers per second per million light-years. This expansion is not due to galaxies actually moving through space, but rather the fabric of space itself stretching.
Moreover, we know that this expansion is uniform, meaning it's occuring in all directions and edges of the universe at the same rate, signified by the scale factor R. If R doubles, the distances between galaxy clusters double, indicating the universe's scale over time has grown. We are not at the center of the expansion; from any viewpoint in the universe, the expansion pattern appears the same.