The expansion of space in the universe, as described by the theory of cosmic expansion, is a consequence of the Big Bang theory and is supported by various observational evidence, such as the redshift of distant galaxies. However, it's essential to understand that cosmic expansion does not violate the principle of special relativity, which states that nothing with mass can travel at or faster than the speed of light in a vacuum (denoted as 'c').
When we say that space is expanding, it means that the distances between galaxies or galaxy clusters are increasing over time. However, it's important to note that this expansion is happening on the cosmological scale, where we are dealing with very large distances, often measured in billions of light-years.
The expansion of space is not a motion through space in the conventional sense. It's not that galaxies are moving away from each other like objects moving through space; rather, it's space itself that is stretching, causing the distances between objects to increase over time.
To illustrate this concept, consider an analogy using dots on an inflating balloon. Draw several dots on a deflated balloon, representing galaxies. As the balloon is inflated, the dots move away from each other, not because the dots themselves are moving through space, but because the space between them is expanding.
The expansion of space is not subject to the restrictions of special relativity because it's not a motion of objects through space, but rather a change in the fundamental structure of the universe itself. This is why cosmic expansion doesn't violate the principle that nothing with mass can travel faster than the speed of light.
However, it's crucial to understand that cosmic expansion only affects the large-scale structure of the universe on cosmological distances. At smaller scales, such as within galaxies or solar systems, the gravitational forces dominate and can keep objects bound together despite the cosmic expansion happening on larger scales. This is why galaxies, stars, and planets within a local region can maintain their relative positions despite the overall expansion of the universe.