The experimental evidence supporting the expansion of empty space comes from observations of the large-scale structure of the universe and is primarily based on the phenomenon known as cosmic expansion. The discovery of cosmic expansion is credited to the observations made by astronomer Edwin Hubble in the 1920s.
Hubble noticed that galaxies were receding from us in all directions, and the farther they were from us, the faster they appeared to be moving away. This observation led to the formulation of Hubble's law, which states that the velocity at which a galaxy is moving away from us is proportional to its distance from us.
Further observations and measurements of the cosmic microwave background radiation, the afterglow of the Big Bang, have also provided additional evidence for the expansion of the universe.
Now, when we say that empty space can expand, we are referring to the idea that the distances between objects in the universe can increase over time. This expansion is not a physical movement of objects within space but rather a stretching of space itself. It is important to note that this expansion occurs on cosmological scales, where the gravitational pull between galaxies and other celestial objects becomes negligible compared to the overall expansion of space.
In this context, "empty space" refers to the vast regions of space between galaxies, where matter and energy densities are extremely low. Even though this space may be devoid of matter, it is not devoid of physical properties. Space itself can possess energy and curvature, and under the influence of gravity, it can expand or contract.
The expansion of space is often visualized using the analogy of an inflating balloon. As the balloon inflates, all the points on its surface move away from each other. Similarly, as the universe expands, the distances between galaxies increase.
It's important to note that while space itself can expand, there are various regions within space where matter and energy can still exist. For example, galaxies, stars, and planets are not expanding because they are gravitationally bound systems. However, the space between these bound systems is expanding.
The concept of the expansion of space has been supported by numerous experimental observations and is a cornerstone of modern cosmology.