The observation that galaxies farther away from Earth are moving away from us faster is a fundamental principle of cosmology known as the Hubble's law. It is based on the observation of the redshift of light from distant galaxies.
The key concept behind this phenomenon is the expansion of the universe. According to the Big Bang theory, the universe originated from an extremely dense and hot state and has been expanding ever since. As space itself expands, it carries galaxies along with it.
The relationship between the recessional velocity of a galaxy (its velocity away from us) and its distance from us was first quantified by Edwin Hubble in the 1920s. He discovered that the recessional velocity of a galaxy is directly proportional to its distance from us. Mathematically, this relationship is expressed as v = H₀d, where v is the recessional velocity, d is the distance, and H₀ is the Hubble constant.
In simpler terms, galaxies that are farther away from us are moving away faster because the expansion of space itself is causing more distant galaxies to recede at higher speeds. It's important to note that this apparent motion is not due to galaxies moving through space like objects in a conventional sense, but rather the expansion of the fabric of space itself.
Imagine an analogy of dots on the surface of an inflating balloon. As the balloon inflates, every dot on its surface moves away from every other dot. The farther apart two dots are, the faster they appear to move away from each other. This analogy helps illustrate the concept of the expanding universe, where galaxies are like the dots on the balloon's surface.
It's worth mentioning that while the recessional velocity increases with distance, there are additional factors that can influence the observed velocities of galaxies, such as peculiar motions due to local gravitational interactions. However, the overall trend of galaxies moving away faster as their distance increases remains a foundational principle of our current understanding of the expanding universe.