The observation that galaxies farther away from Earth are moving away from us faster is a fundamental principle in cosmology known as Hubble's Law. It is derived from the observation that the universe is expanding.
According to Hubble's Law, the recessional velocity of a galaxy is directly proportional to its distance from us. In other words, the farther a galaxy is from us, the faster it appears to be moving away. This relationship is encapsulated by Hubble's constant, denoted as "H0."
To understand why this occurs, it is essential to grasp the concept of the expanding universe. The universe is not static but is undergoing a continuous expansion, meaning that the space between galaxies is increasing over time. Think of it as an inflating balloon, where the dots on the surface represent galaxies. As the balloon expands, every dot moves away from every other dot, and the farther apart they are, the faster they move away.
To illustrate this, consider a simple analogy: imagine painting dots on the surface of an inflating balloon. As you inflate the balloon, all the dots move away from each other. Now, imagine you are standing on one of the dots, observing the other dots. From your perspective, every other dot appears to be moving away from you, and the ones farther away move faster.
This expansion of the universe is described by the Big Bang theory, which states that the universe originated from a highly dense and hot state approximately 13.8 billion years ago. Since then, it has been expanding, causing the galaxies to move away from each other.
It's important to note that Hubble's Law describes a statistical relationship observed on large scales, as the expansion of the universe affects the distribution of galaxies. However, on smaller scales, such as within galaxy clusters, other forces like gravity can dominate and create complex motions.