In 1929, astronomer Edwin Hubble discovered that all galaxies were moving away from us on average and that, stranger still, those that were twice as far away moved twice as fast. And those that were three times farther away moved three times faster and so on. Everything was moving away from us. This is called Hubble’s Law.
What Hubble observed could exist between objects moving at a constant speed in a straight line. For example, imagine participating in a race where each runner chooses their speed and direction randomly at the beginning of the race, but must move at a constant speed in a straight line. Obviously, the distance between two runners remains unchanged only if both are moving at the same speed in the same direction. This situation is excluded from the race for simplicity.
When two runners start moving from the same location, the distance between them increases forever. This means that you see the other runner moving away forever. When many runners start from the same place, everyone in the race sees the other runners moving away forever, and the farther away a runner is from you, the faster he moves away. These phenomena in the race are quite similar to Hubble’s law and can be demonstrated by a computer simulation (I have run this simulation several times in my head).
When each runner starts from a different place, the distance between two runners increases or decreases over time. If it increases, you see the other runner moving away forever. If it decreases, you see the other getting closer until he overtakes you and then moves away forever. When there are many runners and everyone in the race starts from a different location, more and more runners will move away from you over time. If the race lasts for hours, most of the runners will move away from each other, and only a tiny fraction of them will approach you.
These observations in the race, where each runner starts from a different location, correspond precisely to observations in the cosmos: the universe has existed for so long that most galaxies are moving away from each other and only a tiny fraction of galaxies are approaching the Milky Way.
