For most of human history, people believed that the universe was eternal and unchanging. The stars and galaxies appeared fixed, and there seemed to be no reason to imagine otherwise.
But during the twentieth century, astronomers made one of the most remarkable discoveries in the history of science.
The universe is expanding.
This does not mean that galaxies are flying through empty space like fragments from an explosion. Instead, space itself is stretching. As space expands, the distances between galaxies increase.
The idea of an expanding universe was first suggested by solutions to Einstein’s equations of general relativity. At the time, Einstein himself believed the universe must be static and introduced a mathematical term, the cosmological constant, to prevent expansion or contraction.
Soon afterwards, observations changed everything.
In the 1920s, Edwin Hubble discovered that distant galaxies are moving away from us. Moreover, the farther away a galaxy is, the faster it appears to recede. This relationship became known as Hubble’s Law.
The expansion of the universe does not imply that our galaxy occupies a special position. Observers in any galaxy would see other galaxies moving away from them. There is no center of expansion in the ordinary sense. Every region of space participates in the process.
If we imagine reversing this expansion and running cosmic history backwards, galaxies come closer together, temperatures rise, and matter becomes increasingly dense. Eventually we reach a time when the observable universe was compressed into an extremely hot and dense state.
This picture forms the foundation of modern cosmology and is commonly known as the Big Bang model.
Yet the term “Big Bang” can be misleading. It was not an explosion in space. Rather, it marked the beginning of the expansion of space itself.
Even today, the universe continues to expand. In fact, observations suggest that the expansion is accelerating, driven by an unknown phenomenon known as dark energy.
The expansion of the universe reminds us that the cosmos is not a static stage upon which events unfold. It is dynamic and evolving.
Galaxies drift apart, stars are born and die, and the universe itself changes with time.
Perhaps that is one of the most profound discoveries of modern science.
The universe is not a finished structure.
It has a history.
And that history is still unfolding.