Have you ever wondered how the universe came into being? How the stars twinkling in the sky above us came to exist? For centuries, scholars and theologians had debated the origins of the cosmos, but it was the work of a Catholic priest and theoretical physicist, Georges Lemaitre, that truly revolutionized our understanding of the universe.

In 1927, Lemaitre began work on a theory that would come to shape our modern understanding of the cosmos, based on the remarkable observation that space seemed to be expanding. He imagined a time before humans or any living thing inhabited the earth when the universe was just a barren rocky ball around a glowing sun. Before that, the sun ignited in a blaze of nuclear glory, crushed into being by the irresistible force of gravity in a vast cloud of gas and dust. That gas and dust were a consequence of many generations of stars living and dying before our own, igniting, fusing elements, and exploding in terrific supernova that spread their elemental waste products across the cosmos.

But what happened before all of that? Lemaitre imagined a time before the chemical complexity we enjoy, when only hydrogen and helium floated inert in a lightless universe. If space is expanding now, then in the reverse chronology of Lemaitre’s mind, it is contracting. There would be a point in this dark, gassy cosmos before the first star when all the matter in the universe was compressed into a vastly smaller space. Atoms would become packed so tightly that even photons are unable to pass through; the universe would be utterly opaque.

As the contracting reversed universe crushes down even further, the atoms themselves are crushed first into subatomic particles and then into pure energy. In this small, dense universe, there is only radiation. Here, even Lemaitre’s imagination begins to falter. The universe can become even smaller, even hotter, but what happens then? Lemaitre pushed the unanswerable from his mind, content in having grasped 13 billion or so years of cosmological history and unperturbed by the mysteries contained within the first fraction of a second.

But the Big Bang theory of which Lemaitre was unknowingly the first true proponent has been the subject of debate, research, and speculation for nearly 100 years. How did the universe begin? What was the trigger for its incredible expansion? And could our simple matter brains ever even comprehend the real truth?

Lemaitre’s calculations rocked the cosmological world, but they were not embraced by everybody. Before his work, the focus of cosmology had largely been on our own place within the universe, as well as observations to determine its composition and extent. But Lemaitre’s insight opened up a whole new field of study, one that has captured the imagination of scientists and laypeople alike.

The Big Bang theory remains the best explanation we have for the origins of the universe. It tells the story of a moment when all of existence was compressed into an infinitesimally small point, which then exploded and began expanding, creating the universe we know today. While there is still much we don’t know and may never fully understand, the Big Bang theory continues to inspire wonder and awe, reminding us of the incredible complexity and beauty of the universe we call home.