The Big Bang Theory - Explained (expanding universe theory)

The Big Bang Theory is the modern theory for the origin of Earth and the universe, focusing on how the universe came into existence. It's fundamentally about the expanding universe hypothesis, explaining what happened at the very beginning of our universe. Evidence suggests the universe indeed had a beginning, with nothing prior to that moment and then something (the universe) after.

In 1920, Edwin Hubble provided evidence that the universe is expanding, observing that galaxies move further apart over time. An analogy using an inflating balloon with marked points (galaxies) illustrates this expansion. While the distance between galaxies is increasing, the galaxies themselves are not expanding. The balloon analogy is thus only partially correct as real galaxies don't expand individually.

The Big Bang Theory posits that all matter in the universe initially existed in one place as a singular atom – a tiny ball with unimaginably small volume, infinite temperature, and infinite density. The origin of this singularity is unknown, but it's theorized to be similar to the core of black holes, where intense gravitational pressure squashes matter into infinite density.

This tiny ball violently exploded, leading to a huge expansion. The Big Bang marked the beginning of time, space, and matter. In a fraction of a second, the universe grew from smaller than an atom to larger than a galaxy and continues to expand today. This event is estimated to have occurred 13.7 billion years ago.

Within the first second of the Big Bang, energy converted into matter and antimatter, largely destroying each other, but some matter survived. More stable particles like protons and neutrons began to form. Over the next three minutes, the universe cooled enough (below 1 billion °C) for protons and neutrons to form hydrogen and helium nuclei. After 3,000 years, the universe cooled further (to about 4,000 °C), allowing atomic nuclei to capture electrons and form full atoms, making the universe transparent and filled with hydrogen and helium gas.