30 Philosophers, Chapter 1, Touchstone 1 of 80: Big Bang.
Immediately after the earliest state we can model, the universe entered a period of rapid expansion and cooling. This is the Big Bang Expansion: not an explosion in space, but the expansion of space itself.
That distinction matters. The Big Bang did not happen at one location inside an empty universe. There was no outside room waiting for it. Space itself expanded, carrying matter and energy with it. As the universe stretched, it cooled. That cooling allowed the first physical processes to unfold: fields changed, particles formed, forces separated, and eventually the long road toward atoms, stars, galaxies, planets, and life began.
The Big Bang is the first threshold in Big History: the beginning of our known universe. It marks the start of the cosmic story that leads, billions of years later, to chemistry, stars, planets, life, agriculture, science, and us.
The expansion itself is well supported by multiple lines of evidence, including the observed expansion of the universe, the cosmic microwave background, and the large-scale structure of galaxies. The cosmic microwave background is especially important: it is the oldest light we can observe, released about 380,000 years after the Big Bang when the universe cooled enough for light to travel freely. NASA describes this glow as a 2.7 K afterglow, stretched by cosmic expansion and still visible today.
The singularity before this expansion remains highly speculative. But the expansion of the early universe is one of the strongest pillars of modern cosmology.
Big History Thresholds: 1=Big Bang | 2=Stars&Galaxies | 3=Chemicals | 4=Solar System | 5=First Life | 6=TI | 7=Agrarian | 8=Science