hot Big Bang
Run a film of the expanding universe backward and something simple happens: as the cosmos shrinks, everything in it gets squeezed closer together and hotter. Press the same gas into a smaller volume and it heats up, the way a bicycle pump warms as you compress the air inside. Wind the whole universe back far enough and it becomes a uniform, blazing-hot, dense soup of radiation and particles. That is the central idea of the hot Big Bang: the universe began in an extremely hot, dense state and has been expanding and cooling ever since.
The word 'hot' is the heart of the model and what makes it testable. As the universe expanded, its temperature fell in a predictable way, and at each temperature different physics took over. In the first second it was a plasma of fundamental particles; in the first few minutes, while it was still billions of degrees, light atomic nuclei were forged out of protons and neutrons; about 380,000 years in, when it had cooled to a few thousand degrees, electrons finally stuck to nuclei to make neutral atoms and the universe became transparent, releasing a flash of light we still detect today as the cosmic microwave background. Three great pillars of evidence support this: the expansion itself, the abundances of the lightest elements, and that relic afterglow.
The hot Big Bang is not a theory of how the universe came from nothing, nor a description of an explosion at a point in space. It is a theory of what the universe was like once it was already hot, dense, and expanding — and how it evolved from there. The very first instant, the 'bang' itself, lies beyond what the standard model describes; that is the territory of the Planck era and of cosmic inflation. A common misunderstanding pictures the Big Bang as matter flying outward into pre-existing empty space from a central blast. In fact space itself was hot and dense everywhere at once, with no center and no edge; it is space that has been stretching.
The model makes a sharp prediction: a faint glow of leftover heat should fill the whole sky, cooled by expansion to just a few degrees above absolute zero. In 1965 two engineers found exactly that — a hiss of microwaves coming uniformly from every direction, now measured at 2.725 degrees above absolute zero. That relic radiation is the cooled-down fireball of the hot Big Bang itself.
The cosmic microwave background is the cooled afterglow predicted by the hot Big Bang.
The Big Bang was not an explosion at a point in space — it was a hot, dense state of space itself, the same everywhere, with no center. And the model describes what happened after the universe was already hot and expanding, not the literal first instant or 'why' it began.