The Big Bang
The Big Bang explained: how an expanding universe began in a hot dense state, the evidence from redshifts to the microwave afterglow, and what came next.
The Big Bang is the prevailing account of how the universe began: about 13.8 billion years ago all space, matter, and energy were compressed into an extremely hot, dense state that has been expanding and cooling ever since. Galaxies, stars, and planets are the cooled-down end products of that expansion.
An expanding universe
The central observation is that distant galaxies recede from us in all directions, with speed proportional to distance — the Hubble-Lemaître law. Space itself is stretching, carrying galaxies apart like raisins in rising dough. Run that expansion backward and the universe was hotter, denser, and smaller at every earlier epoch, converging on an extremely hot, dense origin. The framework is Einstein's theory of gravity, built on the insights of special relativity: matter and energy curve spacetime, and the equations permit — indeed, demand — a dynamic, evolving cosmos. Note that the Big Bang was not an explosion in space but an expansion of space, with no centre and no edge; every observer sees the same recession in all directions. The cosmic age derived from this expansion — cross-checked against the oldest star clusters — anchors the 13.8-billion-year figure used throughout modern cosmology.
The first minutes: cosmic nucleosynthesis
In its first seconds the universe was a soup of protons, neutrons, electrons, and radiation, far too hot for atoms. As expansion cooled it below a billion degrees, protons and neutrons began fusing — the same fusion physics, running everywhere at once. Within minutes this primordial nucleosynthesis forged the universe's first helium, deuterium, and traces of lithium, then stopped as cooling and dilution shut the reactions down. The predicted abundances — roughly three-quarters hydrogen, one-quarter helium by mass — match observations of pristine gas clouds superbly. Everything heavier came much later, cooked inside stars and scattered by their deaths, as described in stellar nucleosynthesis.
The afterglow and the evidence
Three pillars support the Big Bang account. First, the expansion itself, measured through galactic redshifts and calibrated distance ladders. Second, the light-element abundances, which match nucleosynthesis calculations with no adjustable parameters. Third, the cosmic microwave background: heat left over from the infant universe, released when electrons and nuclei first combined into neutral atoms and the cosmos turned transparent. That afterglow arrives today as faint microwaves from every direction, at a temperature just above absolute zero, with ripples that encode the seeds of all later structure — seeds whose growth required the gravitational scaffolding of dark matter. Together these independent measurements tell one consistent story: a universe about 14 billion years old, expanding and cooling from a hot, dense beginning.
Tags
big bang cosmology expansion universe