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The Big Bang model describes the universe as having originated from an extremely hot, dense state approximately 13.8 billion years ago and expanding ever since. Three independent pillars of evidence support it: (1) Hubble's observation of cosmic expansion, which runs backward to a hot dense origin; (2) the cosmic microwave background (CMB) — a nearly uniform 2.7 K thermal glow from the cooled plasma of 380,000 years after the Big Bang, when the universe first became transparent; and (3) Big Bang nucleosynthesis — observed abundances of hydrogen, deuterium, helium-4, and lithium-7 precisely match predictions of nuclear reactions in the first three minutes. The Big Bang is not an explosion of matter into pre-existing space but the beginning of space-time expansion itself.
Study the timeline of the universe from the Planck epoch through nucleosynthesis, recombination, and the formation of first stars. Understand the CMB as a snapshot of the universe at recombination and how its tiny temperature fluctuations grew into today's large-scale structure.
You already know from Hubble's law that galaxies are receding from us at speeds proportional to their distance, which means the universe is expanding. Now run that expansion backward in time. If galaxies are flying apart today, they were closer together yesterday, and closer still a billion years ago. Extrapolate far enough and everything converges toward an extraordinarily hot, dense state — the Big Bang, approximately 13.8 billion years ago. This is not an explosion that scattered matter into pre-existing empty space. Space itself has been expanding, carrying matter with it, and the Big Bang marks the beginning of that expansion.
The strongest evidence comes from three independent lines. First, the expansion itself, measured through Hubble's law and confirmed by observations of distant supernovae. Second, Big Bang nucleosynthesis: in the first three minutes after the Big Bang, temperatures were high enough for nuclear fusion to occur throughout the universe. The predicted abundances — roughly 75% hydrogen, 25% helium-4, with trace amounts of deuterium and lithium-7 — match observed cosmic abundances with remarkable precision. You know from stellar nucleosynthesis that stars produce heavier elements, but the universe's baseline hydrogen-to-helium ratio was set in those first minutes, before any star existed.
Third and most dramatic is the cosmic microwave background (CMB). For the first 380,000 years, the universe was so hot that atoms could not form — electrons and protons existed as a plasma that scattered photons, making the universe opaque. As expansion cooled the plasma below about 3,000 K, electrons combined with protons to form neutral hydrogen in an event called recombination, and photons could suddenly travel freely. Those photons have been streaming through space ever since, their wavelengths stretched by the expansion of the universe from visible light down to microwaves. Today they form a nearly perfect blackbody spectrum at 2.725 K — the faint afterglow of the early universe, detectable in every direction.
The CMB is not perfectly uniform. Tiny temperature fluctuations of about one part in 100,000, mapped in exquisite detail by satellites like COBE, WMAP, and Planck, correspond to slight density variations in the early universe. These are the seeds of all cosmic structure: regions slightly denser than average gravitationally attracted more matter over billions of years, growing into the galaxies, galaxy clusters, and cosmic web we observe today. The statistical pattern of these fluctuations encodes fundamental cosmological parameters — the age of the universe, the ratio of ordinary matter to dark matter, and the geometry of space — making the CMB the single most informative observation in all of cosmology.