The Big Bang is not an explosion of matter into empty space — it is the expansion of space itself from an extremely hot, dense state about 13.8 billion years ago. The term 'Big Bang' is somewhat misleading; a better mental model is that all of space (not just matter in space) was once compressed into an extraordinarily small, hot, dense state, and has been expanding ever since.
Before ~10⁻⁴³ seconds (the Planck era), our current physics — general relativity and quantum field theory — cannot describe what was happening. At this frontier, quantum gravity (a still-unsolved unification) should apply. The observable universe has a beginning in the observable sense, but whether there was 'anything' before it is a question that may not even be well-posed.
10⁻³⁶ to 10⁻³² seconds — Inflation: A period of exponential expansion stretched the universe from sub-atomic to macroscopic scales almost instantaneously, smoothing out irregularities and seeding the quantum fluctuations that would later grow into galaxies.
First seconds — Quark–gluon plasma cooled into protons and neutrons. The universe was a hot soup of particles. Slight matter–antimatter asymmetry left a residue of normal matter.
3 minutes — Big Bang Nucleosynthesis: Protons and neutrons fused to form the first atomic nuclei: mostly hydrogen (75%) and helium (24%), with traces of lithium and beryllium. This ratio is a key observational test of the Big Bang model.
380,000 years — Recombination: The universe cooled enough (~3,000 K) for electrons to combine with nuclei, forming neutral atoms. For the first time, photons could travel freely — the universe became transparent. This moment's radiation, stretched by expansion, is visible today as the Cosmic Microwave Background (CMB).
~200 million years — The Cosmic Dawn: The universe was dark (no stars yet) until gravity clumped matter into dense knots, igniting the first stars. These first-generation (Population III) stars were likely massive, hot, and short-lived — and their UV light gradually reionised the universe again.
Billions of years — Galaxy formation and evolution proceeds, the universe continues to expand and, since about 5 billion years ago, that expansion has been accelerating due to dark energy.
The CMB is the oldest light we can observe — a snapshot of the universe at 380,000 years of age. It was discovered accidentally in 1965 by Arno Penzias and Robert Wilson, who noticed unexplained microwave noise from all directions in the sky. It corresponds to a blackbody temperature of about 2.725 K today.
The CMB is nearly perfectly uniform, but tiny temperature fluctuations (about 1 part in 100,000) encode the seeds of all cosmic structure. Missions like COBE (1989), WMAP (2001), and Planck (2009) have mapped these fluctuations in extraordinary detail, yielding precise measurements of the universe's age, geometry, composition, and rate of expansion.
In 1929, Edwin Hubble established that galaxies are receding from us with a velocity proportional to their distance: v = H₀d. This relationship — Hubble's Law — demonstrated that the universe is expanding. Run the expansion backwards and you arrive at the Big Bang.
The Hubble constant H₀ describes the rate of expansion. Its precise value is actually a matter of active debate: measurements from the CMB (early universe) give ~67 km/s/Mpc, while measurements using nearby standard candles give ~73 km/s/Mpc. This 'Hubble tension' is one of the biggest open problems in cosmology.
Test what you've just learned.
1.What is the Cosmic Microwave Background (CMB)?
2.What does Hubble's Law state?
3.What were Population III stars?
4.What is the 'Hubble tension'?