The Big Bang — How the Universe Began

What the Big Bang actually was

The Big Bang was not an explosion of matter into pre-existing empty space. It describes the observable universe expanding from an extraordinarily hot, dense early state roughly 13.8 billion years ago. The term "explosion" is a common misconception; what expanded was spacetime itself, carrying matter and radiation along with it. Questions about whether there was a "before" remain open in modern physics.

The Singularity — t = 0

At t = 0, the known laws of physics break down. The density and temperature were effectively infinite — a state called the initial singularity. General Relativity predicts this singularity but cannot describe it, because quantum effects dominate at the Planck scale (10⁻³⁵ m, 10⁻⁴³ s). A complete theory of quantum gravity — still unfinished — would be required to describe the universe at this earliest moment. What we know with confidence begins a fraction of a second after t = 0.

Cosmic Inflation — 10⁻³⁶ to 10⁻³² seconds

A fraction of a second after the Big Bang, the universe underwent a period of exponential expansion called inflation, growing by at least a factor of 10²⁶ in an almost immeasurably brief interval. Proposed by Alan Guth in 1980, inflation solves three deep problems: (1) the flatness problem — why spacetime is geometrically flat to high precision; (2) the horizon problem — why the cosmic microwave background is nearly uniform across regions that should never have been in contact; (3) the absence of magnetic monopoles. Quantum fluctuations during inflation were stretched to cosmic scales, seeding the density variations that would eventually form galaxies.

Nucleosynthesis and the Dark Ages

Between about 10 seconds and 20 minutes after the Big Bang, the universe cooled enough for protons and neutrons to fuse into the first atomic nuclei — mostly hydrogen and helium, with trace amounts of deuterium and lithium. This period is called Big Bang Nucleosynthesis. Its predictions agree well with observed hydrogen, helium, and deuterium abundances, although the primordial lithium abundance remains an open problem. For the next 380,000 years, the universe remained an opaque plasma of electrons and nuclei. When it cooled to about 3,000 K, electrons combined with nuclei (recombination), and the universe became transparent. This released the light we now observe as the Cosmic Microwave Background (CMB). The later interval before the first stars lit up is called the Cosmic Dark Ages.

Galaxy Formation and the Cosmic Web

After recombination, gravity amplified the tiny quantum fluctuations from inflation into the large-scale structure we observe today. Dark matter — which interacts only gravitationally — collapsed first into halos, providing the gravitational scaffolding for ordinary matter to fall into. The first stars (Population III stars — massive, metal-free, extremely luminous) ignited around 200 million years after the Big Bang, ending the Dark Ages. Over billions of years, galaxies formed, merged, and organised into the cosmic web of filaments, sheets, and voids visible in surveys like the Sloan Digital Sky Survey. The Milky Way formed roughly 8–9 billion years ago.

Evidence: why we know the Big Bang happened

The Big Bang model rests on several strong lines of observational evidence. (1) Hubble expansion: galaxies are receding, with more distant galaxies receding faster, as expected in an expanding universe. (2) The CMB: discovered by Penzias and Wilson in 1965, this 2.725 K radiation is the cooled afterglow of the hot early universe, with temperature fluctuations of only about 1 part in 100,000. (3) Big Bang Nucleosynthesis: the observed abundances of hydrogen, helium, and deuterium broadly match predictions, though the primordial lithium abundance remains a known tension. (4) Galaxy evolution: distant galaxies — seen as they were billions of years ago — are systematically younger and more irregular than nearby ones, confirming that the universe evolved from a simpler early state.

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