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Overview
The Big Bang theory is the leading scientific model for the early evolution of the universe. It states that the observable universe was once much hotter and denser and has been expanding and cooling over cosmic time. The theory is supported by galaxy redshifts, cosmic microwave background radiation, and the abundance of light elements.
Expansion of the Universe
Distant galaxies generally recede from us, and the farther they are, the faster they appear to recede. This indicates expansion of space on large scales.
Hot, Dense Early State
Running cosmic expansion backward implies an earlier state that was much hotter and denser than the universe today.
Cosmic Microwave Background
The cosmic microwave background is leftover radiation from the early universe, now cooled to microwave wavelengths.
Light Element Abundances
The early universe produced light elements such as hydrogen, helium, and small amounts of lithium. Observed abundances support Big Bang nucleosynthesis.
Not an Explosion in Space
The Big Bang was not an explosion from one point into empty space. It describes the expansion of space itself from an earlier hot, dense condition.
Cosmic Timeline
The universe evolved from early high-energy conditions into atoms, stars, galaxies, planets, and eventually complex structures.
Common Mistakes
A common mistake is asking where in space the Big Bang happened. In the model, space itself was part of the expanding universe.
Why This Matters in Physics
The Big Bang theory connects cosmology, relativity, particle physics, nuclear physics, galaxies, and the history of the universe.