Definition
The Big Bang is the hot, dense initial state from which the observable universe has been expanding for roughly 13.8 billion years.
It is not an explosion in space but an expansion of space itself. Every point in today's universe traces back to a smaller, denser, hotter past — and, when extrapolated all the way back, to a region where the equations of general relativity predict a singularity of infinite density and zero size.
Why it matters
How it works
In 1929 Edwin Hubble showed that distant galaxies recede with velocities proportional to their distance. Run the expansion film backward and everything converges. Georges Lemaître had already proposed in 1927 that the universe began from a "primeval atom," and Alexander Friedmann had derived the expanding-universe solutions of general relativity in 1922.
The decisive confirmation came in 1965 when Arno Penzias and Robert Wilson detected the cosmic microwave background — a uniform 2.725 K bath of thermal radiation predicted as the cooled afterglow of a once-hot, dense plasma. Big Bang nucleosynthesis (the first three minutes) explains the observed cosmic abundances of hydrogen (~75%), helium (~25%), and trace lithium with no free parameters.
Modern cosmology refines the picture with a brief inflationary epoch (~10⁻³⁶ s after t = 0) that flattens the geometry and seeds structure, and with dark matter and dark energy controlling the late-time expansion. The "bang" itself — the t = 0 singularity — is widely interpreted as a signal that general relativity breaks down, not as a literal point of infinite density.