About 13.8 billion years ago, the universe began in a state of unimaginable density and temperature. Every galaxy, every star, every planet, every atom in your body — all of it emerged from this singular event we call the Big Bang.
The evidence
The Big Bang wasn't always accepted. In the early 20th century, many scientists believed the universe was static and eternal. But in 1929, Edwin Hubble made a discovery that changed everything: the universe is expanding. He observed that distant galaxies are moving away from us, and the more distant they are, the faster they're receding. This relationship — Hubble's Law — is the cornerstone of modern cosmology.
If the universe is expanding now, it must have been smaller and denser in the past. Running the clock backward, everything we see must have originated from a single point of infinite density: the Big Bang.
The smoking gun came in 1965. Arno Penzias and Robert Wilson, working at Bell Labs in New Jersey, detected a faint background hiss in their radio antenna that couldn't be explained. It was coming from every direction in space with equal intensity. They had accidentally discovered the cosmic microwave background radiation — the afterglow of the Big Bang itself.
The first seconds
The universe didn't explode outward in the conventional sense. Space itself expanded, carrying matter and energy along with it. In the first fraction of a second, the universe underwent a period of cosmic inflation that expanded it by a factor of 10²⁶ in less than 10⁻³² seconds.
At one microsecond, the universe was a hot plasma of quarks, gluons, and leptons. As it cooled, quarks combined to form protons and neutrons. At three minutes, the first atomic nuclei formed: mostly hydrogen (75%), helium (25%), and trace amounts of lithium. This process, called Big Bang nucleosynthesis, predicted the exact abundance of light elements — a prediction confirmed by observation.
For the next 380,000 years, the universe was too hot for atoms to form. Electrons bounced around freely, scattering light in all directions. The universe was opaque. Then, as it cooled to about 3,000 Kelvin, electrons finally combined with nuclei to form neutral atoms. The universe became transparent. Light could finally travel freely through space. This light — redshifted from visible to microwave frequencies by the expansion of the universe — is what Penzias and Wilson detected.
The cosmic microwave background
The CMB is the most perfect blackbody radiation ever measured: a uniform temperature of 2.725 K in every direction. But there are tiny fluctuations — variations of about 100 microkelvins — that represent the seeds of all structure in the universe.
These fluctuations were measured with extraordinary precision by the COBE, WMAP, and Planck satellites. The patterns in the CMB encode information about the universe's geometry, composition, and eventual fate.
Dark matter and the universe's recipe
Analysis of the CMB reveals the universe's composition: about 5% ordinary matter, 27% dark matter, and 68% dark energy. The dark matter wasn't part of the hot plasma — it didn't interact with radiation and could begin clumping together earlier. These dark matter halos provided the gravitational scaffolding for ordinary matter to eventually form galaxies.
What came before?
The honest answer is: we don't know. The laws of physics as we understand them break down at the moment of the Big Bang. Some theories suggest the universe emerged from a quantum fluctuation in a pre-existing vacuum. Others propose cyclic models where the Big Bang is just one bounce in an eternal sequence of expansions and contractions. Some physicists argue the question itself may be meaningless — time as we understand it may not exist "before" the Big Bang.