Big Bang

During the first fraction of a second—an era called inflation—the universe expanded exponentially, smoothing and flattening space. After inflation ended, the universe consisted of a highly energetic plasma of particles and radiation.

Within minutes, protons and neutrons combined to form light elements (hydrogen, helium, deuterium, and small amounts of lithium) in a process known as Big Bang nucleosynthesis.

About 380,000 years after the Big Bang, the universe cooled enough for electrons and nuclei to combine into neutral atoms. This allowed photons to travel freely, producing the Cosmic Microwave Background—the oldest light we can observe.

Over time, gravity amplified slight density variations in the plasma, leading to the formation of stars, galaxies, and large-scale structures.

Key evidence supporting the Big Bang includes the expansion of space (observed via galaxy redshifts), the abundance of light elements, and the precise measurements of the Cosmic Microwave Background by missions such as COBE, WMAP, and Planck.

Modern cosmological models (ΛCDM) incorporate dark matter and dark energy, explaining current observations of cosmic acceleration, geometry, and composition.