Big Bang
The Big Bang theory describes the universe's origin from an extremely hot, dense state around 13.8 billion years ago, followed by rapid expansion and cooling that continues to this day.
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.
Sources
APODs including "Big Bang"
COBE All-Sky Map
7 October 2006
This historic all-sky map is based on the first two years of data from NASA's COsmic Background Explorer (COBE) satellite, launched in November of 1989. The map shows minute temperature variations (red is hotter) imprinted on the Cosmic Microwave Background (CMB) radiation by structures in the early Universe. These detailed measurements of the CMB and other COBE results ushered in an age of precision cosmology, and exactly confirmed the predictions of the Big Bang theory. Playing leading roles in the COBE project, for their resulting discoveries John C. Mather (NASA, Goddard Space Flight Center), and George F. Smoot (UC Berkeley) were selected to receive the 2006 Nobel Prize in Physics.