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"
A Slice Through an Artificial Universe
23 June 1998
We live in the era of humanity when most of our Universe is being mapped. To help understand these maps, astronomers computationally estimate the appearance of several possible candidate universes, to which maps of the real Universe can be compared. Pictured above is a slice through one of these artificial universes, displayed so that each part of the universe is seen at the same time after the Big Bang. The above map corresponds to an area nearly ten billion light years across. Highlighted in red are filaments that each contain thousands of galaxies, while darker regions are nearly devoid of galaxies. Our good-sized Milky Way Galaxy would hardly be visible on this map.