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.
30 June 2021
How did the first stars form? To help find out, the SPHINX computer simulation of star formation in the very early universe was created, some results of which are shown in the featured video. Time since the Big Bang is shown in millions of years on the upper left. Even 100 million years after the Big Bang, matter was spread too uniformly across the cosmos for stars to be born. Besides background radiation, the universe was dark. Soon, slight matter clumps rich in hydrogen gas begin to coalesce into the first stars. In the time-lapse video, purple denotes gas, white denotes light, and gold shows radiation so energetic that it ionizes hydrogen, breaking it up into charged electrons and protons. The gold-colored regions also track the most massive stars that die with powerful supernovas. The inset circle highlights a central region that is becoming a galaxy. The simulation continues until the universe was about 550 million years old. To assess the accuracy of the SPHINX simulations and the assumptions that went into them, the results are not only being compared to current deep observations, but will also be compared with more direct observations of the early universe planned with NASA's pending James Webb Space Telescope.