BIGBANG

The Big Bang theory is the prevailing cosmological model describing the origin and evolution of the universe. It posits that the cosmos began from an extremely hot, dense state roughly 13.8 billion years ago and has expanded and cooled ever since. The name does not imply an explosion into preexisting space; rather, space itself expanded from a compact state.

Evidence for the theory includes the observed redshift of distant galaxies indicating expansion, the cosmic microwave background radiation detected in 1965 as a near-uniform glow pervading the sky, and the primordial abundances of light elements consistent with nucleosynthesis in the early universe. The standard model of cosmology, often called Lambda-CDM, incorporates dark energy (Lambda) and cold dark matter, matching a wide range of observations from the CMB to the distribution of galaxies.

The timeline begins in the early universe with conditions described by physics beyond current knowledge, including a brief inflationary period that smoothed and stretched space. Within minutes, light elements formed; after about 380,000 years, electrons and nuclei combined to form neutral atoms, releasing the cosmic microwave background. Over billions of years, matter coalesced into stars, galaxies, and larger structures.

Despite its success, the Big Bang model leaves open questions, such as the nature of dark matter and dark energy, and it challenges physics at t approaching zero where a quantum theory of gravity would be needed. Ongoing observations and theoretical work continue to refine the model and its parameters.