Cosmology

The Big Bang is the prevailing cosmological model: the observable universe began ~13.8 billion years ago in a hot, dense state and has been expanding ever since. Evidence: cosmic microwave background, primordial nucleosynthesis ratios, galaxy redshifts, large-scale structure.

The CMB is a 2.725 K blackbody radiation filling all space — leftover photons from when the universe cooled enough (~380 000 years after Big Bang) for atoms to form and photons to free-stream. Tiny anisotropies (one part in 100 000) seed all later structure.

Dark matter is invisible mass (~85% of all matter) inferred from galaxy rotation curves, gravitational lensing and CMB anisotropies. It interacts gravitationally but not electromagnetically. Candidates include WIMPs, axions and primordial black holes.

The largest-scale structure of the universe — galaxies clustered along filaments separated by huge voids. N-body simulations starting from CMB-era density fluctuations reproduce this web through gravitational collapse over billions of years.

In the first few minutes after the Big Bang, protons and neutrons fused into light nuclei — about 75% hydrogen, 25% helium-4, traces of deuterium, lithium-7. The observed ratios precisely match theoretical predictions and strongly support the hot Big Bang model.