earlyuniverse

The early universe refers to the period shortly after the Big Bang, up to the formation of the first atoms. In this era the cosmos expanded rapidly and cooled from extremely hot, dense conditions, setting the stage for all later structure. Cosmologists describe a sequence of phases defined by energy scales, particle content, and phase transitions that shaped the subsequent evolution of the universe.

Key epochs include the Planck epoch, followed by proposed events such as baryogenesis and various symmetry breakings. The inflationary paradigm posits a brief period of rapid expansion that smoothed and flattened space and amplified quantum fluctuations, seeding later structure. After inflation, the universe reheated and cooled through the electroweak and quantum chromodynamics transitions, altering the particle content and forces that governed interactions.

Big Bang nucleosynthesis occurred within the first few minutes, forging light nuclei such as helium-4, deuterium, helium-3, and traces of lithium. The resulting primordial abundances agree with observations once the baryon density is constrained by measurements of the cosmic microwave background.

At about 380,000 years after the Big Bang, electrons combined with nuclei to form neutral atoms, allowing photons to decouple and travel freely. The radiation released, the cosmic microwave background, is detectable today as a nearly uniform glow with small anisotropies that encode information about the early universe.

Over time, matter began to dominate over radiation, enabling growth of density perturbations into stars and galaxies. The first generations of stars emerged tens to hundreds of millions of years after the Big Bang, leading to reionization and the end of the cosmic dark ages.

Evidence for the early universe comes from the CMB, primordial element abundances, and the large-scale structure of matter, all interpreted within the standard ΛCDM cosmology with an inflationary origin.