First Moments and Cosmic Inflation
Within the first fraction of a second after the Big Bang, the universe underwent rapid exponential expansion known as inflation, smoothing out irregularities and setting the initial conditions for all subsequent structure. This phase is supported by measurements of the cosmic microwave background and large scale galaxy surveys that match predictions from inflationary models according to Forbes.
During the first few minutes, temperatures dropped enough for protons and neutrons to combine into light nuclei in a process called Big Bang nucleosynthesis, producing roughly 75 percent hydrogen and 25 percent helium by mass, with trace amounts of deuterium and lithium as reported by Space.com.
Recombination and the Birth of Light
From Plasma to Neutral Atoms
About 380,000 years after the Big Bang, the universe cooled to roughly 3,000 kelvin, allowing electrons to bind with nuclei and form neutral atoms, a milestone called recombination. This made the cosmos transparent to radiation, releasing the cosmic microwave background that we still detect as a nearly uniform glow across the sky per NASA.
Mapping the Early Universe
Space missions such as the Planck satellite have mapped tiny temperature fluctuations in this background radiation, providing precise values for the age, composition, and geometry of the universe, and confirming that ordinary matter makes up only about 5 percent of the total energy density.
Structure Formation and the Modern Universe
From Gas Clouds to Galaxies
Over hundreds of millions of years, gravity pulled matter into filaments and halos, forming the first stars and galaxies, while dark matter shaped the large scale cosmic web that astronomers observe in galaxy redshift surveys today.
Current Observations and Open Questions
Modern telescopes and missions continue to refine measurements of expansion rate, dark energy, and galaxy evolution, with data from observatories and space agencies feeding into models that describe how the universe transitioned from a hot dense state to the structured cosmos we see now according to ESA.