The Big Bang is the leading scientific explanation for the origin of the universe, describing its expansion from an extremely hot, dense state 13.8 billion years ago. This article explores the evidence, the cosmic epochs that followed, and the open question of what triggered the initial event.
The Simons Observatory is a next-generation cosmic microwave background experiment in the Atacama Desert, designed to probe the earliest moments of the universe—from inflation to the formation of the first stars and galaxies. Using a suite of small and large aperture telescopes, it maps the oldest light in the cosmos with unprecedented precision, targeting questions about the universe’s origin, composition, and evolution.
The Hubble–Lemaître law describes the expansion of the universe, showing that galaxies recede at speeds proportional to their distance. It is a cornerstone of modern cosmology, linking the Big Bang to the cosmic microwave background and the large-scale structure of the cosmos.
The universe was not infinitely small at the Big Bang. Cosmic inflation stretched space to a finite, macroscopic size before the hot Big Bang began. This article traces the universe’s evolution from the Planck epoch to today, explaining key epochs, the cosmic microwave background, and the formation of structure.
The scale factor is a dimensionless number that describes how the universe expands over time. Normalized to 1 today, it allows cosmologists to track the evolution of the cosmos from the Planck epoch to the present, revealing the sequence of radiation, matter, and dark-energy-dominated eras.
The Big Bang theory is the leading scientific explanation for the origin and evolution of the universe. It describes how the cosmos emerged from an extraordinarily hot, dense state 13.8 billion years ago and has been expanding and cooling ever since — from the Planck epoch through inflation, nucleosynthesis, recombination, and the formation of stars and galaxies.
The universe is 13.8 billion years old. This definitive guide explains how astronomers measure the age of the cosmos using the cosmic microwave background, stellar evolution, and the expansion rate, and traces the universe’s evolution from the Planck epoch to the present day.
The lepton epoch was a brief but pivotal period roughly 1–10 seconds after the Big Bang, when electrons, positrons, and neutrinos dominated the universe. It set the stage for neutrino decoupling, the annihilation of electron-positron pairs, and the primordial conditions that led to Big Bang nucleosynthesis.
Acoustic peaks are periodic ripples in the cosmic microwave background temperature, imprinted by sound waves in the early universe. These peaks encode the fundamental physics of the Big Bang and have become a cornerstone of modern cosmology, revealing the composition and evolution of the cosmos.
One second after the Big Bang, the universe had expanded and cooled enough for neutrinos to decouple, ending the quark–hadron transition and setting the stage for nucleosynthesis. This article traces the entire cosmic timeline from the Planck epoch to structure formation, explaining inflation, recombination, the cosmic microwave background, and the emergence of stars and galaxies.
