The Big Bang

The Big Bang Theory is our best explanation for the origin and evolution of the universe. Approximately 13.8 billion years ago, all matter, energy, space, and time exploded outward from an infinitely hot, dense state — a singularity. In the first fraction of a second, the universe expanded exponentially (inflation) and cooled rapidly. The first protons and neutrons formed. Within minutes, nuclear fusion produced the first helium and hydrogen nuclei. After 380,000 years, the universe cooled enough for electrons to combine with nuclei, releasing light for the first time — the Cosmic Microwave Background Radiation, still detectable today. The Big Bang is supported by multiple lines of evidence: the expansion of the universe (galaxies moving away from each other), the Cosmic Microwave Background, and the observed abundance of light elements. The 2011 Nobel Prize in Physics was awarded for the discovery that the universe's expansion is accelerating — driven by mysterious "dark energy." What caused the Big Bang, what happened before it, and what "outside" the universe means are questions we cannot yet answer.