The Big Bang and Cosmic Inflation: How Our Universe Began

Main Idea

About 13.8 billion years ago, everything that exists today, all matter, energy, space, and time, was compressed into an extremely hot, dense state. That state began expanding and cooling, a process that continues today as the universe keeps growing. Within the first tiny fraction of a second, a burst of exponential growth called cosmic inflation stretched space by a staggering factor, smoothing the universe out on large scales while stretching quantum-scale fluctuations up to cosmic size. Those stretched fluctuations became the tiny seeds of uneven density that gravity later amplified into galaxies, galaxy clusters, and the vast cosmic web we observe today.

Reasoning / Evidence

The CMB data solve two problems that a simple expanding universe couldn't explain on its own. Regions of the sky too far apart to have ever exchanged light show nearly identical temperatures (the horizon problem), and the universe's large-scale geometry is measured to be flat to within a fraction of a percent (the flatness problem). Inflation resolves both: a brief period of exponential expansion would stretch a small, uniform patch of space to become our entire observable universe, explaining both the uniformity and the flatness. WMAP and Planck also confirmed a specific prediction of inflation, that the density fluctuations should follow a near-perfect bell curve with a very particular statistical signature, which is exactly what their full-sky maps revealed.