The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. Once neutral atoms form, however, it takes roughly 550 million years for the ‘dark ages’ to end. Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

Ask Ethan: Why Was The Universe Dark For So Long?

The first stars formed almost half a billion years before we could see their light. Here’s why.

Ethan Siegel
Nov 11, 2017 · 6 min read

At the moment of the Big Bang, the Universe was full of matter and radiation, but there were no stars. As it expanded and cooled, you formed protons and neutrons in the first fraction of a second, atomic nuclei in the…