We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

In its early days, the universe was much hotter and denser than it is today. Back then, the universe was too hot even for atoms to form, leaving all matter in a state of floating ions called plasma. Electrons whizzed around freely, having too much energy and too much fun to be committed to a single positive nucleus. But as the universe cooled, there was a brief period when this ceased to be true: the temperature dropped enough that the charged plasma turned into neutral gas, and electrons started to orbit around protons to form atoms and elements. During this transition, the universe went from being opaque to being transparent. Back in the plasma phase, photons couldn’t get very far without bumping into the freely moving electrons and ions. But once the electrons and protons (and neutrons) formed neutral atoms, it was much rarer for photons to interact with them, so the photons could move around more freely. To the photons, the foggy universe suddenly became crystal clear. And because the universe has mostly gotten cooler since then, most of those photons are still flying along untouched. These are the photons that we detect when we measure the cosmic microwave background radiation, and the curious thing is that the temperature of these photons seems to be the same everywhere.