Chapter 2

Introduction

By the end of the previous chapter, the Universe had already come a long way. Space itself was expanding. The first protons and neutrons had formed. Hydrogen and helium nuclei now drifted through the young Universe. The ingredients for everything we see today were finally in place.

But there was still one major problem. There were still no atoms. No stars. No galaxies. And perhaps most surprisingly… No visible light. To understand why, we first need to understand what the Universe actually looked like around 380,000 years after the Big Bang.

The Universe Finally Clears

Fortunately, the Universe had one thing working in its favour. Time. As space continued expanding, everything gradually cooled. Around 380,000 years after the Big Bang, temperatures finally dropped low enough for electrons to slow down and become captured by hydrogen and helium nuclei. For the first time in cosmic history, stable atoms were born. This event is known as recombination.

It's a slightly strange name because the particles had never actually combined before, but the name has remained. Its importance cannot be overstated. Once electrons became locked inside atoms, there were suddenly far fewer free charged particles left to scatter light. Almost instantly (on a cosmic timescale), the fog lifted. Photons that had spent hundreds of thousands of years bouncing around the Universe could finally travel freely across space. The Universe had become transparent. It was the closest thing the cosmos has ever had to someone switching the lights on.

The Oldest Light in the Universe

The photons released during recombination are still travelling today. For the last 13.8 billion years, they have crossed unimaginable distances, passing through expanding space as galaxies, stars and planets slowly formed around them. During that journey something interesting happened.

Because the Universe itself continued expanding, the wavelengths of those photons stretched with it. Originally, this light was energetic enough to be visible and infrared light. Today it has been stretched into microwaves, making it far too low in energy for our eyes to detect. Astronomers call this ancient radiation the Cosmic Microwave Background Radiation (CMBR). 

It fills every direction in the sky. No matter where we point our telescopes, we see the faint afterglow left behind by the moment the Universe first became transparent. It is, quite literally, the oldest light we can ever observe.

The CMB is also one of the strongest pieces of evidence that the Big Bang really happened. Tiny temperature differences within it have allowed scientists to measure the age, composition and evolution of the Universe with astonishing precision. It's remarkable to think that, even now, photons released over 13 billion years ago are still quietly passing through the room you're sitting in.

Looking Ahead

The Universe had finally become transparent. Light could travel freely through space for the first time. Hydrogen and helium filled the cosmos. Yet something was still missing. There were still no stars. No galaxies. No planets.

The Universe contained all the ingredients needed to build them but nothing had yet brought those ingredients together.

So how did a Universe that was almost perfectly smooth eventually produce billions of galaxies and trillions of stars? That story begins next.