time itself—began 13.7 billion years ago in the big bang, and we are now telling the story of the universe with a confidence that has grown considerably over the last 10 years. We think that, just after the big bang, the universe was totally different from what it is today—none of the elementary particles that we know compose the matter of today were present. The universe was an incredibly dense knot of highly curved space-time. Then came an era of cosmic inflation, during which the universe rapidly expanded by a truly enormous factor (at least a factor of 100 trillion trillion in growth). The laws of quantum mechanics suggest that random fluctuations at the time of inflation would have produced microscopic density variations from place to place, which expanded with the universe to became macroscopic variations today. Remarkably, astrophysicists are able to connect the giant filaments and voids in the great cosmic web of galaxies to the seeds from which they grew. However, just as the cause of the current acceleration is unknown, so also is the underlying detailed physics of inflation still a complete mystery.
About 400,000 years after the big bang, the continued expansion and cooling of the universe had dropped the temperature to about 3,000 degrees, which was cool enough for the first hydrogen atoms to form. This is the epoch of recombination. A fundamental change in the universe occurred at that time when the cosmos went from being filled with a plasma that was opaque to light to being filled with an atomic gas through which light could freely pass. It is this freely streaming radiation that we observe at radio wavelengths as the faint glow known as the cosmic microwave background (CMB). The near uniformity of the CMB observed across the sky and the nature of the minute brightness fluctuations we measure in the CMB are just what is expected if inflation occurred. The CMB is therefore a fantastic signal telling us about the early universe.4
Following the recombination and the formation of the first atoms, the early universe was a nearly formless primordial soup of dark matter and gas: there were no galaxies, stars, or planets. The background radiation had a temperature that quickly cooled to a temperature below that of the coolest stars and brown dwarfs known today. This was truly the dark ages. However, things began to change when the slightly denser regions left over from inflation began to contract under the relentless pull of gravity. It took a few hundred million years, but eventually these dense regions gave birth to a variety of objects—the first stars, and black holes that glowed through accretion of matter—so that the universe became filled with light (Figure 2.5).