The Whole Shebang: A State-of-the Universe(s) Report
The Inflationary Universe: The Quest for a New Theory of Cosmic Origins
Before the Beginning: Our Universe and Others
On a summer weekend a few years ago my wife and I visited friends at their ranch in the Glass Mountains of West Texas. After dinner we sat outside on lawn chairs and looked up at the sky. Far from city lights, in clear, dry air, and with the moon down, we could see not only Altair and Vega and the other bright stars that you can see from anywhere on a cloudless summer night, but also an irregular swath of light running across the sky, the Milky Way, as I had not seen it in decades of living in cities.
The view is something of an illusion: the Milky Way is not something out there, far from us—rather, we are in it. It is our galaxy: a flat disk of about a hundred billion stars, almost a hundred thousand light years across, within which our own solar system is orbiting, two thirds of the way out from the center. What we see in the sky as the Milky Way is the combined light of the many stars that are in our line of sight when we look out along the plane of the disk, almost all of them too far away to be seen separately. Staring at the Milky Way and not being able to make out individual stars in it gave me a chilling sense of how big it is, and I found myself holding on tightly to the arms of my lawn chair.
Astronomers used to think that our galaxy was the whole universe, but now we know that it is one of many billions of galaxies, extending out billions of light years in all directions. The universe is expanding: any typical galaxy is rushing away from any other with a speed proportional to the distance between them. As the universe becomes less crowded it is also becoming colder. Observing what is happening now, and using what we know of the laws of physics, we can reconstruct what must have been happening in the past.
Here is the account that is now accepted by almost all working cosmologists. About 10 to 15 billion years ago, the contents of the universe were so crowded together that there could be no galaxies or stars or even atoms or atomic nuclei. There were only particles of matter and antimatter and light, uniformly filling all space. No definite starting temperature is known, but our calculations tell us that the contents of the universe must once have had a temperature of at least a thousand trillion degrees centigrade. At such temperatures, particles of matter and antimatter were continually converting into light, and being created again from light. Meanwhile, the particles were also rapidly rushing apart, just as the galaxies are now. This expansion caused a fast cooling of the particles, in the same way that a refrigerator is cooled by the expansion of the freon gas in its coils. After a few seconds, the temperature of the matter, antimatter, and light …