LECTURE

From about 10 35 of a second after the moment of creation, we can tell a good scienti c story. Something appeared. It was tiny (the size of an atom perhaps) and inconceivably hot. It was expanding rapidly and cooling fast. At this point, energy and matter were indistinguishable. As this “something” cooled, “things” and “forces” appeared, in a series of “phase changes,” rather like the change steam undergoes as it turns into water. Gravity appeared from the chaos, as did the “strong” and “electromagnetic” forces.

From 10 33 to 10 32 seconds, the Universe expanded faster than the speed of light, growing from the size of an atom to the size of a galaxy. This phase is known as “in ation.” Within the rst second, quarks—the building blocks of atomic nuclei— were created in positive and negative forms. These then proceeded to annihilate each other, leaving just a few survivors. These forlorn survivors formed protons (which are positively charged) and neutrons (which are electrically neutral).

Electrons (lighter than protons and negatively charged) were created in an equally violent process. A few minutes after the big bang, the Universe consisted of a hot “plasma” (a mixture of energy and charged subatomic particles, a bit like the center of the Sun today).

About 380,000 years later, the Universe was cool enough for positively charged protons to capture negatively charged electrons, creating the rst, simple atoms of hydrogen and helium. Because the positive charges of protons are cancelled by the negative charges of electrons, atoms are electrically neutral, so electromagnetic radiation (of which light is just one form) could now travel freely through the Universe without getting tangled in networks of electromagnetism. The Universe cooled, entering a “dark age” that lasted several hundred million years.

Other entities and forces were created that we don’t understand. We call this missing stuff “dark matter” or “dark energy.” We know it’s there because studies of the movements of galaxies show that something is exerting a powerful gravitational pull. This stuff may account for up to 95% of the Universe’s total mass. Seventy percent of that total may consist of undetected forms of energy. Twenty- ve percent may consist of matter in forms we cannot detect because they emit no radiation (such as cold, dead stars or