In our culture, the phrase “big bang theory” is often used to mean the idea that the universe was created in one explosive moment (or it’s a TV sitcom or a Styx album). For cosmologists, however, “big bang” means the early expansion of the universe, which might or might not have begun in an instant. The late stages of this process are better understood than the beginning: It ended with a sky full of stars, but at the beginning, even the laws of physics are unknown. Is a point of infinitesimal size and infinite density even possible? No one knows.
The big bang was no ordinary explosion. Not only did matter fly apart, as it does from fireworks, but space itself expanded from a small volume to a large volume. When scientists speak of expanding space, they mean a specific type of space-time curvature. On a curved object, such as a pear, the total length of one dimension varies as a function of the other. Near its stem, a pear’s circumference is small, but this circumference grows, levels out, grows again and shrinks as you go from the top of the pear to the bottom. In the same way, the volume of space grew from early times to late times, if we think of time as a dimension. In fact, the image above shows what this space-time shape would look like if we could see the universe and time from the outside.
Simply due to the shape of the bowl, later times have more elbow room than earlier times. This is why the early universe was so hot and dense. If you go back far enough, the entire universe was as hot as the center of a star. Just as stars fuse heavy elements from lighter ones, the early universe fused helium from hydrogen, and modern measurements of the hydrogen-to-helium ratio (about 4 to 1) agree exactly with the expected temperature (a billion degrees Celsius) and time available for this process (three minutes).
At even earlier times, protons must have formed from quarks, and the Higgs field must have become as asymmetric as it is today. These earlier extrapolations are more uncertain, relying on discoveries about how matter behaves at such high energies (from colliders) and patterns in the distribution of matter (from telescopes). The shape of the earliest moments left its imprint in the cosmic microwave background, the light left over from when the whole universe glowed with heat. Advances in particle physics and cosmology could tell us whether space came from a point, like the bottom of a bowl, or started as a long bee-stinger called inflation (see figure). For all we know, the big bang was not the beginning of the universe, but a transition from some other kind of universe.
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