In Chapter 18, we put forward a history of the universe. The picture is extremely simple. Its inputs are Einstein's equations and the assumptions of homogeneity and isotropy. We also used our knowledge of laws of atomic, nuclear and particle physics. We saw a number of striking confirmations of this basic picture, but there are many puzzles.

1. (1) The most fundamental problem is that we don't know the laws of physics relevant to temperatures greater than about 100 GeV. If there is only a single Higgs doublet at the weak scale, it is possible that we can extend this picture back to far earlier times. If there is, say, supersymmetry or large extra dimensions, the story could change drastically. Even if things are simple at the weak scale, we will not be able to extend the picture all the way back to t = 0. We have already seen that the classical gravity analysis breaks down.

2. (2) There are a number of features of the present picture we cannot account for within the Standard Model. Specifically, what is the dark matter? There is no candidate among the particles of the Standard Model. Is it some new kind of particle? As we will see, there are plausible candidates from the theoretical structures we have proposed, and they are the subject of intense experimental searches.

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