Excerpt

I had to make some quite arbitrary decisions as to what I could include and not include, and I could easily imagine someone else very clever or myself in a different mood discussing a rigorously disjoint set of topics. What I chose to do is to talk about three relatively specific, very important problems in physics, astronomy, and to a lesser extent, mathematics, and try to generalize from these problems. I chose problems which do have a true interdisciplinary component and, at the same time, are among the most important problems we are currently up against. In each of these, I shall go from the specific to the general, so do not be misled by the headings. They refer to the vague generalities that come at the end.

New Sensory Systems

We now have from microphysical considerations what I think is a very good candidate for a complete model of formation of structure in the universe. More and more definite evidence over the last twenty years has been accumulating for the Big Bang cosmology, and it is now quite generally established. The new development over the last ten years or so is that we have obtained a much better idea of some unknowns, some of the parameters in the Big Bang cosmology, which previously had to be put in as initial conditions with no understanding. We now have real physical insights about what they should be. In particular, we have a strong theoretical prejudice based on reasons that I cannot go into in great detail here: that the density of the universe, on the average, should be equal to the critical density. If the universe were slightly more dense than it is, it would eventually collapse. It is just poised on the verge of collapsing. That means the density in terms of Newton’s constant Gn and Hubble’s parameter H is 3H2/8πGn or numerically 10⁻29 g/c3. Ordinary matter consists of about 1024 atoms per cubic centimeter. The density of the universe as a whole is about one atom per cubic meter, a number I find astonishing. Nevertheless, that is approximately the density of the whole, and we want to know whether the universe has that density or slightly less. The theoretical prejudice, which is not contradicted yet by experiments, is that it should be almost precisely equal.

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